SE2051213A1 - A hand-held electrically powered cut-off tool with a kickback mitigation function - Google Patents

Abstract

A hand-held electrically powered cut-off tool (100) for cutting concrete and stone by a rotatable cutting disc (105), the cut-off tool (100) comprising an electric motor (130) arranged to be controlled by a control unit (110) via a motor control interface (120),wherein the control unit (110) is arranged to obtain data indicative of an angular velocity of the cutting disc (105), and to detect a kickback condition based on a decrease in angular velocity, andwherein the control unit (110) is arranged to electromagnetically brake the electric motor (130) in response to detecting a kickback condition.

Description

TITLE A HAND-HELD ELECTRICALLY POWERED CUT-OFF TOOL WITH AKICKBACK IVIITIGATION FUNCTION TECHNICAL FIELD The present disclosure relates to electrically powered hand-held cut-off toolsfor cutting concrete and stone, and in particular to kickback mitigation systemsspecifically tailored for such tools.

BACKGROUND Cut-off tools for processing hard materials such as concrete and stone requirepowerful motors which provide the necessary energy to process the hardmaterials. Electrically powered cut-off tools have recently been introduced.These machines comprise high performance batteries which power hightorque electric motors. Some electrically powered cut-off tools are also powered via cable from electrical mains.

On rare occasions, the rotating cutting disc of the cut-off tool enters into lockingcontact with the object that is processed. Due to the large amounts of kineticenergy stored in the rapidly rotating cutting disc, the disc will be ejected fromthe object and the cut-off tool will move upwards and backwards towards theoperator. This is referred to as a kickback condition, and it may cause severeinjury to the operator. lt is therefore highly desirable to avoid kickback events, and to mitigate the effects of a kickback event if it should anyway occur.

US 10,675,694 B2 discloses a braking device able to quickly stop rotation of arotating cutting disc with high inertia. A brake unit acts on a belt drive of a cut-off tool to efficiently brake a cutting disc from a state of high kinetic energy.

US 8,413,34O B2 discloses a safety guard assembly for mitigating the harmfuleffects of a kickback event. The assembly includes a safety guard, a lockingmechanism, and a weight. ln a kick-back situation, the locking mechanism quickly releases, and the weight forces the guard to swing rapidly down over the guard, thereby providing protection from the saw blade.

However, there is a continuing need for improved kickback mitigation system.

SUMMARY lt is an object of the present disclosure to provide electrically powered hand-held cut-off tools with improved kickback mitigation systems. This object isobtained by a hand-held electrically powered cut-off tool for cutting concreteand stone by a rotatable cutting disc. The cut-off tool comprises an electricmotor arranged to be controlled by a control unit via a motor control interface.The control unit is arranged to obtain data indicative of an angular velocity ofthe cutting disc, and to detect a kickback condition based on an abruptdecrease in angular velocity. The control unit is also arranged to control anelectromagnetic braking of the electric motor in response to detecting akickback condition, and preferably also to actively regulate an energy outtakefrom the electric motor over the control interface during the electromagneticbraking.

This way a kickback condition can be detected very rapidly by the control unit,and the electromagnetic braking can be made powerful enough such that thekickback event can be stopped well before the event becomes dangerous toan operator of the cut-off tool. ln fact, in most cases the kickback mitigationsystems discussed herein are able to halt the kickback event before the cuttingdisc even leaves the object which is being cut. The braking operation ispreferably a controlled braking operation which is regulated by the control unit.This controlled energy outtake from the electric motor reduces the risk forcomponent damage and the like, while still providing efficient kickbackmitigation.

According to aspects, the control unit is arranged to estimate a rotor angle ofthe electric motor based on a measured current over the control interface, andto obtain the data indicative of angular velocity as a difference of the rotor angle over time. Several methods for estimating rotor angle based on measurements of current over the control interface between control unit and electric motor areknown. These methods advantageously do not require external sensors formeasuring rotor angle. For instance, the control unit can be arranged todetermine an angular position of a rotor of the electric motor based on dataindicative of a rotor flux angle of the electric motor, and to obtain the dataindicative of angular velocity as a difference of the rotor angular position overtime. Since the rotor angle determination can be done without signals fromexternal sensors, the entire kickback mitigation system disclosed herein canbe integrated in its entirety in the control unit and electric motor assembly,which is an advantage. Advantageously, there is no need for advanced brakingdevices such as that disclosed in US 10,675,694 B2.

According to aspects, the hand-held electrically powered cut-off tool comprisesan energy dissipating module configured to dissipate energy from the electricmotor during the electromagnetic braking in a controlled manner. This energydissipating module can be used by the control unit to perform braking in acontrolled manner without risking, e.g., too high energy levels in the controlunit circuitry, in the electric machine windings, or on the control interface. Theenergy dissipating module may, for instance, comprise any of a resistance, asuper-capacitor, a cable connection to electrical mains and/or a batteryconfigured with an energy absorption capacity.

According to aspects, the control unit is arranged to obtain the data indicativeof the rotor flux angle of the electric motor based on a measured current overthe control interface. This measurement of current is not associated with anysignificant implementation complexity in the control unit, which is anadvantage. However, the control unit may also be arranged to obtain the dataindicative of a rotational velocity of the cutting disc at least in part from anexternal sensor, such as a Hall effect sensor or the like configured to measurerotations of the electric motor shaft. The external sensor can be used as analternative to the current measurements on the control interface, or incombination with the current measurements on the control interface for increased reliability.

According to aspects, the control unit is arranged to process the data indicativeof the angular velocity of the cutting disc by a first low-pass filter and by asecond low-pass filter, where the first low-pass filter has a larger bandwidthcompared to the second low-pass filter. The first low-pass filter is applied forkickback event detection, and the second low-pass filter is applied otherwise.

This way the normal electric motor control is associated with a larger noisesuppression since a lower bandwidth low-pass filter is used. The kickbackevent detection is preferably more rapid, which is why the higher bandwidthfilter is used. Thus a robust motor control is provided while at the same time arapid kickback detection is enabled.

According to some aspects, the speed regulator function is bypassed whenthe control unit controls the electromagnetic braking during a kickback eventto enable a more rapid braking operation.

According to aspects, the control unit is arranged to determine an angularacceleration associated with the electric motor, and to detect the kickbackcondition based on a comparison between the determined angularacceleration and a detection threshold. This is a relatively low complexitydetection principle which nevertheless provides robust detection performance associated with a high detection probability and a low probability of false alarm.

According to aspects, the control unit is arranged to detect the kickbackcondition also based on an angular velocity associated with the electric motorby conditioning kickback detection based on a minimum angular velocity. Thisway false detections during, e.g., tool start from standstill is avoided, which isan advantage. The detection threshold can be manually configurable orarranged to be automatically configured by the control unit, e.g., independence of tool inertia.

According to aspects, the control unit is arranged to obtain data indicative of atool diameter or indicative of a tool inertia of the rotatable cutting disc, and toadjust the detection threshold based on the data indicative of tool diameter ortool inertia. This way kickback detection can be optimized to suit operation with a given tool. Some more heavy cutting discs may be associated with slightly different behavior in terms of decreased acceleration during kickback eventscompared to lighter cutting discs. lf the tool inertia is approximately known,such differences can be compensated for in order to obtain a more reliable andaccurate kickback detection mechanism. The control unit may for instance bearranged to obtain the data indicative of the tool diameter or tool inertia frommanual input, or based on a calculated or an estimated tool inertia, whereinthe tool inertia is arranged to be determined based on a current drawn by theelectric motor during acceleration from a standstill condition. Thus, a robustmechanism for estimating tool inertia without external sensors or the like isprovided. This mechanism can be applied to any cutting disc attached to thetool which is an advantage.

According to aspects, the cut-off tool comprises a radio frequency identification(RFID) reader, and the control unit is arranged to obtain the data indicative ofthe tool diameter or tool inertia from an RFID device embedded into the cuttingdisc via the RFID reader. This way the tool diameter or tool inertia data isobtained directly from the cutting disc in a reliable manner. When the cuttingdisc is replaced by another cutting disc, the data is automatically updated.

According to aspects, the cut-off tool comprises means for detecting toolidentification data (ID) from an optically readable tag arranged on the tool oron a tool packaging associated with the tool, and to obtain the data indicativeof the tool diameter or tool inertia based on the tool ID. The cut-off tool may,for instance, be configured to contact a remote server or the like to obtain therequired cutting disc data. The call to the remote server may comprise the toolID obtained from the optically readable tag. The cut-off tool may comprise aradio transceiver, and the control unit can be arranged to obtain the dataindicative of the tool diameter or tool inertia from the remote server via the radio transceiver.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor by applying a configurablebraking torque in dependence of a pre-determined time limit for braking the rotatable cutting disc. This means that the control unit does not always need to apply maximum braking force, and thereby spare components such as abraking resistor and other electrical components from increased wear due to hard use.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor to generate a torque determinedin dependence of a direct current (DC) bus voltage of the cut-off tool. This is arelatively simple method for controlling the applied torque. lt also protects largeparts of the control unit circuitry from dangerously high DC levels in an efficient manner.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor to generate a torque determinedin dependence of an energy dissipating capability of the energy dissipatingmodule. The energy dissipating module is associated with a maximum energydissipating capability, i.e., a maximum amount which can be dissipated over aperiod of time. By generating braking torque in dependence of this energydissipating capability, overloading the energy dissipating module can be prevented.

According to aspects, a DC bus voltage of the cut-off tool is regulated byswitching a device associated with an impedance during electromagneticbraking of the electric motor. This switching mechanism provides a relatively simple yet reliable mechanism for regulating the DC voltage.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor to generate a braking torquebelow a maximum braking torque level associated with a maximum rate ofchange in motor shaft angular speed. lf the electric motor is braked to rapidly,the estimate of rotor angle may suffer in accuracy, which in turn will degradebraking capability. By generating a braking torque below the maximum brakingtorque level, the estimate of rotor angle can be kept at an accurate level such that the braking performance is not reduced.

According to aspects, the cut-off tool comprises a support arm and the rotatable cutting disc is arranged to be driven by the electric motor via a belt drive and a geared transmission. This allows for a gear ratio which reducesthe torque requirements on the electric motor and control unit assembly duringthe kickback mitigation braking.

Generally, all terms used in the claims are to be interpreted according to theirordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to "a/an/the element, apparatus, component, means,step, etc." are to be interpreted openly as referring to at least one instance ofthe element, apparatus, component, means, step, etc., unless explicitly statedotherwise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated. Furtherfeatures of, and advantages with, the present invention will become apparentwhen studying the appended claims and the following description. The skilledperson realizes that different features of the present invention may becombined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present disclosure will now be described in more detail with reference to the appended drawings, whereFigure 1 shows an example of an electrically powered cut-off tool;Figure 2 schematically illustrates a general electric motor control system; Figure 3 schematically illustrates a three-phase electric motor control system based on an inverter; Figure 4 is a functional view of an example kickback mitigation system;Figure 5 schematically illustrates a kickback detection system; Figure 6 shows a cut-off tool support arm for a circular cutting blade; Figure 7 is a cross-sectional view of a support arm for a circular cutting blade;Figure 8 illustrates a drive arrangement for a circular cutting blade; Figure 9 is a flow chart illustrating methods; Figure 10 schematically illustrates a control unit; and Figure 11 schematically illustrates a computer program product.

DETAILED DESCRIPTION The invention will now be described more fully hereinafter with reference to theaccompanying drawings, in which certain aspects of the invention are shown.This invention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments and aspects set forth herein;rather, these embodiments are provided by way of example so that thisdisclosure will be thorough and complete, and will fully convey the scope ofthe invention to those skilled in the art. Like numbers refer to like elements throughout the description. lt is to be understood that the present invention is not limited to theembodiments described herein and illustrated in the drawings; rather, theskilled person will recognize that many changes and modifications may be made within the scope of the appended claims.

Figure 1 shows a hand-held electrically powered cut-off tool 100 for cutting intohard materials such as concrete and stone. The tool 100 comprises a rotatablecircular cutting disc 105, which may also be referred to as a cutting blade,mounted on a support arm 150. The cutting disc 105 is normally brought intorotation in a down-cut direction R, i.e., downwards into the object to be cut.The cutting disc 105 is arranged for abrasive operation, by means of cuttingsegments on the periphery of the cutting disc 105, where the cutting segments comprise diamond granules or the like.

An electric motor 130 is arranged to drive the cutting tool via a drivearrangement in the support arm. This motor is powered from an electricalenergy storage device 140, such as a battery or a super-capacitor.Alternatively, the electric motor may be powered from electrical mains viacable. An example drive arrangement for driving the cutting disc 105 by theelectric motor 130 will be discussed in more detail below in connection to Figures 6-8. This drive arrangement is based on a combination of gears and abelt to reduce requirements on the strength of the belt and also to reduce therequirements on electric motor torque. However, normal belt drives with two pulleys and an endless belt can of course also be used.

With reference again to Figure 1, the electric motor is controlled by a controlunit 110 via a motor interface 120. This is also schematically shown in Figure2 and in Figure 3. The motor interface may vary in function and physicalrealization, but the control unit 110 controls electric motor speed over theinterface, and may both accelerate and decelerate, i.e., brake, the electricmotor via the motor interface 120.

The motor is preferably a permanent magnet synchronous motor (PMSM)which is an alternating current (AC) synchronous motor whose field excitationis provided by permanent magnets, and which has a sinusoidal counter-EMF) electromotive force (back EMF) waveform. PMSM motors are known in electromotive force (counter waveform, also known as backgeneral and will therefore not be discussed in more detail herein. For instance,similar electrical motors including associated control methods are discussed in"Electric Motors and Drives" (Fifth Edition), Elsevier, ISBN 978-0-08-102615- 1, 2019, by Austin Hughes and Bill Drury.

The motor 130 may be a three-phase motor as schematically shown in Figure3. ln this case the motor interface 120 comprises three wires for energizing themotor windings. The wires are fed from an inverter 115 which is normallycontrolled by a current command from the control unit 110. An inverter is amodule which generates one or more phases of alternating current, normallyfrom a DC feed. By controlling the frequency and voltage of the phases overthe motor interface 120, the electromagnetic field in the motor can be broughtinto a controlled rotation to generate a positive torque by the motor shaft, whichthen can be used to power the cutting disc 105. The electric motor can also beused to provide negative torque to the motor shaft, i.e., to brake to cutting disc105.

The present disclosure relates to a kickback mitigation function for hand-heldelectrically powered cut-off tools which relies on quickly detecting onset of akickback event, followed by a controlled and resolute application of negativetorque by the electric motor to rapidly brake the cutting disc 105. lt has longbeen thought that electromagnetic braking cannot be applied fast enough andwith enough force to mitigate kickback in high power cut-off tools, mainly sincethe cutting disc stores so much kinetic energy during operation. However, bythe techniques disclosed herein, kickback mitigation in high power cut-off toolsis enabled. The techniques do not require a mechanical brake, which is anadvantage. Another advantage is that there is no requirement of any externalsensors to detect the kickback condition, since the control unit can performreliable detection based solely on the signals over motor interface 120.However, it is appreciated that external sensors can be used to complementthe system in order to provide an increased level of reliability and robustness.Furthermore, the kickback mitigation mechanism which will be described in thefollowing is so fast that it is often able to stop the kickback before the cuttingdisc even leaves the object which is being cut, thereby preventing all harmfuleffects of the kickback event.

Electromagnetic braking of electrically powered hand-held tools has beenproposed before, but for other applications with much less strict requirementson detection delay and generated braking torque. For instance, hammer drillsand the like are associated with a significantly smaller kinetic energy and arethus much more easily braked. US 10,675,747 B2 and US 7,055,620 B2 showelectromagnetic braking systems for mitigating effects of stuck drill bits, whichare not directly applicable for kickback mitigation in high powered cut-off tools.

US 10,630,223 B2 describes another example of a power tool comprisingmeans to brake a rotatable tool automatically based on electromagneticbraking principles. The disclosure describes a mechanism for detecting akickback condition based on an external sensor, such as a Hall effect sensor.The sensor is configured to measure a number of rotations of a rotatable worktool. lf the number of rotations has decreased significantly from one timeinstant to another, then a kickback condition is detected, and a response is 11 triggered. This polling operation is most likely not fast enough to respond to acut-off tool kickback condition in a timely manner, i.e., within a few millisecondsfrom onset of the kickback condition. The braking action described in US10,630,223 B2 mainly comprises disconnecting the electric motor from thepower source in order to avoid damage to a workpiece, which is a ratherrudimentary form of braking not likely to be able to cope with the large amountsof kinetic energy present in hand-held electrically powered cut-off tools. Also,there is no disclosure of any actively regulated or controlled braking function.Rather, the braking operation relies on mechanical switching of resistances.Once triggered, the counter EMF of the system described in US 10,630,223B2 will depend on the angular velocity of the electric motor shaft, so the appliedbraking torque will be a function of the rotational velocity of the tool and cannotbe controlled. To summarize, the mechanisms disclosed in US 10,630,223 B2are not ideal for mitigating kickback effects in high power cut-off tools wherethe amount of kinetic energy is very large compared to other types of powertools such as drills, grinders, and handheld saws.

To provide a kickback mitigation function which is suitable also for highpowered cut-off tools associated with significant tool inertia, that responds fastenough and with sufficient braking force, there is disclosed herein a hand-heldelectrically powered cut-off tool 100 for cutting concrete and stone by arotatable cutting disc 105. The cut-off tool 100 comprises an electric motor 130arranged to be controlled by a control unit 110 via a motor control interface120. The control unit 110 is arranged to obtain data indicative of an angularvelocity of the cutting disc 105, and to detect a kickback condition based on adecrease in angular velocity. The control unit 110 is arranged to controlelectromagnetic braking of the electric motor 130 in response to detecting akickback condition, and optionally also to actively regulate an energy outtakefrom the electric motor 130 over the control interface 120 during theelectromagnetic braking.

The detection mechanism is based on monitoring the angular velocity of thecutting disc 105. lf an abrupt decrease in velocity is seen, such as a high levelof retardation in electric rotor angle or cutting disc angle, a kickback condition 12 is detected. The details of the kickback event detection mechanism will bediscussed in more detail below. lmmediately after a kickback event has beendetected by the control unit 110, the electric motor is forcefully braked in orderto mitigate the effects of the kickback event. This braking involves an activecontrol of the energy outtake from the electric motor in order to provide a strong braking force without damaging the electrical components of the cut-off tool.

As mentioned above, the kickback detection and braking of the cutting disc isoften so rapid as to stop the blade before it even leaves the object which isprocessed. Thus, the upwards and backwards motion K in Figure 1 can oftenbe avoided altogether. Even if some kickback motion occurs, the energytransferred from the cutting disc 105 to the machine body 101 will be reducedto a level as to mitigate the harmful effects of the kickback event. Notably, theelectric motor is not just disconnected from the power source 140 as in manyof the prior art documents. Rather, the energy outtake from the electric motoris actively regulated to provide a strong enough braking action to halt thekickback event.

For instance, according to some aspects, the current from the motor isregulated actively during braking, and it can therefore be maximizedindependently of electric motor angular speed. This means that braking can becontrolled during the braking process to always maintain strong braking effect.lf a resistance is simply switched in as in some of the prior art, the counter-EMF of the electric motor will determine the braking force, thus leaving much room for improvement in braking capability.

The limiting factor in providing a strong braking torque is the energy dissipatingcapability of the electrical system. To brake the cutting disc 105, its kineticenergy must be transferred away from the cutting disc and dissipated by thesystem. The energy transfer must also be fast enough since otherwise thekinetic energy is transferred into the machine body 101 to generate thekickback movement K. This energy transfer is done electrically in the proposeddesign. Thus, no friction brakes or other complicated mechanical structures are required to provide the necessary braking torque. 13 Figure 4 shows a functional view of an example kickback mitigation system. Aspeed command is obtained, e.g., from the trigger 160 of the tool 100. Thisspeed command is input to a processor 410 which will be discussed in moredetail below in connection to Figure 10. The processor 410 converts the speedcommand into a current command which is sent to an inverter 420, which inturn controls the electric motor 130 via the motor interface 120. ln case themotor 130 is a three-phase motor, the control interface 120 comprises threewires with respective phases. An energy dissipator 430 is connected to theinverter 420. This dissipator is configured to consume surplus energy in thesystem, i.e., to dissipate energy from the electric motor 130 during theelectromagnetic braking, thereby protecting electrical components and themotor 130 itself from dangerously high voltages. The energy dissipatingmodule 430 may comprise any of a resistance, a super-capacitor and/or abattery configured with an energy absorption capacity. According to someaspects, the dissipator module 430 is a resistance configured to be switchedby the processor 410 in dependence of a DC voltage of a DC bus which feedsthe inverter with power. This way the DC voltage level on the DC bus can beregulated to always be close to a target level, or set-point level, despite a largesurge of energy coming from the electric motor 130 via the motor interface 120during the hard braking required to mitigate a detected kickback event.

According to a first example, the resistance is switched in if the DC bus voltageexceeds a first threshold and is switched out if the DC bus voltage goes belowa second threshold. The first threshold is preferably configured higher than thesecond threshold, which effectively means that the switching mechanism isassociated with hysteresis. This hysteresis provides a robust detection mechanism.

According to another example, a regulator such as a PID regulator is arrangedwith a set-point or target DC bus voltage value. This target DC bus voltagevalue is compared with the actual DC bus voltage value and the difference isused to determine a duty cycle for switching the resistance. 14 According to some aspects, the energy dissipating module 430 is notified whenkickback condition is detected, whereupon the energy dissipating module canprepare to absorb surplus energy before that surplus energy arrives at the DCbus. The energy dissipating module 430 may, e.g., preemptively switch in theresistance or lower the relevant voltage thresholds or target voltage values forperforming the switching. ln this case it may be advantageous to alsodisconnect the power source, since otherwise current from the power source may be drawn.

A current measurement taken in connection to the motor interface 120 is fedback to the processor 410, whereby a closed loop motor control system isformed. According to some aspects, the control unit 110 is configured toelectromagnetically brake the electric motor 130 by applying a configurablebraking torque in dependence of a time limit for braking the rotatable cuttingdisc 105. This means that the applied braking torque can be configured for aparticular tool in order to mitigate kickback events. Some tools may requiremore torque in order to be braked fast enough while other cutting tools mayrequire less force. Thus, the risk to the electric motor from braking too hard orto other electric components can be reduced.

According to other aspects, the control unit 110 is configured toelectromagnetically brake the electric motor 130 at a torque determined independence of a DC bus voltage of the cut-off tool 100. This way overload tothe DC bus can be avoided, which is an advantage. For instance, the DC busvoltage can be regulated by switching a device associated with an impedance, such as a resistor, during electromagnetic braking of the electric motor 130.

The processor maintains an estimate of rotor angle. There are many knownways to estimate rotor angle in an electric machine, e.g., based on the currentmeasurement as schematically illustrated in Figure 4. For instance, in "ElectricMotors and Drives" (Fifth Edition), Elsevier, ISBN 978-0-08-102615-1, 2019,Austin Hughes and Bill Drury discuss the topic at length.

One example of a method for estimating rotor angle based on a current measurement made on the control interface 120 will now be described. The method also uses a reference voltage associated with the electric motor, i.e.,the reference voltage upon which the current regulator mechanism is based. ltis assumed that the reference voltage (such as the reference voltage used bythe current regulator for the electric motor) is sufficiently similar to the actualaverage voltage over the phases of the electric motor over a time window of interest.

The reference voltage and control interface current are first transformed into acomplex stationary domain, i.e., the motor current tab and motor referencevoltage vab are represented as complex numbers. This is often referred to as a Clarkes transform.iab = ia +j * ibVab = Va +j * vbBased on these vectors, the complex valued magnetic flux of the stator ï-ibsflb I lpsfl +j * lpsß is estimated by integrating a difference between applied voltage and resistivevoltage drop, adjusted by a damping factor which is proportional to a previouslyestimated stator flux. The damping factor is added mainly to make the estimated rotor angle value more robust.

Given the stator magnetic flux, a winding-induced flux is subtracted (derivedbased on a product of motor winding inductance and motor current) in order to obtain the complex rotor magnetic flux ï-ibnab I lpifla +j * Ipryb Thus, let R represent motor resistance and L represent motor inductance, then the electric motor equations in the complex ab-plane (after Clarkes transform) are given by_ 61/1va = R =i< La + di"â'Uh = R * l-b + Ipsb dt 16 which can be rewritten as 61/1di" va - R =i< La61/1 .d? =vb-R*Lb These values could be directly integrated to obtain stator flux. However, adamping term is preferably introduced to stabilize the estimated rotor angle.One example of such a damping term operation is ïps,a[k]+: (vaUd _ R * iaUÜ] _ K * ïps,a[k _ lbdt1.115,1» [k]+= (Vblkl _ R * iblk] _ K * 1.115,1» [k _ ïbdf where K is a damping factor, K * :pm [k - 1] is the damping term referred toabove, k is a time index, and dt is a time step of the recursion. The winding- induced flux is subtracted asï-ibna [k] = Iibsfl [k] _ L * ia [k]ï-ibfqb [k] = lpsß [k] _ L * ib [k] This value is then optionally filtered by a low-pass filter or the like to suppressnoise and distortion. lf filtering is applied, then some delay compensation maybe necessary to account for delays introduced by the filtering and also otherdelays incurred by, e.g., computation and the like.

The rotor angle can be found as an angle of the estimated rotor flux tpnab, i.e.,a rotor flux angle. This angle can be determined, e.g., using a signed arcus tangent function, also known as an atan2 function "[15] I atanzüpffib [km Iibna + ß where a is the rotor angle and where ß is an angle compensation configured to compensate for introduced delays, e.g., by filtering operations.

According to another example, the control unit 110 is arranged to determinean angular position of a rotor of the electric motor, i.e., a rotor angle, based ondata indicative of a rotor flux angle of the electric motor, and to obtain the data indicative of angular velocity as a difference of the rotor angular position over 17 time, i.e., a time derivative or time difference value. The control unit 110 may,for instance, be arranged to obtain the data indicative of the rotor flux angle ofthe electric motor based on a measured current over the control interface(120), or electromagnetic force (EMF) associated with the electric motor 130. based on a measured or otherwise determined counter To improve the estimate of both rotor position and velocity, filtering can beapplied to reduce measurement noise. Such filtering may comprise, e.g.,normal low-pass filtering or more advanced filtering techniques such asKalman filtering and the like. However, too much noise suppressing filteringmay increase detection delay which is undesired.

According to some aspects, the control unit 110 is arranged to process thedata indicative of the angular velocity of the cutting disc 105 by a first low-passfilter and by a second low-pass filter. The first low-pass filter has a largerbandwidth compared to the second low-pass filter. The first low-pass filter isapplied for kickback event detection, and the second low-pass filter is appliedotherwise. This way, during normal operation, noise suppression is high, butthe system is not able to respond quickly to changes and it would incur toomuch delay in detecting a kickback event.

According to other aspects, the control unit 110 is configured toelectromagnetically brake the electric motor 130 at a braking torque below amaximum braking torque level associated with a maximum rate of change inmotor shaft angular speed. This means that the motor braking will be done ina controlled manner, which allows, e.g., to maintain an accurate estimate ofthe rotor angle in Figure 4. lf the rate of change in motor shaft angular speedgoes above a pre-determined or configurable threshold, then the braking force can be reduced. lt is appreciated that the arrangements for detecting angular position andvelocity without external sensors can be implemented independently of thebraking methods used in the tool. Thus, there is also disclosed herein a hand-held electrically powered cut-off tool 100 for cutting concrete and stone by a rotatable cutting disc 105. The cut-off tool 100 comprises an electric motor 130 18 arranged to be controlled by a control unit 110 via a motor control interface120. The control unit 110 is arranged to determine an angular position of arotor of the electric motor 130 based on an estimated rotor flux angle of theelectric motor, and to obtain data indicative of an angular velocity of the cuttingdisc 105 based on a rate of change of the angular position of the rotor overtime. The control unit 110 is arranged to detect a kickback condition based on105,electromagnetically brake the electric motor 130 in response to detecting akickback condition. a decrease in angular velocity of the cutting disc and to An example functional view of the kickback detector module 440 is shown inFigure 5. This module operates on rotor angle data which is first differentiatedonce 510 to obtained rotor speed and then again 520 to obtain rotoracceleration. The differentiation is optionally associated with a filteringoperation to suppress measurement noise. However, it is appreciated that allsuch noise suppressing filtering increases detection delay which isundesirable, thus, a balance should be made between detection delay andnoise suppression ability in the system, as discussed above.

Kickback detection is performed by an evaluation module 530 which comparesthe rotor acceleration to a detection threshold. lf a sufficiently large negativeacceleration is detected, then a kickback event is detected, and a brakecommand is issued by the evaluation module 530. ln other words, the controlunit 110 is arranged to determine an angular acceleration associated with theelectric motor 130, and to detect the kickback condition based on a comparisonbetween the determined angular acceleration and a detection threshold.

According to some aspects, the control unit 110 is arranged to detect thekickback condition also based on an angular velocity associated with theelectric motor 130 by conditioning kickback detection based on a minimumangular velocity. This angular velocity may, e.g., be the estimated rotor speedwhich results from the first differentiator 510, perhaps with some additionalfiltering applied. The conditioning may, e.g., comprise requiring a certain minimum initial velocity in order to detect a kickback event. The rationale for 19 this Conditioning being that a severe kickback event normally does not takeplace at low rotational cutting disc velocities. Also, the estimate of rotoracceleration may be associated with large errors during a start-up phase whenthe cutting disc is accelerated from stand-still or from a low velocity.

The detection threshold may according to one example be manuallyconfigurable. ln this case configuration data is manually input to the evaluationmodule 530, where it is used to determine the detection threshold.

According to other aspects, the control unit 110 is arranged to obtain dataindicative of a tool diameter or tool inertia of the rotatable cutting disc 105, andto adjust the detection threshold based on the data indicative of tool diameteror tool inertia. This data may, for instance, be obtained by the control unit 110as manual input. Herein, data indicative of a tool diameter also indicates anamount of inertia associated with the cutting disc 105. The larger the inertia,the more kinetic energy must be handled during a kickback event. This meansthat different types of tools having different weights and different tool diametersmay require different detection threshold in order for the kickback mitigationfunction to provide the desired performance.

The control unit 110 can also be arranged to obtain the data indicative of thetool diameter or tool inertia based on a calculated or estimated tool inertia. Thistool inertia can, for instance, be determined based on a current drawn by theelectric motor during acceleration from a standstill or low velocity condition,i.e., it can be determined from the current measurement on the controlinterface 120 in Figure 4 during acceleration of the cutting disc 105. Theprocessor 410 may comprise a look-up table or the like which allows fortranslating between estimated tool inertia values and suitable detectionthresholds. Alternatively, an analytic function can be used to determinesuitable detection thresholds from the estimated tool inertia. Also, low-passfiltering operations used to determine, e.g., rotor acceleration values, can beconfigured in dependence of such estimated tool inertia. This is because ahigh inertia tool is expected to change rotor velocity somewhat more slowly which may warrant a reduced filtering bandwidth to suppress more noise when estimating, e.g., rotor speed and rotor acceleration.

The cut-off tool may furthermore comprise a radio frequency identification(RFID) reader. ln this case the control unit 110 can be arranged to obtain thedata indicative of the tool diameter or tool inertia from an RFID deviceembedded into or otherwise arranged in connection to the cutting disc 105 viathe RFID reader. According to another example, the tool data may be storedon a remote server. lf the cut-off tool comprises a radio transceiver, the controlunit 110 can be arranged to obtain data indicative of the tool diameter or tool inertia from a remote server via the radio transceiver.

The cut-off tool may also comprise other means for identifying, e.g., the typeof tool. Such means for identification may comprise optically readable tagssuch as QR-codes, or punch-card like symbols which can be read optically andused to index a database on, e.g., the remote server, to obtain the data indicative of the tool diameter or tool inertia.

Of course, the data indicative of the tool diameter or tool inertia can also bemanually input to the control unit 110.

The rotational data can also be obtained from an external sensor, such as aHall effect sensor arranged to measure a rotational velocity of a shaft in thedrive arrangement, such as a motor shaft or a pulley shaft, or even the shaftof the cutting tool 105 itself. This rotational data can be used in combinationwith the rotor angle estimate data obtained directly from the electric motor, orit can be used in place of this data as an alternative source of information bywhich the kickback detection can be performed.

With reference to Figures 6-8, the cut-off tool 100 may comprise a belt drivearrangement in the support arm 150 configured to provide a drive ratio whichreduces the rotational speed of the electric motor drive shaft down to a speedsuitable for processing concrete, e.g., about 3500-4500 revolutions per minute(rpm). This is an advantage since electric motors operating at reduced enginespeeds are more costly and often also weighs more than standard motors operating around 9000-10000 rpm. Such gear ratios necessitate using a 21 smaller pulley at the motor drive shaft to drive a larger pulley connected to thework tool. However, if the larger pulley is co-axially attached directly to therotatable work tool, then the attainable cutting depth may be reduced by thelarge belt pulley.

The drive arrangement illustrated in Figures 6-8 is based on a combination ofa drive belt portion and a gear transmission portion. The belt drive portioncomprises a first pulley 610 and a second pulley 630 with a drive belt 620 inbetween. To reduce blade speed with respect to a rotational speed of the firstpulley, the second pulley has a larger pitch diameter than the first pulley. Thisdrive ratio increases torque and reduces speed making the rotatable work toolsuitable for dry cutting operation. The drive arrangement also comprises a geartransmission portion as shown in Figure 8. The gear transmission portioncomprises a first gearwheel 810 and a second gearwheel 820. The firstgearwheel 810 is co-axially connected to the second pulley 630 and thesecond gearwheel 820 is arranged to be co-axially connected to the rotatablework tool 105. Thus, as the first pulley 610 is rotated, the belt 620 drives thesecond pulley 630 in the same direction of rotation. The second pulley, beingco-axially connected to the first gearwheel 810, then drives the first gearwheelin the same direction of rotation as the first pulley 610. The first gearwheel 810is radially connected to the second gearwheel 820, and therefore drives thesecond gearwheel in an opposite direction of rotation. Thus, the direction ofrotation of the first pulley and the direction of rotation R of the work tool 105are opposite to each other. This is not a problem when using an electric motoras a power source, which can be configured to rotate in any direction. Thus,the disclosed drive arrangements are especially suited for use with electric motors.

The gear transmission portion is dimensioned to support a braking action bythe electric motor to stop rotation by the rotatable work tool from a rotationvelocity of about 50 m/sec in 5 ms, for a given belt dimension. Effectively thismeans that, due to the gear transmission portion, the power source can be parameterized more aggressively for a braking operation, without placing 22 undue requirements on the belt drive portion, and the belt in particular, which is an advantage.

According to some aspects, a ratio of the first gearwheel 810 pitch diameterand the second gearwheel 820 pitch diameter is between 0,4 and 0,6, andpreferably 0,56. According to an example, the first gearwheel 810 has a pitchdiameter between 20 and 35 mm, preferably 28 mm, and the secondgearwheel 820 has a pitch diameter between 40 and 60 mm, preferably 50mm. Regarding the belt drive portion, the first pulley 610 may be associatedwith a pitch diameter between 30 and 40 mm, preferably 35.4 mm, and thesecond pulley 630 may be associated with a pitch diameter between 60 mmand 70 mm, preferably 64.85 mm. According to aspects, a ratio between apitch diameter of the first pulley and a pitch diameter of the second pulley isbetween 0,4 and 0,6, and preferably about 0,55. Various types of drive be|tscan be used in the belt drive portion, such as a v-belt or a toothed belt.

Thus, according to some aspects, the cut-off tool 100 comprises a support arm150, wherein the rotatable cutting disc 105 is arranged to be driven by theelectric motor 130 via a belt drive 610, 620, 630 and a geared transmission810, 820.

There is also disclosed herein a hand-held electrically powered cut-off tool 100for cutting concrete and stone by a rotatable cutting disc 105. The cut-off tool100 comprises an electric motor 130 arranged to be controlled by a control unit110 via a motor control interface 120 as illustrated in, e.g., Figure 2 and Figure3. The control unit 110 is arranged to determine an angular position of a rotorof the electric motor 130, or, equivalently, of the electric motor shaft, based ona current over the control interface 120, and to obtain data indicative of anangular velocity of the cutting disc 105 based on a rate of change of the angularposition of the rotor (or shaft) over time. The control unit 110 is arranged todetect a kickback condition based on a decrease in angular velocity of thecutting disc 105, and to control electromagnetic braking of the electric motor130 in response to detecting a kickback condition. 23 Figure 9 is a flow chart illustrating methods, there is illustrated a method in acontrol unit 1 10 for mitigating kickback in a hand-held electrically powered cut-off tool 100 arranged for cutting concrete and stone by a rotatable cutting disc105, wherein the cut-off tool 100 comprises an electric motor 130 arranged tobe controlled by the control unit 110 via a motor control interface 120. The method comprisesobtaining S1 data indicative of an angular velocity of the cutting disc 105,detecting S2 a kickback condition based on a decrease in angular velocity, and electromagnetically braking S3 the electric motor 130 in response to detectinga kickback condition.

Optionally, the method also comprises actively regulating S4 an energyouttake from the electric motor 130 over the control interface 120 during theelectromagnetic braking.

Figure 10 schematically illustrates, in terms of a number of functional units, thegeneral components of a control unit 110. Processing circuitry 1010 is providedusing any combination of one or more of a suitable central processing unitCPU, multiprocessor, microcontroller, digital signal processor DSP, etc.,capable of executing software instructions stored in a computer programproduct, e.g. in the form of a storage medium 1030. The processing circuitry1010 may further be provided as at least one application specific integratedcircuit ASIC, or field programmable gate array FPGA.

Particularly, the processing circuitry 1010 is configured to cause the device180 to perform a set of operations, or steps, such as the methods discussedin connection to Figure 9 and the discussions above. For example, the storagemedium 1030 may store the set of operations, and the processing circuitry1010 may be configured to retrieve the set of operations from the storagemedium 1030 to cause the device to perform the set of operations. The set ofoperations may be provided as a set of executable instructions. Thus, theprocessing circuitry 1010 is thereby arranged to execute methods as hereindisclosed. 24 The storage medium 1030 may also comprise persistent storage, which, forexample, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

The device 110 may further comprise an interface 1020 for communicationswith at least one external device. As such the interface 1020 may compriseone or more transmitters and receivers, comprising analogue and digitalcomponents and a suitable number of ports for wireline or wireless communication.

The processing circuitry 1010 controls the general operation of the control unit110, e.g., by sending data and control signals to the interface 1020 and thestorage medium 1030, by receiving data and reports from the interface 1020,and by retrieving data and instructions from the storage medium 1030.

Figure 11 illustrates a computer readable medium 1110 carrying a computerprogram comprising program code means 1120 for performing the methodsillustrated in Figure 9, when said program product is run on a computer. Thecomputer readable medium and the code means may together form acomputer program product 1100.

Claims (26)

1. A hand-held electrically powered cut-off tool (100) for cutting concreteand stone by a rotatable cutting disc (105), the cut-off tool (100) comprising anelectric motor (130) arranged to be controlled by a control unit (1 10) via a motorcontrol interface (120), wherein the control unit (1 10) is arranged to obtain data indicative of an angularvelocity of the cutting disc (105), and to detect a kickback condition based on a decrease in angular velocity, and wherein the control unit (1 10) is arranged to control an electromagnetic brakingof the electric motor (130) in response to detecting a kickback condition.

2. The hand-held electrically powered cut-off tool (100) according to claim1, wherein the control unit (110) is arranged to actively regulate an energyouttake from the electric motor (130) over the control interface (120) during theelectromagnetic braking.

3. The hand-held electrically powered cut-off tool (100) according to claim1 or 2, wherein the control unit (110) is arranged to estimate a rotor angle ofthe electric motor (130) based on a measured current over the control interface(120), and to obtain the data indicative of angular velocity as a difference of the rotor angle over time.

4. The hand-held electrically powered cut-off tool (100) according to anyprevious claim, wherein the control unit (110) is arranged to determine the rotorangle of the electric motor based on data indicative of a rotor flux angle of theelectric motor, and to obtain the data indicative of angular velocity as adifference of the rotor angle over time.

5. The hand-held electrically powered cut-off tool (100) according to anyprevious claim, wherein the control unit (110) is arranged to process the dataindicative of the angular velocity of the cutting disc (105) by a first low-passfilter and by a second low-pass filter, where the first low-pass filter has a largerbandwidth compared to the second low-pass filter, wherein the first low-pass 26 filter is applied for kickback event detection, and where the second low-pass filter is applied otherwise.

6. The hand-held electrically powered cut-off tool (100) according to anyprevious claim, comprising an energy dissipating module (430) configured todissipate energy from the electric motor (130) during the electromagneticbraking.

7. The hand-held electrically powered cut-off tool (100) according to claim6, wherein the energy dissipating module (430) comprises any of a resistance,a super-capacitor, a cable connection to electrical mains and/or a batteryconfigured with an energy absorption capacity.

8. The hand-held electrically powered cut-off tool (100) according to anyprevious claim, wherein the control unit (110) is arranged to obtain the dataindicative of a rotational velocity of the cutting disc (105) at least in part from an external sensor.

9. The hand-held electrically powered cut-off tool (100) according to anyprevious claim, wherein the control unit (110) is arranged to determine anangular acceleration associated with the electric motor (130), and to detect thekickback condition based on a comparison between the determined angular acceleration and a detection threshold.

10. The hand-held electrically powered cut-off tool (100) according to claim9, wherein the control unit (110) is arranged to detect the kickback conditionalso based on an angular velocity associated with the electric motor (130) by conditioning kickback detection based on a minimum angular velocity.

11. The hand-held electrically powered cut-off tool (100) according to any ofclaims 9-10, wherein the detection threshold is manually configurable.

12. The hand-held electrically powered cut-off tool (100) according to any ofclaims 9-11, wherein the control unit (110) is arranged to obtain data indicativeof a tool diameter or indicative of a tool inertia of the rotatable cutting disc(105), and to adjust the detection threshold based on the data indicative of tool diameter or tool inertia. 27

13. The hand-held electrically powered cut-off tool (100) according to claim12, wherein the control unit (110) is arranged to obtain the data indicative of the tool diameter or tool inertia from manual input.

14. The hand-held electrically powered cut-off tool (100) according to claim12, wherein the control unit (110) is arranged to obtain the data indicative ofthe tool diameter or tool inertia based on a calculated or estimated tool inertia,wherein the tool inertia is arranged to be determined based on a current drawnby the electric motor during acceleration from a standstill condition.

15. The hand-held electrically powered cut-off tool (100) according to claim12, wherein the cut-off tool comprises a radio frequency identification, RFID,reader, and wherein the control unit (110) is arranged to obtain the dataindicative of the tool diameter or tool inertia from an RFID device embeddedinto the cutting disc (105) via the RFID reader.

16. The hand-held electrically powered cut-off tool (100) according to claim12, wherein the cut-off tool comprises means for detecting tool identificationdata, ID, from an optically readable tag arranged on the tool or on a toolpackaging associated with the tool, and to obtain the data indicative of the tooldiameter or tool inertia based on the tool ID.

17. The hand-held electrically powered cut-off tool (100) according to anyprevious claim, wherein the cut-off tool comprises a radio transceiver, andwherein the control unit (110) is arranged to obtain data indicative of the tool diameter or tool inertia from a remote server via the radio transceiver.

18. The cut-off tool (100) according to any previous claim, wherein the controlunit (110) is configured to control the electromagnetic braking of the electricmotor (130) by applying a configurable braking torque in dependence of a pre-determined time limit for braking the rotatable cutting disc (105).

19. The cut-off tool (100) according to any previous claim, wherein the controlunit (110) is configured to control the electromagnetic braking of the electricmotor (130) to generate a torque determined in dependence of a direct current,DC, bus voltage of the cut-off tool (100). 28

20. The cut-off tool (100) according to claim 6, wherein the control unit (110)is configured to control the electromagnetic braking of the electric motor (130)to generate a torque determined in dependence of an energy dissipatingcapability of the energy dissipating module (430).

21. The cut-off tool (100) according to any previous claim, wherein a directcurrent, DC, bus voltage of the cut-off tool (100) is regulated by switching adevice associated with an impedance during electromagnetic braking of theelectric motor (130).

22. The cut-off tool (100) according to any previous claim, wherein the controlunit (110) is configured to control the electromagnetic braking of the electricmotor (130) to generate a braking torque below a maximum braking torque level associated with a maximum rate of change in motor shaft angular speed.

23. The cut-off tool (100) according to any previous claim, comprising asupport arm (150), wherein the rotatable cutting disc (105) is arranged to bedriven by the electric motor (130) via a belt drive (610, 620, 630) and a gearedtransmission (810, 820).

24. A method in a control unit (110) for mitigating kickback in a hand-heldelectrically powered cut-off tool (100) arranged for cutting concrete and stoneby a rotatable cutting disc (105), wherein the cut-off tool (100) comprises anelectric motor (130) arranged to be controlled by the control unit (110) via amotor control interface (120), the method comprising obtaining (S1) data indicative of an angular velocity of the cutting disc (105), detecting (S2) a kickback condition based on a decrease in angular velocity,and controlling (S3) electromagnetic braking of the electric motor (130) in responseto detecting a kickback condition.

25. The method according to claim 24, further comprising actively regulating(S4) an energy outtake from the electric motor (130) over the control interface(120) during the electromagnetic braking. 29

26. A hand-held electrically powered cut-off tool (100) for cutting concreteand stone by a rotatable cutting disc (105), the cut-off tool (100) comprising anelectric motor (130) arranged to be controlled by a control unit (1 10) via a motorcontrol interface (120), wherein the control unit (110) is arranged to determine an angular position ofa rotor of the electric motor (130) based on a current over the control interface(120), and to obtain data indicative of an angular velocity of the cutting disc(105) based on a rate of change of the angular position of the rotor over time, wherein the control unit (110) is arranged to detect a kickback condition basedon a decrease in angular velocity of the cutting disc (105), and to controlelectromagnetic braking of the electric motor (130) in response to detecting akickback condition.