GB2269025A

GB2269025A - Torque controller for electrical tool.

Info

Publication number: GB2269025A
Application number: GB9215760A
Authority: GB
Inventors: Manuel Ocaua Perdiguero
Original assignee: Delco Chassis Overseas Corp
Current assignee: Delco Chassis Overseas Corp
Priority date: 1992-07-24
Filing date: 1992-07-24
Publication date: 1994-01-26
Anticipated expiration: 2012-07-24
Also published as: GB9215760D0; GB2269025B

Abstract

A torque controller for an electrical tool, such as an electric screwdriver, includes a current sensing circuit 14 for sensing the current required by the motor M of the tool, a control unit T2 - T4, 52, 54 for disconnecting the tool power supply 12 from the motor when the sensed current has reached a maximum desired motor current, and a regulator 20 for setting the maximum desired motor current. A timer 18 may delay the commencement of sensing of motor current until after an initial stabilisation period of the motor. Alternatively, a mechanical coupling (fig 4) may delay transmission of torque to the tool until after an initial rotation has taken place. The current sensor may include a galvanometer A which indicates instantaneous torque to the operator and also includes a limit switch which effects the disconnection of the power supply. <IMAGE>

Description

TORQUE CONTROLLER FOR ELECTRICAL TOOL The present invention relates to a torque controller for controlling the output torque of an electric tool, such as an electric screwdriver or other similar device.

Electrically operated screwdrivers are now commonplace, both in manufacturing situations and in the home for do-it-yourself applications. Torque control systems used with some of these screwdrivers have included mechanical torque wrenches and sophisticated electromechanical systems.

A problem associated with mechanical torque wrenches is that they generally do not produce an accurate or easily adjustable torque, so are not suitable for precision work. Moreover, their precision is dependant upon the skill of the operator and it is also necessary to look at the tool as well as the workpiece in order to monitor the torque being produced.

Although sophisticated electromechanical systems can provide accurate torque, they are not readily adaptable for incorporation into existing screwdrivers and are generally very expensive.

The present invention seeks to provide an improved torque controller for an electrically operated tool.

According to an aspect of the present invention, there is provided a torque controller for limiting the torque of an electrical tool, which tool includes a motor and a power supply connectable to the motor, the torque controller comprising a current sensor for sensing the current requirement of the motor; a regulator for setting a maximum desired motor current; and a switch for disconnecting the power supply from the motor when the current requirement of the motor reaches or exceeds the maximum desired motor current.

It has been found that the output torque produced by the motor of an electrical tool is related to the current requirement of the motor.

Thus, by measuring and controlling the current of the motor, the torque output of the motor can be controlled.

The present invention can provide precision in selection of the maximum desired tightening torque, repeatability in the torques produced and cancellation of the torque produced in starting and stopping of the tool.

The torque controller preferably comprises delay means adapted to prevent the current sensor from sensing the current requirement of the motor for a predetermined period after commencement of rotation of the motor.

Until revolutions have stabilised, the current requirement of a d.c. motor is generally greater than after stabilisation. Therefore, disconnection of the current sensor on starting up of the motor can prevent erroneous torque readings being made.

Advantageously, the delay means comprises a delay switch connected across the current sensor and adapted to short-circuit the current sensor during the predetermined period. The delay switch may be an inductance controlled resistive switch connected in parallel with the current sensor, the delay means including an inductor and a current source for causing current to flow through the inductor after the predetermined period so as to cause the resistance of the resistive switch to increase so as to couple electrically the current sensor to the motor. Alternatively, the delay means may comprise a delay switch connected between the motor and the current sensor and a timer for closing the switch after the predetermined period.

In an alternative embodiment, the controller may comprise delay means adapted substantially to prevent or reduce any retarding torque being applied to the motor during a predetermined period after commencement of rotation of the motor. In this case, the delay means may comprise a connecting portion connectable' to a mechanical output of the tool and a tool head, the tool head being rotatable relative to the connecting portion during the predetermined period.

With any of the above types of delay means, the current sensor may comprise a galvanometer and sensing means for sensing a predetermined current reading of the galvanometer representative of the maximum desired motor current, the sensing means being coupled to the switch. This provides a convenient and simple system for setting the maximum desired motor torque.

Preferably, the regulator comprises a potentiometer connected in series between the motor and the galvanometer, and a resistance connected in parallel with the potentiometer and the galvanometer, the potentiometer being adjustable so as to adjust the proportion of current flowing into the galvanometer. The potentiometer can provide the means for setting the maximum desired motor torque.

In an embodiment, the current sensor comprises a transistor circuit including a transistor, the regulator comprising a potentiometer connected between the base and emitter of the transistor, the transistor being adapted to actuate the switch to connect the motor to the power supply when the sensed current is less than the maximum desired motor current and to actuate the switch to disconnect the motor from the power supply when the sensed current is greater than the maximum desired motor current.

Advantageously, the controller comprises a supply indicator for indicating when the power supply has been connected to the motor. The controller may also/alternatively comprise a disconnection indicator for indicating when the switch has disconnected the power supply from the motor.

Some embodiments of the present invention are described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a first embodiment of torque controller; Figure 2 is a front elevational view of a galvanometer of the torque controller of Figure 1; Figure 3 is a circuit diagram of a second embodiment of torque controller; and Figure 4 is an enlarged an exploded view of an embodiment of mechanical delay device for use with modified versions of the torque controllers of Figures 1 and 3.

In Figures 1 and 3 examples of specific values and types of circuit components are given.

Other values and types can be chosen readily by the skilled person as appropriate.

Referring to Figure 1, the circuit of a torque controller 10 is shown for use in controlling, and more precisely in limiting, the output torque of a motor M of an electric tool (not shown), such as a screwdriver, which is connectable to a power source 12. The torque controller 10 is adapted for use with a d.c. powered tool, although can be readily modified, as will be apparent to the skilled person, to control a.c. powered tools.

The motor M is connected to a d.c. power supply 12 through a switch bank 11 which includes an on/off switch S1 and a direction reversing switch COM1 for changing the direction of rotation of the motor M. The switch bank 11 is a hermetically sealed unit, on account of which the negative supply of the battery is extracted through the motor terminals TER1 and TER2. However, since the polarity of these terminals changes according to the direction of rotation of the motor M, the diodes 38 and 39 are provided to ensure that the terminal TERS is always at the correct polarity. More specifically, one of the diodes 38, 39 will always be conducting. In this way, the negative supply of the battery is obtained in order to supply the circuit shown without being affected by any change of polarity at terminals TER1 and TER2.

The switch S1 comprises two portions, each connected across a respective terminal TER1 or TER2 of the motor M so as to cut off the motor M completely from the power supply 12 when the switch S1 is open.

The principal components of the circuit of the torque controller 10 include a variable ammeter 14, a timer circuit 18, a regulating potentiometer 20, and a control circuit 50.

The variable ammeter 14 is connected so as to measure the output current of the motor M, thereby to obtain a measure of the output torque produced by the motor M. The variable ammeter 14 includes a galvanometer 15 having first and second terminals across which is connected a calibration potentiometer 22, together with first and second oppositely disposed diodes 24,26 for use in preventing excessive voltages being applied across the galvanometer 15. A regulating potentiometer 20 is connected to the first terminal of the galvanometer 15 and to an output terminal of the ammeter 14, while a resistor 17 is connected between the output terminal of the ammeter and the second terminal of the galvanometer 15, so as to be connected in parallel with the potentiometer 20 and galvanometer 15.

The galvanometer 15 is provided with a single throw switch 16, or other suitable detector, for detecting when the galvanometer needle reaches a predetermined position, representative of the maximum desired torque. This is shown in more detail in Figure 2, in which it can be seen that the switch 16 is disposed close to the end of travel of the needle so as to be closed by the galvanometer needle when it reaches the predetermined position. The closure of the switch 16 is used in another part of the circuit of the torque controller 10, described below, to disconnect the motor 10 from the power supply 12, thereby switching off the tool once the maximum desired torque has been produced.

Connected across the galvanometer 15 is an inductance controlled resistive switch 28 of the timer circuit 18. The resistive switch 28 has a low resistance when there is no current flowing through associated inductor 30 but a high resistance when current is flowing through the inductor 30.

The timer circuit 18 is connected to the positive terminal of the power source 12 by a positive line 32 and is also connected to the negative terminal of the power source 12, through one part of the switch S1, by a negative line 34 and a resistor 36.

Also included in the timer circuit 18 is a transistor T1, which has its emitter connected to the negative line 34 and its collector connected to one end of the inductor 30, such that in use current is fed to the inductor when the transistor T1 is biased on. The base of the transistor T1 is biased by first and second series-connected resistors 40,42, the first resistor being connected between the base of the transistor T1 and the negative line 34 and the second resistor being connected between the base and the collector of the transistor T1. Also biasing the base of the transistor T1 is a capacitor 44, connected between the base of the transistor T1 and the negative line 34.

The timer circuit 18 is provided with first and second protection diodes 46,48, the first being connected to the emitter of the transistor T1 and the positive line 32 and the second being connected in parallel across the inductor 30. These protection diodes 46,48 serve, in particular, to protect the transistor T1 from any voltage surges which may be produced by the inductor 30.

The control circuit 50 is connected to the motor M and to the ammeter 14 and includes three cascaded transistors T2,T3,T4, a thyristor 52 and two resistors 54,56. The transistor T4 is connected in series with the motor M and the resistor 17. The resistor 54 is connected between the positive terminal of the power supply 12 and the base of transistor T2, while the thyristor 52 has a first cathode connected to the base of transistor T2 and its anode connected to the negative terminal of the power supply 12. The resistor 56 is connected to the anode and to a second cathode of the thyristor 52.

The second cathode of the thyristor 52 is also connected to the switch 16 of the galvanometer 15.

Two visual indicators are provided in the controller 10, which in this embodiment are a green light emitting diode 60 connected between the positive and negative terminals of the power supply 12 through a resistor 62, and a red light emitting diode 64 connected between the base of the transistor T2 and the positive terminal of the power supply 12 through a resistor 66.

The torque controller of Figure 1 is operated as follows. After assembly of the controller, the calibration potentiometer 22 is adjusted so as to calibrate the galvanometer 15 in order that at the scale provided on the galvanometer 15 is representative of motor output torque.

Whenever it is desired to use the tool, the regulating potentiometer 2Q is first adjusted with reference to a scale associated with the potentiometer 20 to the maximum desired motor torque.

If such a scale is not provided, the regulating potentiometer 20 is adjusted after the switch S1 is closed, as is explained in further detail below.

The adjustment of the regulating potentiometer 20 adjusts the resistance of the current path formed by the regulating potentiometer 20 and the galvanometer 15 (together with the calibrating potentiometer 22, diodes 24,26 and resistance switch 28) so as to adjust the proportion of current required by the motor M which is supplied through the galvanometer 15, as opposed to the proportion of current supplied through the resistor 17. In this manner, when the motor M is made to rotate with no load connected to it, the galvanometer needle will move to an initial position along its scale, determined by the minimum current required by the motor M and by the setting of the regulating potentiometer 20.Thus, this initial setting of the galvanometer needle will affect the additional amount of current that the motor M can sink before the galvanometer needle closes the switch 16, and thereby the amount of torque that can be produced by the motor M before the switch 16 is closed.

In operation, when the on/off switch S1 and direction reversing switch COM1 are closed to connect the motor M to the power supply 12, a potential difference is also applied across the positive and negative lines 32,34, causing the capacitor 44 to begin charging, through the inductor 30 and the second series connected resistor 42. Until the capacitor has become sufficiently charged, the voltage across it will be insufficient to cause the transistor T1 to conduct. During this time, the current flowing through the inductor 30 is relatively small and does not affect the resistance of the resistive switch 28, which will remain conductive to short circuit the variable ammeter 14. However, once the voltage across the capacitor 44 is sufficient to bias the transistor T1 on, the current flowing through the inductor 30 increases significantly to cause a large increase in the resistance of the resistive switch 28 such that it effectively becomes an open circuit, allowing the variable ammeter 14 to operate. The delay in biasing on the transistor T1 is determined by the values of the inductor 30, resistor 42 and capacitor 44.

Thus, the timer circuit 18 acts to disable the variable ammeter 14 for a predetermined period, which is chosen to be sufficient to allow the output current from the motor M to stabilise after closure of the switches S1 and COM1. In this manner, erroneous current readings by the galvanometer 15 can be prevented.

During the period when the galvanometer 15 is kept inoperative by the resistive switch 28 and when it is in its initial setting, the needle of the galvanometer 15 is at its rest position. Thus, the switch 16 is open, causing the voltage at the base of the transistor T2 to be high such that the transistor T2 conducts, causing the transistors T3 and T4 to conduct also and connecting the motor M to the positive and negative terminals of the power supply 12.

Closure of the switches S1 and COM1 also activates the light emitting diode so as to indicate that the power supply 12 has been connected to the motor M.

On applying a load to the motor M, the current sunk by the motor will increase so as to overcome the retarding torque provided by the load.

The appropriate proportion of the additional current required will be supplied to the motor M through the regulating potentiometer 20 and galvanometer 15.

Thus, the galvanometer needle will move along its scale towards the switch 16 until it reaches and closes the switch 16.

On closure of the switch 16, the voltage at the base of transistor T2 goes low, causing the transistor T2 to cease conducting, which by cascading action through transistor T3 causes the transistor T4 to cease conducting, thereby disconnecting the power supply 12 from the motor M. At the same time, the variable ammeter 14 becomes deactivated, causing the needle to return to zero.

The drop in voltage at the base of transistor T2 activates the light emitting diode 64 so as to indicate that the maximum desired torque has been reached. This is particularly useful when inserting screws into a workpiece, to ensure that the screw is tightened to the desired amount and no further.

The light emitting diodes 60 and 64 remain activated until the switch S1 is opened or the power supply is otherwise removed.

As will be apparent to the skilled person, the above-described circuit can be readily adapted for use with an existing tool, in which case the resistor 17 and transistor T4 would be connected in the tool's power line.

Figure 3 shows another embodiment of torque controller circuit, also suitable for use as a separate control unit for connection to an existing screwdriver or other similar tool. The only modification necessary in the tool, if not already provided, is to open the circuit formed by the power supply 112, the motor M and the switch S1', so that the resistor 117 and the transistor T14 of the torque controller 100 can be coupled serially into this circuit.

In practice, the terminals TER3 and TER4 of the motor circuit are connected to a suitable socket, while the positive and negative voltage lines V+, Vof the torque controller circuit have corresponding terminals connected to a matching plug.

The torque controller 100 includes a timer 118 and an adjustable control circuit 115. The timer 118 is similar to the timer 18 of the embodiment of Figure 1 and includes a transistor T11 having its collector connected to the V+ line of the controller 100 and its emitter connected to the control circuit 115. The base of the transistor T11 is connected between first and second series connected resistors 142,140, the first of these resistors 142 being also connected to the positive voltage line V+, while the second of these resistors 140 is also connected to the emitter of the transistor T11. Connected in parallel with the second series connected resistor 140 is a capacitor 144.

As with the timer circuit 18 of the embodiment of Figure 1, on connection of the power supply 112 to the supply lines V+ and V-, the transistor T11 remains non-conducting until the capacitor 144 charges sufficiently, during which period no positive supply voltage reaches the control circuit 115 due to the effective open circuit provided by the transistor T11. Once the capacitor 144 has become sufficiently charged, the transistor T11 becomes conductive so as to couple the regulator to the positive supply line V+.

The control circuit 115 includes a calibration potentiometer 122 connected in series with a resistor 121 and a regulating potentiometer 120, together with a transistor T12 and a capacitor 123. The calibration potentiometer 122 is also connected to the emitter of the transistor T11, while the regulating potentiometer 120 is also connected to the emitter of transistor T12. The base of transistor T12 is connected between the calibration potentiometer 122 and the resistor 121, while the capacitor 123 is connected across the base and the emitter of the transistor T12.

The collector of transistor T12 is connected to the base of transistor T13 and to a biasing resistor 150, which is also connected to the positive supply line V+. The emitter of transistor T13 is connected to the negative supply line V-, while its collector is connected to the base of p-n-p transistor T15 through a low resistance resistor 152.

The collector of transistor T15 is connected to the base of transistor T14. A protection diode 154 is provided between the emitter and collector of transistor T14.

In use, the calibration potentiometer 122 and the regulating potentiometer 120 are set so that after the time delay for connecting the positive supply line V+ to the calibration potentiometer 122 has elapsed, the transistor T12 remains non-conducting until the retarding torque on the motor M exceeds the desired maximum value. In this manner, the voltage at the collector of the transistor T12 remains high, which causes the transistor T13 to conduct and thus to bring the voltage of the base of the transistor T15 low, thereby causing this transistor to conduct. Conduction of transistor T15 biases transistor T14 into conduction, thereby closing the circuit of the motor M and power supply 112.

However, should the retarding torque applied to the motor M exceed its predetermined desired maximum value, determined by appropriate adjustment of the regulating potentiometer 120, the current flowing through the motor M will increase to such an extent that the increase in voltage across the positive and negative supply lines V+ and V-, and hence at the base of transistor T12, will bias the transistor T12 into conduction, thereby biasing the transistors T13, T15 and T14 off and disconnecting the power supply 112 from the motor M.

Thus, as with the embodiment of Figure 1, the torque controller 100 ensures that the torque output of the motor M does not exceed a predetermined maximum desired torque, and that the motor M is disconnected from its power supply once it has produced the maximum desired torque.

The circuit will then remain in this state indefinitely until the power supply, through switch Sl,'is cut off.

Referring to Figure 4, there is shown an enlarged and expanded view of an embodiment of mechanical time delay device which can replace the timer circuits 18 and 118 of Figures 1 and 3.

The time delay device 200 is adapted to be fitted to a chuck of the tool in place of the conventional tool head. In the example shown, the device 200 is adapted for use with an electric screwdriver and includes a flat-headed screwdriver piece 210.

The device 200 includes a connecting rod 212 which in use is secured to the chuck of the electric screwdriver. A circular opening 214 is provided in the connecting rod 212 for receiving a first end 230 of a coil spring 216, as is described in further detail below.

Integrally formed with the connecting rod 212 is a substantially circular cylindrical hollow body portion 218 which includes an open end 220 and a transverse aperture 222 which extends around a substantial part of the body portion 218.

A tool head 224 of substantially circular cylindrical shape has formed integrally therewith the flat-headed screwdriver piece 210. An opening 226 is provided in the tool head 224 for receiving a pin 228.

The device is assembled by inserting the tool head 224 into the body portion 218 such that the opening 226 in the tool head 224 is visible through the transverse aperture 222. The pin 228 is secured in the aperture 226 such that the pin extends through the aperture 222 beyond the perimeter of the body portion 218. The coil spring 216 is then slipped over the body portion 218. Finally, the first end 230 of the coil spring is inserted into the opening 214 in the connecting rod 212, the second end 232 of the coil spring 216 is then rotated to twist the coil spring and then secured to the pin 228. In this manner, the coil spring 216 biases the pin 228, and consequently the tool head 224, to one end of the aperture 222.

The direction of biasing of the spring 216 is such that when the screwdriver is initially operated in a manner as to tighten a screw, the tool head 224 rotates relative to the connecting rod 212, until the pin 228 has rotated a sufficient amount to reach the other end of aperture 222, when the connecting rod 212 and the tool head 224 effectively become locked together In this manner, the output of the motor can be stabilised during substantially the first revolution of the connecting rod 212 without any significant torque being imparted to the tool head 224.

As the device 200 ensures that substantially no load is applied to the motor M when the motor M first begins to rotate, any fluctuations in the current requirement of the motor M will be relatively small and not sufficient to trigger the torque controller 10,100 into disconnecting the power supply 12,112 from the motor M.

In place of the screwdriver piece 210, the device 200 could be provided with a universal fitting for receiving a plurality of different tool pieces.

As will be apparent to the skilled person, the above embodiments could be modified to operate in digital form and/or under the control of a microprocessor. The embodiments could also be adapted to control a.c. motors.

Claims

1. A torque controller for limiting the torque of an electrical tool, which tool includes a motor and a power supply connectable to the motor, the torque controller comprising a current sensor for sensing the current requirement of the motor; a regulator for setting a maximum desired motor current; and a switch for disconnecting the power supply from the motor when the current requirement of the motor reaches or exceeds the maximum desired motor current.

2. A torque controller according to claim 1, comprising delay means adapted to prevent the current sensor from sensing the current requirement of the motor for a predetermined period after commencement of rotation of the motor.

3. A torque controller according to claim 2, wherein the delay means comprises a delay switch connected across the current sensor and adapted to short-circuit the current sensor during the predetermined period.

4. A torque controller according to claim 3, wherein the delay switch is an inductance controlled resistive switch connected in parallel with the current sensor, the delay means including an inductor and a current source for causing current to flow through the inductor after the predetermined period so as to cause the resistance of the resistive switch to increase, thereby to couple electrically the current sensor to the motor.

5. A torque controller according to claim 2, wherein the delay means comprises a delay switch connected between the motor and the current sensor and a timer for closing the switch after the predetermined period.

6. A torque controller according to claim 1, comprising delay means adapted substantially to prevent or reduce any retarding torque being applied to the motor during a predetermined period after commencement of rotation of the motor.

7. A torque controller according to claim 6, wherein the delay means comprises a connecting portion connectable to a mechanical output of the tool and a tool head, the tool head being rotatable relative to the connecting portion during the predetermined period.

8. A torque controller according to any preceding claim, wherein the current sensor comprises a galvanometer and sensing means for sensing a predetermined current reading of the galvanometer representative of the maximum desired motor current, the sensing means being coupled to the switch.

9. A torque controller according to claim 8, wherein the regulator comprises a potentiometer connected in series between the motor and the galvanometer, and a resistance connected in parallel with the potentiometer and the galvanometer, the potentiometer being adjustable so as to adjust the proportion of current flowing into the galvanometer.

10. A torque controller according to any one of claims 1 to 7, wherein the current sensor comprises a transistor circuit including a transistor, the regulator comprising a potentiometer connected between the base and emitter of the transistor, the transistor being adapted to actuate the switch to connect the motor to the power supply when the sensed current is less than the maximum desired motor current and to actuate the switch to disconnect the motor from the power supply when the sensed current is greater than the maximum desired motor current.

11. A torque controller according to any preceding claim, comprising a supply indicator for indicating when the power supply has been connected to the motor (M).

12. A torque controller according to any preceding claim, comprising a disconnection indicator for indicating when the switch has disconnected the power supply from the motor.

13. A torque controller according to claim 11 or 12, wherein the supply connected indicator and/or the disconnection indicator is a light emitting diode.

14. A torque controller substantially as hereinbefore described with reference to and as illustrated in Figure 1 or 3 of the accompanying drawings.