Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P6/00—Arrangements for controlling synchronous motors or other dynamo-electric motors using electronic commutation dependent on the rotor position; Electronic commutators therefor
  • H02P6/06—Arrangements for speed regulation of a single motor wherein the motor speed is measured and compared with a given physical value so as to adjust the motor speed

Definitions

• This invention relates to a constant speed direct current motor and particularly hut not exclusively to a motor for use in driving tape records and cartridge machines.
• Hysteresis synchronous motors are currently used in tape recorder and cartridge machine capstan drives. Where there is a constant frequency A.C. mains supply such drives exhibit excellent constant speed characteristics.
• D.C. motors for this purpose are not new; however, speed control is poor leading to serious problems with timing stability. Nevertheless D.C. motors have inherent advantages over A.C. synchronous motors:
• the Clark motor is primarily designed for use on machines where it is necessary to have both accurate positional and speed information, for example in editing machines where information concerning the precise position of the tape is required.
• the Clark motor could be used in tape recorder and cartridge machines but this would not make sense technically because the motor would produce unnecessary positional information, and also because the cost would be increased significantly due to the digital circuitry.
• U.S. patents 3,660,238 to Anderson and 3,656,039 to Konrad which are related to each other and which disclose the use of dynamic braking in relation to D.C. motors.
• the dynamic braking is achieved by applying a short circuit to the armature which permits the generator voltage to be dropped across the armature to produce a dynamic braking effect.
• U.S. patent 3,896,357 to Tamikoshi discloses the use of a tachometer in relation to a D.C. motor to control the speed of the motor and to eliminate electric noise.
• U.S. patent 3,448,359 to Engel discloses the use of an oscillator in the creation of an error voltage to control the operation of a D.C. motor.
• patent 3,754,157 to Girault discloses a circuit generating squarewave signals which are supplied to the windings of a motor to control the speed of the motor.
• U.S. patents 2,810,084 to Sprando and 3,737,693 to Mishima relate, generally, to "inside-out" motors.
• U.S. patent 3,541,361 to Nola discloses a brushless D.C. tachometer which uses Hall effect switches.
• a constant speed brushless D.C. motor including at least a pair of Hall effect switches (known per se) buried in the motor stator windings for sensing the direction and position of the rotating motor magnetic field, a feed back loop comparator means in said loop, said comparator means being adapted to sense and compare motor negative electromotive force (back e.m.f.) and motor potential whereby the speed of said motor is held subs tantially cons tant .
• the present motor includes two sources of speed control namely, the fine outer speed control loop and a coarse inner speed control loop utilizing back e.m.f. generated by the motor in operation.
• the motor is adapted to generate pulses and includes means for counting said pulses, comparator means for comparing frequency of said motor pulses with a standard frequency pulse (representing a steady desired motor speed) also providing a balancing potential at said comparator means.
• the negative generated motor voltage (back e.m.f.) is converted to current which is required to exactly offset the motor current fed to a comparator means whereby current to the Hall effect switches is switched off effectively removing power from the motor windings.
• This speed control is relatively coarse.
• a fine speed outer control loop is superimposed over the previously described coarse speed control.
• a group of integrated pulses produced by the motor directly proportioned to its speed of rotation is compared with a separate group of pulses having a constant frequency which exactly equates with the motor speed desired by comparator means thus achieving a frequency lock.
• a down/up counter or integrator error is fed into the back e.m.f. speed control to provide fine tuning.
• the motor pulses are conveniently produced by a variable reluctance type tachometer to produce a number of voltage pulses in each motor revolution, these pulses are converted to a voltage, this voltage is compared with one of opposite sign created by oscillator means. Thus a fine correction current is produced if the two voltages are not exactly matched. Additional velocity feedback means is also optionally provided to stabilize the fine control loop. Conveniently control of the number of pulses produced by the oscillator allows in turn fine control of the motor speed. Thus the motor is frequency locked to the oscillator controlled fine speed control loop and can be run at any desired constant speed according to the oscillator setting.
• Figure 1 is a generalized schematic circuit diagram of the motor control circuit.
• FIGS. 2A and 2B are detailed circuit diagrams of the motor control circuit.
• Figure 3 is a sectional view of the motor construction.
• Figure 4 is a schematic view of the motor fields.
• Figure 5 is a more detailed view of a Hall effect switch.
• the motor is of a multiple speed brushless D.C. outside rotor type having a rotor 1 and a stator 2 mounted internally of the rotor.
• the motor is well suited to driving a capstan wheel of tape recording apparatus.
• the rotor 1 has an open top allowing self-aligning bearings 3 to be mounted on the shaft in close proximity to the major weight distribution of the rotor.
• the stator includes quadrature windings L 1 , L 2 , L 3 and L 4 generally indicated as 5 in Figure 3.
• the rotor includes a pair of permanent magnet poles 6.
• Hall effect switches HSl and HSl are mounted within the stator windings and are responsive to the magnetic poles whereby the switches sense the location and direction of movement of the poles as the rotor rotates about the stator. HALL EFFECT SWITCHES.
• these switches are miniature four terminal devices which sense the direction of a magnetic field directed perpendicular to the plane of the switch which in the drawing is the plane of the paper.
• the support bar 7 and ball bearings 8 allow the motor to be operated at any desired direction between vertical and horizontal. Slots 10 are equi-spaced around the periphery of the rotor. In this application there are 125 slots. Mounted in the support bar 7 a reluctance tachometer 9 is adapted to detect the presence of each of the 125 slots during rotation of the rotor 1. Thus 125 voltage pulses are produced in every revolution. SPEED CONTROL.
• two feedback loops are employed which comprise a coarse speed control inner loop and a fine speed control outer loop.
• the coarse loop is designed to hold motor speed to about 1% to 2% regulation while the fine loop reduces this regulation to about 0.1%.
• the speed control device will be described having reference to Figure 1 which shows a block diagram of the circuit.
• the field windings 2 (L 1 , L 2 , L 3 , L 4 ) of the motor stator are alternatively switched by the magnetic
• Hall effect switches 3 thereby creating a rotating field leading the rotor field thereby forcing the rotor to rotate.
• Switching transistors are controlled by the
• the controller includes an inner loop 11 which utilizes the motor back electromotive force as a measure of the motor speed. This circuitry will be described in greater detail later.
• the outer loop incorporates a tachometer 9 mounted on the motor giving 125 pulses per motor revolution.
• a stable oscillator 12 combines with a programmable divider 13 to measure the speed error as indicated by the number of pulses generated by the tachometer and those of the oscillator.
• An error signal 14 is generated where there is a speed error which is fed back to the inner loop summing junction.
• the error signal 14 is differentiated to provide an acceleration feedback signal which is also applied to the system summing junction to improve the stability of the system.
• a dynamic brake 15 is provided and is used whenever the speed of the motor exceeds that normally encountered, e.g., a practical excess speed of say 2% will cause the brake to operate. This is normally encountered when the selected speed is reduced or the motor is switched off.
• Coils L1, L2 , L3 and L4 generally designated 2 are the four quadrature windings on the motor stator.
• "Hall effect" switches HS1 and HS2 are buried in the quadrature windings to sense the direction and position of the permanent magnet rotor field.
• Figure 1 shows the phase relationship of windings relative to said Hall effect switches.
• Complimentary Darlington amplifiers Q5 - Q1, Q6 - Q2, Q7 - Q3 and Q8 - Q4 amplify the Hall effect switch outputs to create a field which always leads the permanent magnet field by an average of 90 degrees and hence creates an accelerating torque on the rotor.
• this motor would behave as a series D.C. motor, where speed is limited only by motor losses and friction (typically 4000 - 6000 R.P.M.).
• two feedback loops are employed, via coarse inner loop 11 and a fine outer loop.
• the cpastan (not shown) tape drive diameter is chosen to give a surface speed of 71 ⁇ 2 inch/sec. (19.05 num. /sec.) when the capstan speed is 10 revs. /sec. which is equivalent to a motor speed of 600 R.P.M.
• the inner loop is a voltage loop generated by diodes D1 to D4 which pick up the negative generated motor voltage, the magnitude of which is proportional to the speed of the motor.
• the fine control involves an analog comparison of frequency between a tachometer and a stable oscillator 12 (U1e and U1d). As explained in more detail hereinbelow, the tachometer 9 generates 125 pulses per revolution of the motor. The stable oscillator 12 generates a separate group of pulses having a constant frequency which exactly equates with the motor speed desired.
• these pulses are summed at a summing junction, and an analog error signal is produced.
• This analog error signal is then used to control the speed of the motor to the fine control outer loop circuit. It can be appreciated that although the error signal is generated from a digital source, namely the pulses of the tachometer 9 and the stable oscillator 12, the operation of the fine control outer loop using the error signal is entirely analoge and does not require the presence of digitalization circuitry.
• the tachometer 9 is a variable reluctance type which generates 125 pulses per revolution of the motor. These pulses are amplified by U3d to delivera square wave at test point 4 (TP4) having a frequency of 1250 Hz when the motor is running at 600 R.P.M.
• TP4 test point 4
• capacitor C4 and diode D9 inject a precise negative charge onto capacitor C6, so that the faster the motor spins the more negative the voltage on C6.
• Capacitor C4 "differentiates" this square wave to produce 1250 Hz pulses, and diode D9 removes or absorbs the negative going pulses tending to charge capacitor C6 negatively.
• capacitor C3 is "differentiating" a square wave derived from oscillator Uld and Ule and transistor Q11.
• the positive going pulses from C3 are removed or absorbed by D8 tending to charge capacitor C6 positively once per oscillator period.
• Comparator U3A (comparator 9), connected as a high gain operational amplifier, amplifies the voltage on capacitor C6, to give an overall fine correction voltage at test point TP3 which is supplied via resistor R13 to thesumming junction of comparator &3b.
• Thenett effect is that if the motor speed is low the output from the comparator U3a at test point TP3 is high (10V), and conversely, if the mo t or speed is too high the voltage at TP3 is low (0V). Additional velocity feedback is also supplied via capacitor C9 and resistor 14 to stabilize the fine control loop.
• Oscillator Ule and Uld is a very stable temperature compensated CMOS oscillator whose frequency is adjusted by RV5. As the motor controller has been designed to operate at three harmonically related speeds (1:2:4), the oscillator 8 must be divided by 2 when operating at the medium speed and divided by 4 when operating at the low speed.
• Inverter Ulf is wired as a buffer to illuminate LED1 to indicate when fine control loop has "frequency locked” the motor. DYNAMIC BRAKING OPERATION.
• Brushless D.C. motors do not have the facility of "powering" speed reduction but can only rely on mechanical losses to reduce speed.
• diode D11 removes drive from the power stage while dynamic braking is in process.
• the input to diode DIO (RUN) will start the motor if LOW or stop the motor if HIGH.
• Stayt input via R12 can be used as a "phase lock" input if motor was used in a
• the motor requires an unregulated 30 to 40 volt 1 amp supply to operate.
• An in circuit 12 volt regulator supplies regulated power to critical circuits.
• RUN PIN 24 is connected to a cartridge tape micro switch (not shown) and requires a voltage of less than 1 volt to stop the motor or greater than 12 volts to start it.
• START PIN 22 receives a high voltage (20-24V) when the pressure roller (not shown) is engaged. This gives a slight increase in operating power via R12, to compensate for friction losses due to loading of pressure roller on motor. SELECT HIGH SPEED (PIN 27)
• a low voltage (less than 3 volts) on this point causes U1C output to be at 12 volts, enabling potentiometer RV1 and disabling counters U2A and U2B thus delivering 2500 Hz to the comparator as the reference frequency.
• the comparator (U3A) is only stable when the tachometer is delivering 2500 Hz i.e. 20 revolutions/sec. or capstan speed of 15 inches/sec.
• a low voltage (less than 3 volts) on this point enables medium speed potentiometer (RV2) and disables counter U2B (via D7), thus delivering 1250 Hz frequency to comparator U3A which is only stable when capstan motor is running at 10 revolutions/ sec. or 7 1 ⁇ 2 inches/sec. capstan speed.
• SELECT LOW SPEED PIN 25
• a low voltage (less than 3 volts) on this point enables potentiometer RV3 and allows divider U2A and U2B to divide oscillator (UlE and UID) frequency by 4, thus delivering 625 Hz to the comparator U3A which is stable when motor speed is 5 revolutions/sec.
• RV1 is used to preset the fast forward speed to 20 revolutions per second (15 inches/sec. capstan surface speed).
• RV2 is used to preset the coarse control loop speed at 10 revolutions per second
• RV3 is used to preset for low speed (3-3/4 inches/sec. capstan surface speed).
• RV5 in the oscillator 12 is used to trim the normal speed to the required accuracy.
• RV4 is used to balance the drive to the two Hall effect pairs HS1 and HS2 this is set to minimize wow and flutter.
• a light emitting diode LED6 is a useful device to ascertain at a glance the performance of the system at normal speed via:
• the invention has been particularly described when having reference to a three speed machine. However, there are two speed applications, e.g., in the playing of tape cartridges and such operation is achieved with only minor modification of the circuit.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Control Of Motors That Do Not Use Commutators (AREA)

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Family Cites Families (5)

• 1979
  • 1979-09-17 WO PCT/US1979/000751 patent/WO1981000796A1/en unknown
• 1981
  • 1981-03-23 EP EP19800900278 patent/EP0035496A1/de not_active Withdrawn

Non-Patent Citations (1)

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