GB2285321A

GB2285321A - DC motor speed controller

Info

Publication number: GB2285321A
Application number: GB9326071A
Authority: GB
Inventors: Brian Drury
Original assignee: IBIS DESIGNS
Current assignee: IBIS DESIGNS
Priority date: 1993-12-21
Filing date: 1993-12-21
Publication date: 1995-07-05
Also published as: GB9326071D0

Abstract

A d.c. motor speed controller comprising an electronically-controlled switch 6 arranged to be connected in series between a motor 1 and a d.c. supply for applying pulsed d.c. drive current to the motor; voltage measuring means 7 for measuring a generator voltage generated by the motor at a given time after a drive pulse has been applied to the motor; and means 5 for controlling the switch 4 to apply a next drive pulse to the motor in dependence on the measured generator voltage. The controller may be applied to a cordless hand drill. Torque may be limited by counting the number of pulses for which the speed remains below the desired speed, and cutting out if this number exceeds a threshold. <IMAGE>

Description

DC MOTOR SPEED CONTROLLER This invention relates to a control technique for regulating the speed of d.c. electric motors.

Various electrical circuits and techniques are used for the purpose of controlling the speed of d.c. motors; these can normally be shown to fall into one of the following categories: 1. DC Tacho A d.c. Tachometer is mechanically coupled to the output shaft. This allows a speed-proportional d.c.

voltage to be produced for accurate speed control.

However, the d.c. tachometer is bulky and expensive and has a limited life span.

2. Digital Encoder A speed-proportional pulse stream is generated.

This is processed by an electronic frequency-to-voltage converter circuit for speed control. Again however, the encoder is expensive and space consuming. It also has a poorer speed consistency than the d.c. tacho at low speed ranges.

3. IXR Compensation Motor current rises in a linear fashion with rising motor load. Control electronics monitor the motor current and change the motor voltage accordingly.

Although this technique is less expensive and uses less space than the d.c. tacho and digital encoder it does not provide precise speed control, particularly at low speed.

According to one aspect of the present invention, there is providing a d.c. motor speed controller comprising an electronically-controlled switch arranged to be connected in series between a motor and a d.c.

supply for applying pulsed d.c. drive current to the motor; voltage measuring means for measuring a generator voltage generated by said motor at a given time after a drive pulse has been applied to said motor; and means for controlling said switch to apply a next drive pulse to said motor in dependence on said measured generator voltage.

According to another aspect, there is provided a method of controlling the speed of a d.c. motor, comprising applying a drive current pulse to the motor; after the end of the pulse, measuring the armature generator voltage of the motor; and deciding, from the measured generator voltage, whether or not to apply a next drive pulse to the motor.

The concept of this invention will be illustrated with reference to Figs. 1 and 2.

Figure 1 shows a d.c. motor 1, a power source 2, Voltmeter 3 and a switch 4 which is assumed to be controlled to supply pulses of current to the motor.

When the switch 4 is in the open position the Voltmeter 3 displays the sum of the supply and the armature (generator) voltage of the motor 1. As the generator voltage is in anti-phase with the supply voltage the sum will be (V supply + (-V generator). The result will vary in a linear fashion with motor speed. When the switch 4 is in the closed position the motor 1 will consume electrical power; the Voltmeter 3 will display zero at this time.

The voltage waveform developed at the junction of the switch 4 and motor 1 is depicted in figure 2. The motor 1 is shown initially stationary. During the period that the switch 4 is closed the observed voltage falls to zero. When the switch is opened a transient back-emf is observed. After the back-emf has fallen to zero a speed-dependent settling time can be seen. After the settling time the armature generator voltage will be seen. This generator voltage is in opposition to the applied supply, therefore the observed voltage will equal the supply voltage minus the armature voltage.

If the switch 4 is pulsed on for a very short period and the motor Voltage (speed) is noted, a decision can be made to provide another pulse of power or not. If the speed is below the required speed another pulse is applied. If the speed is equal to or above the required speed, the next pulse is not applied. In either event another Voltage measurement is made after the pulse period and another decision is made. This process is repeated continually.

The controller preferably works on the burst fire principle. However, other types of control can be used, such as PWM control.

Additional components can be added to provide a motor brake to reduce the motor speed as required. This arrangement comprises a switch, preferably a p-type FET, connected across the motor and driven by the controller.

This shunts the armature current through the motor whenever the motor speed exceeds the required speed for more than a preset period. When the motor speed has been reduced by braking to the required speed, power may be applied after a preset period to maintain the new speed.

A further feature which may be added is a torque limiting function. A counter is arranged to detect how many times the motor is determined to be below the desired speed within a given period. Any instance of the motor attaining the correct speed within this period will cause the count to be returned to zero. A high count indicates that the motor is automatically cut-off.

This is particularly desirable in e.g. hand drills where the motor may stall if the drill bit is too big.

The ideal time at which the armature voltage is to be read will vary according to the characteristics of the motor. The time taken for the voltage to settle after the back-emf pulse produced when the switch is closed varies according to the speed of the motor. In a preferred embodiment the time at which the voltage is read may be automatically varied such that the voltage is read earlier when the speed is greater. This enables the motor to be operated over a wide range of speeds.

Alternatively, the motor could be restricted to operating within a limited speed range.

The stability of the motor may be increased by adding a flywheel.

The invention is advantageous in that it results in a much higher torque than other systems for the same size small d.c. motor.

The invention has large number of possible applications and is particularly suited to cordless electric drills which have a limited amount of space for a speed control system.

A specific embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, wherein: figure 1 shows schematically a d.c. motor drive circuit for the purpose of explaining the principle of the invention; figure 2 shows the voltage waveform across the switch of the circuit of Figure 1; figure 3 shows in block diagram form an embodiment of the control circuit according to the present invention; figure 4 is a flow chart of the basic speed control process according to the present invention; figure 5 is a circuit diagram of a practical control circuit according to the present invention; figure 6 is a flow chart of the main body of a control program performed by the microprocessor of Fig.

5; figure 7 shows the delay subroutine of Fig. 6; and figure 8 shows the potcheck subroutine of Fig. 6.

Figure 3 shows a microcontroller 5 connected to a power MOSFET switch 6. The power MOSFET 6 is able to switch the supply voltage to the motor 1 on and off under instruction from the microcontroller 5. A comparator 7 is arranged to monitor the armature voltage and compare this with the voltage derived from the potentiometer 8. Figure 2 depicts the voltage waveform which develops at the Drain connection d of the MOSFET 6.

This simple arrangement is all that is required to implement the basic control program shown in figure 4.

No attempt is made here to give an indication of time.

In a practical application the duration of the drive period and subsequent delays are crucial for optimum performance.

Figure 5 shows a complete and practical circuit.

Additional circuitry not described in the preceding text includes the five bit digital-to-analog converter R3 to R12. The analog output voltage is used for comparison with the voltage produced by the potentiometers VR1 and VR2. This mechanism allows the microcontroller 5 to determine the position of both potentiometers.

MOSFET's TR3 and TR4 provide a low impedance driver stage to ensure a rapid rise and fall time of the control waveform applied to the output MOSFET TR6. TR1 provides a voltage gain stage thereby allowing the full supply voltage to bias the output MOSFET.

TR5 is a p-channel power MOSFET, this can be controlled from ICl via TR2. When TR5 is on, a low impedance is presented to the motor and a braking effect is observed.

Voltage regulation of the supply to IC1 and IC2 is achieved with the regulator IC3.

Referring to Figure 6, the program is a perpetual loop with the lower half being event driven depending on the outcome of the test for speed attainment. If the measured armature generator voltage is greater than or equal to the voltage determined by the speed potentiometer VR1 then the program will continue down the left leg. Alternatively-the right leg will be followed if correct speed has not yet been attained.

The very simple delay routine mechanism is required to produce the two delays shown in Figure 6.

In the potcheck subroutine of Figure 8, the potentiometer settings are determined and a value denoting each position is maintained in RAM. The torque potentiometer setting is used to set the limit for torque shutdown. If a stream of successive drive pulses are applied to the motor without a single missing pulse and the total exceeds the limit set by the torque potentiometer then torque shutdown will occur.

Claims

1. A d.c. motor speed controller comprising an electronically-controlled switch arranged to be connected in series between a motor and a d.c. supply for applying pulsed d.c. drive current to the motor; voltage measuring means for measuring a generator voltage generated by said motor at a given time after a drive pulse has been applied to said motor; and means for controlling said switch to apply a next drive pulse to said motor in dependence on said measured generator voltage.

2. The controller of claim 1, further comprising comparator means for comparing the measuring generator voltage with a reference voltage, wherein the result of the comparison is used to determine whether or not the next drive pulse should be applied.

3. The controller of claim 2 wherein said reference voltage is proportional to the desired. speed of the motor, and the measured voltage is proportional to the actual speed of the motor.

4. The controller of any preceding claim wherein said means for controlling said switch comprises a microprocessor.

5. The controller of any preceding claim wherein said switch comprises a field effect transistor.

6. The controller of any of claims 2 to 5, further comprising means for adjusting said reference voltage.

7. The controller of any preceding claim, further comprising braking means for interrupting the power supply to the motor when the motor speed exceeds a given value.

8. The controller of claim 7 wherein said braking means comprises a second switch arranged to shunt the current through the motor to cause braking.

9. The controller of any preceding claim further comprising torque detection means for measuring the motor torque and means for automatically cutting off the motor when the torque exceeds a predetermined threshold.

10. The controller of claim 9 wherein said torque detection means comprises a counter which counts the number of times the switch is controlled to apply a next pulse, within a predetermined period of time and determines that the torque has exceeded the threshold if the count exceeds a given level within the predetermined period.

11. A method of controlling the speed of a d.c. motor, comprising applying a drive current pulse to the motor; after the end of the pulse, measuring the armature generator voltage of the motor; and deciding, from the measured generator voltage, whether or not to apply a next drive pulse to the motor.

12. The method of claim 11 wherein the measured generator voltage is compared to a reference voltage and the result of the comparison is used to determine whether or not to apply a next drive pulse.

13. The method of claim 11 or 12 wherein the generator voltage measurement is made a predetermined time after the end of the applied drive pulse.

14. The method of any of claims 11 to 13 wherein the drive pulse is applied by closing a switch for a short time and the generator voltage measurement is made a predetermined time after the switch has been opened again.

15. The method of claim 14 wherein the measured generator voltage is indicative of the motor speed and is compared to a reference voltage indicative of the desired speed, wherein no further pulse is applied until it is determined from the comparison that the motor speed has fallen below the desired speed in which case a next drive pulse is applied to the motor.

16. The method of claim 14 or 15 wherein the predetermined time is variable according to the speed of the motor.

17. A d.c. motor speed controller substantially as hereinbefore described with reference to the accompanying drawings.

18. A method of controlling the speed of a d.c. motor, substantially as hereinbefore described with reference to the accompanying drawings.