GB2058405A - Universal Motor Control System - Google Patents

Abstract

A motor drive comprises an electronic switch 11 connected in series with a universal motor 10 across an a.c. supply (LN), and the switch 11 is activated in timed relationship with the supply frequency by a feedback control system incorporating logic 15 for predetermining the speed/torque characteristic of the drive by delivering an error signal to adjust the timed relationship at which the electronic switch 11 is activated according to the relative magnitudes of motor voltage Vs and current Is. The logic 15 may be hard wired or within a preprogrammed microprocessor. <IMAGE>

Description

SPECIFICATION Universal Motor Control System This invention relates to the control of universal motors particularly those used to drive power tools such as heavy drill hammers and similar portable tools.

Known portable power tools have been made to be supplied from a single a.c. supply voltage nominally constant in value but with a relatively small range (100-1 20V for example). This has been a disadvantage where a tool is required to work at a variety of different locations each having a different supply voltage, and requires the use of additional transformer equipment. Failure to use this transformer equipment may lead to damage of the tool. In addition, especially in tools with powerful drives, the torque characteristics of the drive is such that, if the tool bit jams the reaction on the operator may be dangerously large. Because of this the drive normally incorporates a clutch designed to slip at a pre-set over-torque condition, usually about 50% greater than normal full load.This is subject to wear, and, as it must be set for a torque substantially greater than full load to prevent malfunctioning, the protection it gives to the operator is limited. In addition, the current taken by the drive motor under these conditions is such that, in a stalled drive, the motor can be damaged by over-heating in a relatively short period of time. A further disadvantage lies in the hand operated controller for the tool. This may be a simple switch which is operated every time the tool is driven. Due to the onerous nature of the application a higher than normal failure rate of this controller is found.

In portable power tools having a variable speed drive the operator may adjust the speed setting by pressure on the controller which is in the form of a trigger actuating a potentiometer which gives a signal to alter the firing angle of a thyristor in the motor supply line. Here again the heavy duty and general tendency to misuse can cause a greater than normal failure of this controller.

It is an object of the present invention to provide an improved motor drive and control system for a universal motor whereby the disadvantages outlined above may be obviated or mitigated when the improved control system is used in a portable power tool.

According to the present invention there is provided a motor drive comprising an electronic switch connected in series with a universal motor across an a.c. supply, wherein the switch is activated in timed relationship with the supply frequency by a feedback control system incorporating logic means for predetermining the speed/torque characteristic of the drive by delivering a signal to adjust said timed relationship at which the electronic switch is activated according to the relative magnitudes of motor voltage and motor current.

The present invention also provides a control system for a universal motor connected to an a.c.

supply, comprising an electronic switch for connection in series with the motor, sensing means for sensing motor voltage and motor current, an actuating switch, logic means having inputs connected to receive from the sensing means signals representative of the sensed motor voltage and motor current, to receive a control signal from the actuating switch and being arranged to emit an error signal at its output, a ramp generator arranged for deriving a repetitive ramp waveform from the a.c. supply, comparator means for comparing the ramp waveform with the error signal and having its output connected to control operation of said electronic switch.

The logic means may take any one of a plurality of configurations which are predetermined at the design stage to constrain the operational speed/torque characteristics of the universal motor. These configurations may be of analogue form. Alternatively the logic means may be in the form of a preprogrammed microcomputer in which case a selector switch may be provided whereby a selected logic configuration may be used by the operator.

The electronic switch may be in the form of an alternistor, triac, single or reverse-parallel thyristors of a V Mos device.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 shows torque/speed characteristics for a universal motor drive; Fig. 2 illustrates a universal motor drive in block form; Fig. 3 illustrates a detail of the Fig. 2 drive to achieve the Fig. 1 characteristics; Figs. 4, 8 and 10 iilustrate further torque/speed characteristics which can be implemented by the circuits of Figs. 5, 6, 7 and 9 when used in the Fig.

2 drive; Fig. 11 illustrates a universal motor drive incorporating a microprocessor; and Fig. 1 2 is a flow diagram of a typical microprocessor programme for implementing the Fig. 1 torque/speed characteristics.

One suitable set of torque/speed characteristics for the universal motor of a power tool incorporating a speed control switch is illustrated in figure 1. For each setting of the speed control switch the no-load speed of the tool is determined by the internal load of the device and the universal motor characteristics. Although the inherent characteristic of the universal motor gives a high speed at no torque, in practice in a power tool drive there is a sufficient internal load on the motor due to the internal mechanism to ensure that the no-load speed is not greatly higher than that at full load. Different settings of the speed control switch provide a family of characteristics such as a, b, c, d, in fig. 1 for each of which the maximum torque when load is applied is limited to a different constant value.On the maximum setting of the speed control switch torque is limited to normal full-load torque so that, in the event (for example) of a jammed tool bit, the reaction on the operator is maintained at a safe level.

The power supply required to give a set of characteristics as in fig. 1 has to provide a substantially constant a.c. voltage for the sloping part of the characteristic and a substantially constant current over the constant torque part of the characteristic.

Fig. 2 illustrates in block form a typical embodiment of the invention using analogue circuits. The universal motor 10 is supplied from an a.c. source L-N in series with an electronic switch 11 and a current sensing shunt 12. Signals Vs, ls are taken from the motor voltage (Vs) and from the shunt (is), rectified by rectifiers 13, filtered by filters 14 to remove 100 Hz and applied as signals V and I to a logic circuit 1 5 which also receives a reference signal from a speed control switch 16; the logic circuit 1 5 computes an error signal in a manner to be explained, and this is integrated by integrator 17 and compared by comparator 18 with the output of a ramp generator 1 9. The ramp waveform is derived from the supply line L of source L-N and is initiated at each cycle of the supply by a zero crossing detector 20. Comparison of the error and ramp values times the firing of the electronic switch 11 through a suitable driver circuit 21.

In its simplest form the logic circuit 1 5 is as shown in Fig. 3 where diodes a, b operate as a maximum signal selector the output of which is applied to a summing junction to which the reference signal from the control switch 1 6 is applied. The logic circuit of Fig. 3 provides the speed/torque characteristics illustrated in Fig. 1.

The maximum signal selector of the logic circuit 1 5 ensures that control of the electronic switch 11 is from the sensed voltage Vs until the sensed current Is has increased to the constanttorque value. As a consequence the voltage supplied from source L-N to the motor 10 is limited to that set by the electronic switch 11.

Suitably rated the switch 11 enables the motor 10 to be supplied from a wide range of voltage (e.g. 100 to 250V) so that damage due to use on incorrect voltages is prevented and the use of extra transformer apparatus eliminated. The sloping part of the speed/torque characteristics of Fig. 1 is obtained from the universal motor at constant voltage at a level determined by the setting of control switch 1 6. When the current signal in the maximum signal selector of Fig. 3 exceeds the voltage signal the effect is to maintain constant current and therefore constant torque at a level determined by the setting of control switch 1 6. It will also be apparent that the nett speed-torque characteristic at the tool bit has a shape shown, for example by the curve b2 of Fig.

1, 50 the effective torque increases as speed falls.

It will now be obvious that the speed/torque set of characteristics of Fig. 1 may be shaped in a variety of ways to suit the requirements of the load, for example, the characteristics a, b, c, d, in figure 4, having a common full-load torque limit may be obtained by the logic circuit of Fig. 5 which compares the voltage feedback signal V alone with the speed set signal from control switch 1 6 and the current feedback signal I with a reference level, the greater of the two error signals then being selected and transmitted directly to the error integrator 1 7.

A constant speed/constant torque characteristic (as in a2, b2, c2, d2, in Fig. 4) can be obtained from the logic circuit of Fig. 6 by modifying the voltage feedback signal V with a proportion of the current feedback signal I prior to the maximum signal selector so as to increase the voltage supplied to the motor as load initially increases. If it is desired that the constant torque portion of the characteristics is proportional to the control signal this can be achieved by the logic circuit of Fig. 7.

Similarly the shape of the 'constant' torque part of the characteristic may be altered, for example to reduce the effective torque on stall.

Figure 8 characteristic (a) has a constant torque portion followed by a reducing torque portion as speed reduces towards the stalled condition. In this case the effective speed/torque characteristic at the tool bit would be as shown by curve a2. This characteristic can be achieved by the logic circuit of Fig. 9.

A further useful set of characteristics is shown in Fig. 10. A heavy duty drill drive with speed/torque characteristics as in d (Fig. 10) can be used with small high speed tool bits without risk of damage to the small tool bits if the torque is limited as shown by characteristic (a). Here again the effective torque may be altered by shaping back the speed/torque curve at lower speeds as in curve a,.

Similar characteristics to those discussed above may be obtained by using a digital control of the electronic switch 11. Such a control may be obtained from a microprocessor programmed to give the required characteristics. Fig. 11 shows a block diagram of a circuit using a small micro computor chip 30, e.g. Intel 8022, which has processing facilities and inbuilt analogue-tadigital convertors and memory.

The voltage and current sense signals V5 and 15 are each fed to the internal analogue-to-digital convertors of the microcomputer 30 via a passive bias/divider network 31. The processor has a regulated supply Vcz derived from the supply line L and accepts signals from the speed control switch 1 6 as in the analogue circuit.

A typical programme for speed/torque ch'aracteristics of the type of Fig. 1 is shown in Fig. 12. This is stored in the microcomputer internal memory. For larger programmes a unit with external memory could be used.

The main programme loop is entered after power is applied, or after the calculation of the firing angle of the electronic switch at the start of a new cycle.

The motor voltage and current are alternately sampled and continuously totalled from the start of each cycle. A register is also incremented by one each time a loop is completed, the number resulting at the end of the mains supply cycle being proportional to the period of that cycle.

The start of each mains supply cycle (defined as the line voltage going from negative through zero to positive) interrupts the programme from its normal sample and accumulate loop and directs it to a firing angle calculation routine. This routine compares the accumulation of current samples with the accumulation of voltage samples. The current sample accumulation is suitably scaled to allow direct comparison with the voltage signal. If the voltage sample total is less than the scaled current sample then the device will operate to maintain a constant current to the motor, if greater then the device will maintain a constant voltage supply to the motor.

The firing angle for the electronic switch 11 to obtain this effect is determined using either the voltage or current sample accumulation as required, the firing angle from the last cycle and the main supply period count. The calculation is used to determine the firing angle in the next mains supply cycle.

The particular combination of these quantities to produce the firing angle signal tailors the overall transfer function as required. The use of the mains cycle period count makes the calculation always proportional to period and so independent of the frequency of the incoming supply voltage.

The number determining the firing angle is entered at the beginning of the mains supply cycle and one is subtracted from it each time a loop is completed. When the result becomes zero the electronic switch is switched on. It is assumed that the electronic switch will commutate off naturally at the end of each half cycle.

To avoid the requirement of calculating the firing angle each half cycle the half cycle time obtained from the last period is added to the firing angle time already calculated to obtain the firing angle for the second half cycle. This is quite satisfactory since the mains period will not vary significantly between adjacent cycles. Also the relatively slow response of the mechanical system means that slight variations in half cycle firing angle are unimportant.

Proportional speed control from the control switch 1 6 is obtained by using the speed control setting to vary the A/D converter reference voltage. By reducing this reference voltage the A/D converter produces the same magnitude of numbers for progressively smaller inputs. In this way the speed stalled torque of the motor is reduced proportionally.

The electronic switch 11 of the embodiments enables the use of proportional speed control switches 1 6 without moving or sliding contacts. A Hall-effect device used with a permanent magnet which is moved by the finger trigger on the tool gives a sufficiently proportional signal to enable the speed/torque characteristics to be set without moving contacts. The differential voltage obtained across the Hall-effect speed control switch as the magnetic field is altered is amplified and fed to the logic circuit of the analogue embodiment or to the reference pin of the micro-computer. Other forms of contactless control switches such as a high frequency impedance altered by moving a core, or similar device may be used.

It will be seen from the above that the control systems described will enable the universal motor to work from a wide range of input voltages. They can be arranged to give a wide variety of speed/torque characteristics best suited to the load, and these characteristics can be made to make unnecessary the slipping mechanical clutches required in conventional designs of portable power tools. In addition the characteristics can be shaped to give increased safety to the operator and to protect the motor under stall conditions and on higher voltage supplies. A further advantage is that the control system enables the use of contactless control switches for speed and torque so reducing the incidence of switch failure.

Claims (7)

Claims

1. A motor drive comprising an electronic switch connected in series with a universal motor across an a.c. supply, wherein the switch is activated in timed relationship with the supply frequency by a feedback control system incorporating logic means for predetermining the speed/torque characteristic of the drive by delivering a signal to adjust said timed relationship at which the electronic switch is activated according to the relative magnitudes of motor voltage and motor current.

2. A motor drive as claimed in claim 1, wherein the logic means defines a family of speed/torque characteristics and an operatoractuated switch is provided for selecting one of said family to predetermine the characteristic of the drive.

3. A motor drive as claimed in claim 1 or claim 2, wherein the logic means is formed by analogue circuitry.

4. A motor drive as claimed in claim 1 or claim 2, wherein the logic means is formed by a microcomputer.

5. A motor drive as claimed in any preceding claim, wherein the speed/torque characteristics are as illustrated and described with reference to any one of Figs. 1,4,8 and 10 of the accompanying drawings.

6. A motor drive as claimed in claim 1 and substantially as hereinbefore described with reference to any one of the embodiments illustrated in the accompanying drawings.

7. A control system for a universal motor connected to an a.c. supply, comprising an electronic switch for connection in series with the motor, sensing means for sensing motor voltage and motor current, an actuating switch, logic means having inputs connected to receive from the sensing means signals representative of the sensed motor voltage and motor current, to receive a control signal from the actuating switch and being arranged to emit an error signal at its output, a ramp generator arranged for deriving a repetitive ramp waveform from the a.c. supply, comparator means for comparing the ramp waveform with the error signal and having its output connected to control operation of said electronic switch.