GB2230664A

GB2230664A - Current drive circuit

Info

Publication number: GB2230664A
Application number: GB8906448A
Authority: GB
Inventors: David Anthony Haigh; Anthony Michael Folkard
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1989-03-21
Filing date: 1989-03-21
Publication date: 1990-10-24
Also published as: GB8906448D0

Abstract

A bridge-type current drive circuit is provided for supplying a load such as a torque motor 6. The drive circuit has a first output stage 2 for providing voltage drive to one terminal of the torque motor 6 and a second output stage 3 for providing current drive to the other terminal. The drive circuit is thus capable of continuing to supply current to the torque motor (6) when either motor terminal is short-circuited to ground. <IMAGE>

Description

CURRENT DRIVE CIRCUIT The present invention relates to a current drive circuit, for instance for driving a torque motor.

According to the invention, there is provided a bridgetype current drive circuit, comprising a first output stage arranged to provide voltage drive to a first terminal of a load and a second output stage arranged to provide current drive to a second terminal of the load.

It is thus possible to provide a current drive circuit which continues to function when either one of the load terminals is short-circuited to ground. If the first terminal of the load is short-circuited to ground, the second output stage continues to provide a true current drive using ground as a return. The drive voltage is limited by the power supply voltage of the second output stage and voltage drops occurring in the second output stage. The first output stage preferably includes output current limiting means for limiting the output current through the short circuit to a safe value, e.g.

one which does not damage the first output stage.

If the second load terminal is short-circuited to ground, the first output stage continues to control the current through the load to ground using the voltage/impedance characteristic of the load. The drive voltage is limited by the output voltage of the first output stage and, in this mode, the drive current is dependent on the impedance of the load; the voltage is sufficient to provide an approximate drive so that the load remains operational. The output current 9f the second output stage through the short circuits iimited because of the current drive property, and this may be 'fficient to prevent damage if a short circuit occurs.However, further output current limiting means may be provided to limit the output current of the second output stage through the short circuit to a safe value.

Current drive fault tolerance is thus increased and susceptibility to ground faults is reduced, making such a drive circuit particularly suitable for applications where fault tolerant operation is important. For instance, such a current drive circuit may be used to drive half of a dual wound torque motor in aerospace applications, such as in a helicopter engine control apparatus.

During normal operation of the drive circuit in the absence of ground faults, the load current is determined by the second output stage and the voltage at the first load terminal is determined by the first output stage.

It is normal for the first and second output stages to share a common power supply. In normal operation, it is preferable for the load to be driven by the voltage drive output stage with the current drive output stage providing correction for variation in load impedance and additional potential when the voltage drive output stage is saturated. In the event of a ground fault such that one of the load terminals is short-circuited to ground, the drive to the load will at worst be degraded. In many applications, this will still give adequate performance until the fault can be repaired, thus providing a degree of fail-safe capability which is important in some applications, such as in actuators for aircraft.

Preferably, each of the first and second output stages includes a phase compensation network. This improves the transient response of driving current through complex loads. For instance, phase advance may be incorporated to improve the transient response of driving an inductive torque motor.

The invention will be further described, by way of example, with reference to the accompanying drawing, which is a circuit diagram of a torque motor drive circuit constituting a preferred embodiment of the invention.

The motor drive circuit shown in the accompanying drawing is arranged as four inter-connected functional units comprising a digital to analog converter 1, a voltage drive output stage 2, a current drive output stage 3, and a current monitor 4. The drive circuit is shown connected via double pole contacts of a select relay 5 to one half of a dual wound torque-motor 6 via connectors 7 and 8. The relay has a coil connected between a common line 9 and an input connector 10 for receiving a signal to control the operation of the relay 5. Each of the connectors 7 and 8 is connected to the positive and negative supply lines 11 and 12, respectively, by normally reverse biased diodes 13 to 16 which allow the energy stored in the inductive torque motor 6 to be dissipated when the motor is de-selected by opening the contacts of the relay 5.

The digital to analog convertor 1 is based on an integrated circuit 12 bit multiplying digital to analog convertor 17 of type number AD7542 which is available from Analog Devices. The convertor integrated circuit 17 has data inputs DO to D4, address inputs AO and Al, and a write enable input /WR which are connected to respective terminals of an input connector 18. Each 12 bit word is supplied to the convertor integrated circuit 17 as three bytes of four bits and is stored under control of address and write signals. A clear input CtR is connected to a second positive supply line 19 and digital ground and chip select /CS terminals are connected to a digital common line 20. A voltage reference input VR is connected to a reference voltage line 21.First and second outputs 01, 02 are connected to the inverting and non-inverting inputs, respectively, of an integrated circuit operational amplifier 22, the second output 02 and an analog ground pin AGND being connected to an analog ground line 23.

The operational amplifier 22 functions as an integrator and is provided with an integrating capacitor 24 connected between the inverting input and output of the amplifier 22. The output of the amplifier 22 is connected to a first terminal of a resistor 25 whose second terminal is connected to a feed-back input F/B of the convertor integrated circuit 17 and to first terminals of a resistor 26 and a capacitor 27, the second terminal of which is connected to the ground line 23.

The second terminal of the resistor 26 is connected to the inverting input of an operational amplifier 28 and via a resistor 29 to the voltage reference line 21. The non-inverting input of the amplifier 28 is connected via a resistor 30 to the analog common line 23. The output of the amplifier 28 is connected via a parallel combination of a resistor 31 and a capacitor 32 to the inverting input.

The convertor integrated circuit 17, the operational amplifier 22, and the associated components perform the digital to analog conversion, the convertor integrated circuit operating in the conventional bipolar mode. The operational amplifier 28 and associated components perform a level shifting function by mixing a proportion of the analog signal with a proportion of the reference voltage on the line 21, so as to scale and adapt the output signal of the digital to analog convertor 1 to the output stages 2 and 3.

The voltage drive output stage 2 is based on an integrated circuit power operational amplifier 33 such as type number LH0041 which is available from National Semiconductors. The operational amplifier 33 is provided with an output series resistor 34, a negative feed back resistor 35, output current limiting resistors 36 and 37 connected between the positive and negative supply pins of the amplifier 33 and the positive and negative supply lines 11 and 12, respectively, and a compensating capacitor 38. The non-inverting input of the operational amplifier 33 is connected to the output of the digital to analog convertor 1 via a network comprising a resistor 39 in parallel with a series circuit comprising a capacitor 40 and a resistor 41.

The current drive output stage 3 is also based on an integrated circuit operational amplifier 42, for instance of type number LH0041. The amplifier 42 is provided with output current limiting resistors 43 and 44 and a compensating capacitor 45. The amplifier 42 is further provided with input resistors 46 and 47 and feedback resistors 48, 49 and 51 to form a voltage to current convertor arrangement. The inverting input of the amplifier is connected via a parallel combination of the resistor 46 and a capacitor 50 to the output of the digital to analog convertor 1. The current sensing resistor 51 is connected between the output of the amplifier 42 and the connection between output of the current drive output stage 3 and the resistor 49.

The terminals of the current sensing resistor 51 are connected to the inputs of the current monitor 4. The current monitor 4 is embodied as a standard differential voltage amplifier comprising an operational amplifier 52, resistors 53 to 57, and capacitors 58 to 60. The current monitor thus amplifies the voltage drop across the current sensing resistor 51 and provides a current sensing signal at a connector 61, which may be connected to a control circuit for providing open circuit detection of motor current.

In use, the motor drive circuit is connected to a control circuit by the connectors 10, 18 and 61 for controlling the torque of the motor 6. The voltage drive output stage has a non-inverting transfer function whereas the current drive output stage 3 has an inverting transfer function so as to provide, in effect, a bridge drive arrangement for the motor 6. The voltage to current function or transconductance of the current drive output stage 3 and the voltage gain of the voltage drive output stage 2 are set (by appropriate choice of values of the resistors 35, 39, and 46 to 49) so that, for the required motor current of the motor 6, the drive is provided by the voltage drive output stage 2 with the current drive output stage 3 providing a trim to correct for changes in the torque motor impedance and also providing additional potential when the voltage drive output stage 2 is saturated.The motor 6 presents a substantially inductive load to the output stages 2 and 3, and the components 40, 41 and 50 provide phase advance so as to improve the transient response. During normal operation, the motor current is measured by the sensing resistor 51 and the current monitor 4, and is used by the control circuit to monitor the current for safety purposes, i.e.

checking correct operation of the drive. Selection and de-selection of the motor is controlled by control signals supplied to the relay 5 via the connector 10.

In the event of a short circuit to ground from the terminal of the motor 6 connected via the connector 8 and one of the relay contacts to the output of the current drive output stage 3, motor current continues to be supplied by the voltage drive output stage 2, the motor current passing through the motor and to ground through the short circuit. Normal demanded current can still be supplied, but the maximum motor current is reduced because the load represented by the motor 6 now appears between the output of the output stage 2 and ground instead of being shared between the output stages 2 and 3. Thus, the maximum current will be approximately half of that available from the circuit during normal operation. However, the motor can still be driven although with reduced performance which, in many cases, is acceptable and is certainly preferable to complete loss of motor drive.The fault current supplied by the current drive output stage 3 through the short circuit to ground is limited by the transconductance of the output stage 3 and is prevented from exceeding a safe value by the current limiting resistors 43 and 44. Thus, the drive circuit continues to operate, albeit with de-graded performance, and does not suffer damage.

Conversely, in the event of a short circuit to ground of the motor terminal connected via the connector 7 and the relay contact to the output of the voltage drive output stage 2, the current drive output stage 3 continues to supply motor current through the motor 6 and via the short circuit to ground. The transconductance of the output stage 3 is not substantially affected by the fault and the output stage 3 can continue to supply the desired motor current for desired currents of limited values.

However, when higher currents are required, the output of the current drive output stage 3 saturates, thus limiting the maximum current which can be supplied to the motor 6.

Thus, the performance is de-graded, but the drive circuit and motor can continue to operate. The fault current supplied by the voltage drive output stage 2 through the short circuit to ground is limited by the current limiting resistors 36 and 37 so as to prevent damage to the output stage 2. Thus, degraded performance remains available with no risk of damage to the drive circuit.

Claims

1. A bridge-type current drive circuit comprising a first output stage arranged to provide voltage drive to a first terminal of a load and a second output stage arranged to provide current drive to a second terminal of the load.

2. A circuit as claimed in Claim 1, in which the first output stage includes current limiting means for limiting output current of the first output stage to a predetermined maximum value.

3. A circuit as claimed in Claims 1 or 2, in which one or each of the first and second output stages includes a phase compensation network.

4. A circuit as claimed in Claim 3, in which the or each phase compensation network is arranged to provide phase advance.

5. A bridge-type current drive circuit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.