Method of and circuit for testing an electrical actuator drive stage

A circuit is provided for testing a drive stage of an actuator, such as a motor of an EPAS system. A power supply circuit comprises a contact for supplying normal drive stage current and a resistor for supplying reduced drive stage current before the normal current is supplied. A measuring circuit measures a drive stage electrical parameter such as supply voltage and a comparator compares this with an acceptable value. If the measured parameter corresponds to a current through the drive stage which is different from an expected value, a fault is signaled and the contact is prevented from closing.

FILED OF THE INVENTION
 The present invention relates to a method of and circuit for testing an
 electrical actuator power electronic drive stage. Such a circuit may be
 used for testing a drive stage of a vehicle, for instance for driving an
 electric motor of an electric power assisted steering (EPAS) system.
 BACKGROUND OF THE INVENTION
 DE 2 751 116 discloses a testing arrangement for testing vehicle lighting
 circuits. Constant current sources are switched so as to supply currents
 depending on the expected load to different lighting circuits. The
 voltages across the lighting circuits are measured and a fault is
 indicated if the voltage is higher than a preset expected value. This
 arrangement therefore detects bulb failure in such lighting circuits.
 DE 3 842 921 discloses an arrangement for monitoring the currents drawn by
 electrical loads. The load current is measured by switching a current
 sensing resistor into the load circuit and measuring the voltage across
 the resistor. This voltage is compared with a threshold which indicates
 whether the load current is acceptable.
 DE 4 338 462 discloses a control system for electrical consumers in a motor
 vehicle. Power is applied to the electrical consumers via a constant
 current source while switching off the battery voltage for a short time.
 The voltage across the consumer is monitored and a fault is indicated if
 it has an unacceptable value during the short time when the constant
 current source is connected in place of the battery voltage.
 SUMMARY OF THE INVENTION
 According to a first aspect of the invention, there is provided a circuit
 for testing an actuator drive stage, comprising a power supply circuit for
 supplying reduced current to the drive stage before supplying normal
 current thereto, a measuring circuit for measuring an electrical parameter
 within the drive stage, and a comparator for comparing the measured
 parameter with an acceptable value to signal a fault condition when the
 measured parameter corresponds to a current through the drive stage which
 is different from an expected value.
 Preferably the power supply circuit comprises a contact for supplying the
 normal current and at least one resistor connected in parallel with the
 contact for supplying the reduced current before the contact is closed.
 The resistor may be constructed from two or more discrete components
 connected in series. A switch may be provided for disconnecting the at
 least one resistor from the contact.
 A controller, such as a microcontroller, may be provided for closing the
 contact in the absence of a signalled fault condition. The controller may
 be arranged to switch on at least one active device of the drive stage
 while the reduced current is supplied to the drive stage.
 The controller may be arranged, during supply of the reduced current, to
 switch on at least part of the drive stage, to switch off the drive stage,
 and to signal a fault in the actuator if a drive stage output voltage
 falls below a threshold value in less than a predetermined time period.
 The measuring circuit may comprise a circuit for measuring the supply
 voltage to the drive stage. The measuring circuit may comprise a circuit
 for measuring an output voltage of the drive stage. The measuring circuit
 may comprise a potential divider whose output is connected to an
 analogue-to-digital converter, which may be provided within the
 controller.
 According to a second aspect of the invention, there is provided a method
 of testing an actuator drive stage, comprising supplying reduced current
 to the drive stage before supplying normal current thereto, measuring an
 electrical parameter within the drive stage, signalling a fault condition
 if the measured parameter corresponds to a current through the drive stage
 which is different from an expected value, and supplying normal current to
 the drive stage in the absence of signalling of a fault condition.
 It is thus possible to provide an arrangement which signals a fault in an
 actuator drive stage or in an actuator connected thereto before normal or
 full power is supplied to the drive stage. It is therefore unnecessary for
 excessive current, arising for instance from a short circuit in the drive
 stage or the actuator, to have to be broken, for instance by a contact of
 an electromagnetic relay, if a fault exists within the drive stage of the
 actuator. It is further unnecessary to rely on storage of a fault
 indication detected during previous operation of the actuator.
 The invention will be further described, by way of example, with reference
 to the accompanying drawings, in which:

Like reference numerals refer to like parts throughout the drawings.
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 illustrates part of a typical EPAS system for use in a vehicle. A
 three-phase star-connected brushless permanent magnet motor 1 is connected
 via a geared drive to a steering column or rack of a vehicle steering
 system (not shown). The torque applied by a vehicle driver, for instance
 to a steering wheel, is measured and used to control the current supplied
 to the motor 1 so as to assist the steering of the vehicle.
 The motor 1 is connected to a drive stage 2 for supplying drive current
 demanded by a microcontroller unit (MCU) 3. The MCU has inputs (not shown)
 for receiving signals from, for instance, a torque sensor measuring driver
 torque in the steering system and possibly for receiving signals relating
 to steering angle, vehicle speed, and other vehicle parameters. The drive
 stage 2 comprises "top" power devices 4, 5 and 6 and "bottom" power
 devices 7, 8 and 9 arranged as a three-phase bridge drive circuit with
 outputs 10, 11 and 12 connected to the three-phase inputs of the motor 1.
 Each of the power devices 4 to 9 may comprise a power transistor of
 bipolar or field effect type. The top and bottom power devices are
 connected to the outputs of top and bottom driver circuits 13 and 14,
 respectively, whose inputs are connected to outputs of the MCU 3.
 A first power supply line 15 of the drive stage 2 is connected via a
 contact 16 of a relay and a supply line 29 to a vehicle supply, such as a
 battery. The relay further comprises an electromagnetic coil 17 connected
 to an output of the MCU 3. A second supply line 18 is connected via a
 current sensing resistor 19 to a ground connection 20 of the vehicle
 supply. A differential amplifier 21 has differential inputs connected
 across the resistor 19 and supplies an output signal to the MCU 3
 representing the current through the drive stage 2.
 A filter circuit 22 is connected between the power supply lines 15 and 18
 of the drive stage 2 for smoothing the current drawn from the vehicle
 supply. The filter circuit 22 comprises a series-connected resistor 23 and
 capacitor 24. A potential divider comprising resistors 25 and 26 is
 connected between the supply lines 15 and 20. The output of the potential
 divider is connected to an input of the MCU 3. A further potential divider
 comprising resistors 27 and 28 is connected between ground and the supply
 line 29. The output of the potential divider is connected to another input
 of the MCU 3.
 The MCU 3 may be powered via a supply line which is switched on by an
 ignition key of the vehicle or may be constantly powered from the vehicle
 battery. The MCU 3 contains a microprocessor and associated RAM and ROM,
 analogue-to-digital converters for converting the outputs of the potential
 dividers to digital code, and suitable drive arrangements for supplying
 current demand signals to the driver circuits 13 and 14. The MCU 3 also
 has an output interface for driving the coil 17 of the electromagnetic
 relay. The ROM of the MCU 3 stores software for controlling operation of
 the EPAS system and for performing diagnostic tests.
 During normal operation of the EPAS system, when the vehicle electrical
 system is switched on by the ignition key, the MCU 3 supplies power to the
 relay 17 so as to close the contact 16. Power is thus supplied to the
 drive stage 2 via the contact 16. The supply voltage is measured by the
 MCU 3 by means of the potential divider comprising the resistors 27 and 28
 which ensure that the voltage to be measured is within an acceptable range
 for the MCU 3. Similarly, the voltage across the drive stage supply lines
 15 and 20 is measured via the potential divider comprising the resistors
 25 and 26 and the current through the drive stage 2 is measured by means
 of the resistor 19 and the amplifier 21. Diagnostic tests are performed
 while the vehicle is operating so as to check that the motor 1 is not
 being incorrectly driven. If a fault is diagnosed, the motor is isolated
 by turning off the power devices 4 to 9 and removing power from the coil
 17 of the relay so as to open the contact 16. Power assistance is
 therefore disabled so as to protect the vehicle and driver against
 undesired assistance torques.
 Problems may arise at the start of subsequent operation of the vehicle and
 hence of the EPAS system. If the drive stage 2 has failed with a short
 circuit, then a large current will flow through the contact 16 when the
 relay is switched on. This will be detected by the diagnostic tests and
 the contact 16 will be opened. However, this is undesirable because
 opening the relay contacts when a large current is flowing can damage the
 contact 16.
 If the fault in the drive stage 2 occurred during previous operation of the
 system, the MCU 3 may be arranged to store a fault code in non-volatile
 memory and to recognise this when the system is again switched on so as to
 prevent closing of the contact 16. However, fault diagnosis is required to
 err on the side of safety and an incorrect diagnosis may have occurred, in
 which case the system will remain off when there is no real fault.
 Further, although the fault diagnosis may have been correct, the
 non-volatile memory may fail or be incorrectly reset by service personnel,
 so that the contact 16 may still be required to break a large current and
 may therefore suffer damage.
 The circuit shown in FIG. 2 overcomes these problems by testing the drive
 stage 2 for short circuits and other faults at the start of operation of
 the system before the contact 16 is closed. The circuit shown in FIG. 2
 differs from that shown in FIG. 1 in that a resistor 30 is connected in
 parallel with the relay contact 16 so as to supply reduced current to the
 output stage 2 during initial diagnostic tests. Such tests are made when
 the system has been activated before the start of a journey but before the
 relay contact 16 has been closed. It is also possible for the tests to be
 performed at the end of the journey and the results stored in non-volatile
 memory.
 FIG. 3 illustrates a possible modification according to which the resistor
 30 is replaced by series-connected resistors 30a and 30b. This minimises
 the risk of a failed resistor 30 short-circuiting the contact 16.
 FIG. 4 illustrates another possible modification according to which the
 resistor 30 is connected in series with a switching device 31, such as a
 bipolar or field effect transistor, controlled by the MCU 3. This
 arrangement allows the resistor 30 to be disconnected so that other tests
 on operation of the relay may be performed. For instance, such tests may
 include a check on the relay switching time.
 FIG. 5 illustrates another variation which allows the individual phase or
 output voltages of the drive stage 2 to be measured. For the purpose of
 illustration, only one limb of the bridge drive stage has been shown
 comprising the power devices 4 and 7.
 A resistor 32 is connected across the power device 4 whereas a potential
 divider comprising resistors 33 and 34 is connected across the power
 device 7 and sense resistor 19. The output of the potential divider is
 connected to an input of the MCU 3. The resistor network 32-34 is used to
 bias this phase voltage around the centre and the potential divider
 reduces the voltage to a suitable level for an analogue-to-digital
 converter within the MCU 3.
 In order to prevent current passing to the drive stage 2 if the supply
 voltage polarity is erroneously reversed, a diode 35 may be connected in
 series with the resistor 30 or the resistors 30a and 30b as shown in FIG.
 6. Power supply to the MCU 3 is separately protected. Thus, even if the
 MCU 3 were able to be powered, the supply voltage at the supply line 29
 will be found to be outside the normal limits when monitored by the MCU 3
 so that the contact 16 will not be closed.
 The diagnostic tests which are made before the contacts are closed depend
 on the measurements that are available. There are a number of different
 possible combinations of tests comprising alternative configurations of
 the following measurements:
 1. supply voltage measurements across the lines 29 and 20;
 2. drive stage supply voltage measurements across the lines 15 and 20;
 3. motor phase voltage measurements across the lines 10 and 20, 11 and 20,
 and 12 and 20;
 4. supply filter network oscillation measurement;
 The diagnostics that can be performed with each measurement are described
 below. All of the diagnostics except the filter network oscillation are
 based on the following sequence of operations:
 a) wait for any transients to settle then check that the measured voltages
 are within normal limits with all of the power devices turned off and the
 relay 16, 17 turned off;
 b) turn on each top power device 4, 5, 6 one at a time, wait for any
 transients to settle and then check that the measured voltages are within
 normal limits;
 c) turn off the top devices 4, 5, 6 and then turn on each bottom power
 device 7, 8, 9 one at a time and check that the measured voltages are
 within normal limits (after waiting for any transients to settle);
 d) turn on each top and bottom pair 4+7, 5+8, 6+9, one at a time to get
 direct (shoot-through) paths, wait for transient fluctuations to settle
 and then check that the measured voltages are within limits.
 It is possible to go through the full test sequence and record the results
 to provide a clear indication of any of the failure modes that may have
 occurred. It is better to stop the tests as soon as any failure mode is
 revealed. In either case, if a failure mode is discovered, the system
 should be returned to the safest possible state by turning off all of the
 power devices and inhibiting operation of the relay 16, 17.
 In the tables below, V.sub.Ink is the measured drive stage voltage between
 the lines 15 and 20, V.sub.ph is any measured phase output voltage,
 V.sub.sup is the measured supply voltage, s/c is shorthand for short
 circuit, o/c is shorthand for open circuit.
 Diagnostics with Drive Stage Voltage Measurement (Using Resistors 25, 26)

Normal Abnorml
 Test result result Possible causes
 All power V.sub.Ink high V.sub.Ink low Motor supply link s/c to ground
 devices Motor supply link o/c
 off V.sub.Ink measurement top resistor o/c
 V.sub.Ink measurement bottom
 resistor s/c
 At least one top and one
 bottom power device s/c
 Single top V.sub.Ink high V.sub.Ink low Motor star point s/c to ground
 power Bottom power device s/c
 device on V.sub.Ink measurement top resistor o/c
 V.sub.Ink measurement bottom
 resistor s/c
 Single V.sub.Ink high V.sub.Ink low Top power device s/c
 bottom V.sub.Ink measurement top resistor o/c
 power V.sub.Ink measurement bottom
 device on resistor s/c
 Single top V.sub.Ink low V.sub.Ink high Cround link o/c
 & bottom Top power device o/c
 pair on Bottom power device o/c
 Diagnostics with Phase Voltage Measurement (Using Resistors 32-34)

Normal Abnormal
 Test result result Possible causes
 All V.sub.ph V.sub.ph pulled high Ground link o/c
 devices centred Motor phase s/c to supply
 off Motor star point s/c to supply
 Top device s/c
 V.sub.ph pulled low Motor supply link s/c to ground
 Motor supply link o/c
 Motor phase s/c to ground
 Motor star point s/c to ground
 Bottom device s/c
 Single All V.sub.ph all V.sub.ph centred Top device o/c
 top pulled
 device high some V.sub.ph centred Motor phase o/c
 on V.sub.ph pulled low Motor supply link s/c to ground
 Motor suppiy link o/c
 Motor phase s/c to ground
 Motor star point s/c ground
 Single V.sub.ph pulled all V.sub.ph centred Bottom device o/c
 bottom low
 device some V.sub.ph centred Motor phase o/c
 on V.sub.ph pulled high Ground link o/c
 It is also possible to recognise faults in the phase voltage measurement
 divider network and the connections to the power devices.
 Diagnostics with Link Voltage Measurement and Supply Voltage Measurement

Abnormal
 Test Normal result result Possible causes
 All devices V.sub.Ink less V.sub.Ink nearly Relay contacts closed or s/c
 off than V.sub.sup equal to V.sub.sup
 Once the charge in the capacitor 24 has settled, the voltage drop across
 the contact 16 is determined by the potential divider comprising the
 resistor 30 and the measuring resistor network such as the resistors 25
 and 26 or the resistors 32 to 34. If the difference between the measured
 drive stage voltage V.sub.Ink and the measured supply voltage V.sub.sup is
 too small, a short circuit fault on the relay 17 such that the contact 16
 is unexpectedly closed can be diagnosed before the system is made
 operational. Normal operation can then be inhibited and a warning issued.
 However, there are two specific situations in which this test might give
 an erroneous result as follows.
 If the supply voltage has not been connected to the line 29 for a
 sufficient time, then the capacitor 24 will not be fully charged to its
 usual pre-operation state. This might arise if, for instance, a mechanic
 has just reconnected the vehicle battery. If the contact 16 were closed in
 this state, a relatively large current would flow into the capacitor 24
 which might damage the contact 16. However, this state can be detected by
 the rapid fall in the voltage drop from an anomalously high starting value
 and closure of the contact 16 can be delayed until the voltage drop has
 fallen low enough.
 If the driver has just switched on the ignition after a brief switch-off
 interval, then the capacitor 24 may still be charged almost to the supply
 voltage. This can be detected by starting a timer when the driver switches
 off the ignition. if the driver switches the ignition on again after only
 a brief interval, then no diagnosis of a shorted relay contact can be
 made.
 Diagnostics with Link Voltage Measurement and Filter Network Oscillation
 The above diagnostics are unable to detect a short-circuited motor winding.
 Because the motor winding resistance is small, a large current is needed
 to obtain a measurable voltage drop. Therefore it is hard to measure the
 motor winding resistance with the small current that flows through the
 resistor 30 during the diagnostic tests.
 Instead of measuring the resistance, it is possible to measure the effect
 of the inductance of the motor winding using energy stored in the filter
 network 22. The operations required are:
 (i) charge up the filter capacitor 24 via the resistors 23 and 30;
 (ii) turn on one top device 4 and one bottom device 8 simultaneously to
 draw current through the motor 1;
 (iii) wait for the time it takes the voltage in a short-circuited winding
 to fall to zero;
 (iv) measure the drive stage supply V.sub.Ink and/or phase voltages
 V.sub.ph ;
 (v) repeat steps (i) to (iv) for each path through the motor 1;
 (vi) if any voltage measured in step (iii) is near to zero, then the
 diagnostic test indicates a short-circuited winding and the relay contact
 16 should not be closed.
 If the motor winding inductance is present, then the inductance will oppose
 the capacitor discharge and so the drive stage supply V.sub.Ink and phase
 voltages V.sub.ph will still be close to the supply voltage V.sub.sup
 after the time it takes for a short-circuited winding to decay to zero.
 This diagnostic test is illustrated in FIG. 7, which illustrates the decay
 of the voltage across a normal motor winding by curve 40 and the decay
 across a short-circuited winding by the curve 41. In the case of a short
 circuited winding, the voltage decays to zero in a time t whereas the
 voltage across a normal winding decays much more slowly. Thus, by
 measuring the drive stage supply voltage or the phase voltage after an
 appropriate time delay following the step (ii), the measured voltage
 provides an indication of whether the winding is short-circuited.
 Alternatively, the time taken for the voltage to fall to zero or near to
 zero may be measured to assess whether the winding is short-circuited.
 Prior to measurement of the effect of the motor inductance, correct
 operation of the filter circuit 22 may be diagnosed as follows. The MCU 3
 switches a load between the supply lines 15 and 18 that will tend to
 discharge the capacitor 24. For instance, such a load may be the coil of a
 solenoid operated clutch, the coil of another relay, or a resistor. An
 excessive rate of rise of the voltage difference between the drive stage
 supply voltage V.sub.Ink and the supply voltage V.sub.sup (i.e. the
 voltage drop across the resistor 30) indicates too little capacitance in
 the reservoir capacitor 24 or an excessive load. Too small a rate of rise
 indicates too much capacitance or insufficient loading. If this diagnostic
 test is performed satisfactorily so as to imply that the capacitance of
 the capacitor 24 is correct, then the effect of the motor inductance may
 be measured as described hereinbefore.
 The forward voltage drop of the diode 35 (when present) and the time
 constant of the capacitor 24 and the quiescent loading network comprising
 the resistors 25, 26, 32, 33 and 34 can be deducted from measurement of
 the voltage drop (V.sub.sup -V.sub.Ink) across the resistor 30 during the
 recovery phase as follows.
 The MCU 3 switches off the load as soon as the voltage drop rises above
 (VD1+VD1 margin) where, for example, VD1=3 volts and VD1 margin=0.2 volts.
 As soon as the voltage drop falls below VD1, a timer is started. The timer
 values TD2 and TD3 are stored when the voltage drop falls below VD2 and
 VD3, respectively, where, for example, VD2=2.37 volts and VD3=1.74 volts.
 The following equations can then be us ed to estimate the forward voltages
 drop Vf of the diode and the time constant TC of the capacitor 24 and
 resistors:
 ##EQU1##
 Vf=AVD-BF (V.sub.sup -AVD
 where BF is a biasing factor equal to the resistance of the resistor 30
 divided by the total load due to the resistors 25, 26, 32-34 which pull
 down the drive stage supply voltage V.sub.Ink.