Current control for an inductive load

Abstract: A current control for an inductive load (10) includes a switch device (11) and a sensor (12) in series with the load. A voltage comparator (15) is connected to operate when the current in the load reaches a desired average value. A digital control (18) measures the time taken for the current to reach this value and maintains the switch device conductive for a further period based on this measured time. Accurate average current control is obtained even if it is not possible to measure the load current when the switch device is non-conductive.

Description: This invention relates to a current control for an inductive load. The force produced by an electro-mechanical actuator in which there is a cyclically fluctuating current is a function of the average magnetic flux in the air gap of the actuator and hence is related to the average current flowing in the actuator winding (or windings). To control an actuator to give a particular force, therefore it is advantageous to maintain a correct level of average current in the winding(s) to provide reliable actuation without excessive power consumption. Efficient operation of electro-mechanical actuators is conventionally achieved by using pulse width modulation of the voltage supply to the actuator. This involves closing a switch element periodically to cause current in the winding(s) to increase and then opening the switch to allow the current to be diverted through a recirculation diode or other recirculation element, so that the current decays until the next pulse is commenced. With this arrangement, it is difficult in conventional control circuits to measure the average current flow, since the current sensing element required has to be able to monitor the current during both current growth and current decay periods. Where the recirculation diode is separate from the winding, this is possible, but complex and expensive analog circuit elements will be required to effect the monitoring. Where the recirculation diode is built into the winding and has no separate terminals, current metering is even more difficult. It is therefore an object of the present invention to provide a simple but effective average current control which avoids the difficulties mentioned above. In accordance with the invention there is provided a current control for an inductive load comprising a switch device and a current sensing element connected in series with the load, a voltage comparator connected to the sensing element and arranged to operate when the current in said sensing element is equal to a desired average value, and a digital control circuit connected to said voltage comparator to receive an input therefrom and to said switch device to control the state of conduction thereof, said digital control circuit operating to turn said switch device on periodically, to measure the duration of a first interval from turn-on to the moment of operation of the voltage comparator when the current in the load reaches the desired average value, and to maintain the switch element in its turned-on condition for a second interval of duration calculated as a function of the duration of said first interval. If the intervals referred to are short compared with the time constants for current growth and decay, the growth and decay can be regarded as substantially linear, so that an acceptable level of accuracy can be obtained by basing the duration of the second interval on the duration of the first. The calculation used is such that, in steady state conditions, the duration of the second interval is equal to the duration of the first interval. To enable the control to operate stably, however, the calculation is such that the duration of the second interval is equal to the average of the durations of the first intervals in the current cycle and the preceding cycles. In the accompanying drawings: Figure 1 is a circuit diagram showing one example of the invention; Figure 2 is a graph showing waveforms in the circuit of Figure 1; Figure 3 is a flow chart illustrating the operation of the example shown in Figure 1; and Figure 4 is a circuit diagram showing a second example of the invention. Referring firstly to Figure 1, the inductive load is in the form of a solenoid 10 with which there is associated a current recirculation diode 10a. This diode is, in fact, built into the solenoid so that it is not possible to separate the connections between the solenoid and the diode, thereby making it impossible to use a current sensing element which can sense the current level in the solenoid when externally supplied current is interrupted. The control includes a field effect transistor switch device 11 and a current sensing resistor 12 connected in series with the solenoid between a supply rail 13 and a ground rail 14. One end of the resistor 12 is connected to the supply rail 13, so that the voltage at the other end of the resistor is linearly related to the current flowing in the solenoid when the switch device 11 is conductive. The control also includes a voltage comparator 15 which has its inverting input connected by a resistor 16 to said other end of the resistor 12 and to the supply rail 17 (acting as a noise filter) to the supply rail 13. The output terminal of the voltage comparator 15 is connected to an input terminal of an ASIC 18 which incorporates an arithmetic unit (not shown). The ASIC has one output terminal which is connected by a resistor 19 to the gate of the switch device 11. Another output terminal of the ASIC provides a reference voltage to the non-inverting input terminal of the voltage comparator 15 in accordance with commands received by the ASIC and calculations made in the arithmetic unit thereof. The generation of the reference voltage is not described in detail herein but can be set dynamically and statically to minimise the actuator power consumption. Suffice it to say that the reference voltage applied to the non-inverting input of the voltage comparator 15 is set to the voltage to which said other end of the resistor 12 will drop when the instantaneous current flowing in the resistor 12 is equal to the desired average current in the solenoid 10. The relevant part of the program of the ASIC is shown in Figure 3. The routine shown is called periodically at set time intervals and is responsible for monitoring the output of the voltage comparator and controlling the switch device accordingly. Each time the routine is called the variable t is incremented (20) and the value of t is compared (21) with the repetition cycle time t cyc . If the cycle time has not expired the routine tests (22) whether the system is in the first phase of its cycle. If it is, then the routine tests (23) whether the output of the voltage comparator 15 is high or low. If the output is low the routine terminates. If it is high, a calculation (24) is carried out to update the stored value of a variable t av by adding three times the stored value to the current value of t and dividing the sum by 4. The value of a variable t off is then calculated as the sum of the new value of t av and t . The value of the phase variable is then set (25) to 2 so that on the next cycle, decision (22) will cause a jump to a different branch of the routine. When phase is equal to 2, the routine tests (26) whether the period defined by the variable t off has expired. If it has, then phase is set (27) to 3 and the switch is turned off. When phase is set to 3, the routine tests (29) whether t cyc has expired. If it has t is set to zero, phase is set to 1, and the switch is turned on. Thus, in a normal cycle of operation, as shown in Figure 2, the switch device 11 is turned on at the beginning of the cycle. The current level thus starts to rise. When the current level reaches the desired average value, the output of the voltage comparator 15 goes high and the duration of the on period of the switch is then calculated. The switch is held on until the (second) calculated period expires, and then the switch device is turned off again until it is time for a new cycle to commence. Following any change in the desired average current, or any disturbance in the mechanical load on the actuator, the inclusion in the calculation of the total on time of a dominant term based on previous durations of the first interval ensures that the control adjusts itself to the changed conditions in a stable manner. In the second example shown in Figure 4, the solenoid 10 is connected between the supply rail 13 and the switch device 11. The resistor 12 is connected between the switch device 11 and the ground rail 14 and it is the non-inverting input of the voltage comparator 15 which is connected by resistor 16 to the resistor 12. The inverting input of the voltage comparator 15 is connected to receive the reference voltage signal from the ASIC 18. The operation of the second example is exactly the same as that of the first example.

Claims: A current control for an inductive load (10) comprising a switch device (11) and a current sensing element (12) connected in series with the load, a voltage comparator (15) connected to the sensing element and arranged to operate when the current in said sensing element is equal to a desired average value, and a digital control circuit (18) connected to said voltage comparator to receive an input therefrom and to said switch device to control the state of conduction thereof, said digital control (18) circuit operating to turn said switch device on periodically, to measure the duration of a first interval from turn-on to the moment of operation of the voltage comparator when the current in the load reaches the desired average value, and to maintain the switch element in its turned-on condition for a second interval of duration calculated as a function of the duration of said first interval. A current control as claimed in Claim 1 in which said digital control circuit (18) operates so as in steady state conditions to set the duration of the second interval to be equal to the measured duration of the first interval. A current control as claimed in Claim 2, in which said digital control circuit (18) maintains a variable representing the average value of said duration of the first interval and updates such variable each time a new value of the duration of the first interval is determined. A current control as claimed in Claim 3, in which updating of the average value is carried out in accordance with the expression t av = (3*t' av + t)/4 where t av is the new average value, t' av is the old average value and t is the new measured duration of the first interval. A current control as claimed in any preceding claim in which the digital control circuit (18) provides a signal representing the desired average value to the voltage comparator.

Inventors: POTTER STEPHEN, TURVEY SIMON, POTTER, STEPHEN, TURVEY, SIMON
Applicants: LUCAS INDUSTRIES LTD, LUCAS INDUSTRIES LIMITED
Application year: 1998
Main classification: H01H47
IPC codes: H01H47