Electrical transmission or interconnection systems – Switching systems – Condition responsive
Patent
1997-06-16
1998-06-09
Shoop, Jr., William M.
Electrical transmission or interconnection systems
Switching systems
Condition responsive
307125, 327108, 327110, 327131, 363 41, 361 93, 361152, H02M 308
Patent
active
057639639
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a circuit arrangement for monitoring a control circuit including a driver transistor which is used to adjust in an inductive load, for example, a coil, a predetermined average current value by a clocked, pulse-width modulated actuation (PWM) of the driver transistor, and wherein current flow is induced by the inductive load in the blocking intervals of the driver transistor during the PWM-actuation through a circuit branch which is parallel to the inductive load and has a switch.
Prior art hydraulic brake systems with electronic anti-lock control (ABS) or traction slip control (TCS) include electrically or electromagnetically operable hydraulic valves actuated by way of drivers having driver transistors in their output stages. This arrangement is an example of a control circuit which has a driver transistor with inductive load. The driver transistor is used to control the current flow through the inductive load impedance, i.e. the coil of a hydraulic valve. When the driver transistor is blocked, the energy stored in the coil usually tends to maintain the coil current, with the result of a correspondingly high disconnecting voltage across the coil. Arrangements have been disclosed which conduct the coil current, after the driver transistor is blocked, through a circuit branch that is parallel to the inductive load and includes a switching transistor. An (average) current value may be adjusted on a predetermined level by clocked actuation of the driver transistor, by closing the switch in the circuit branch parallel to the load during the blocking interval of the driver transistor, and by modulating the pulse/pulse-break ratio of the actuating pulses or the actuating clock. In circuits of the type at issue, the predetermined current value or nominal current value is generally defined by comparison with an internal reference value. Complex integrated circuits are used which set both the reference value and the switching thresholds of the transistors that determine the pulse/pulse-break ratio in PWM operations. The difficulty involved is that variations of the reference value and the thresholds will go unnoticed and that errors, in particular discrepancies of the coil current from the nominal value, cannot be detected. Also, the IC internal current monitoring circuits depend on the internal reference value. Therefore, the limit values of the internal current monitoring system will shift in the some manner when the reference value changes.
An object of the present invention is to improve a circuit arrangement of the above type so that the coil current or the current which flows through the inductive load is monitored, irrespective of possible measures inside the circuit, in a simple manner, with little structure and great reliability in operation.
It has been found that this object is achieved by the circuit arrangement described in claim 1. The special circuit features include a current measuring device which is connected parallel to the driver transistor by way of a switching element. During a test period, the current which is induced by the inductive load during the locking intervals is conducted to the current measuring device.
Thus, the present invention provides an additional, external current measuring and current monitoring device which is independent of the actual control circuit and the associated monitoring circuits. The operation of the current measuring and current monitoring device is based on the continued flow of the coil current after the driver transistor is blocked due to the inductance, and on directing the current to the measuring device. When the measuring device includes an ohmic resistor through which the diverted current must flow, a voltage proportional to the current will drop across the resistor. The voltage exceeds a predetermined value or is within predetermined limits only if the control circuit is intact. For example, a voltage drop which is in excess of the supply voltage of the control circuit is likely to occur only due to the disconnection of
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English Translation of the International Preliminary Examination Report of International Application No. PCT/EP95/03656.
Fey Wolfgang
Zydek Michael
ITT Manufacturing Enterprises Inc.
Lewis J. Gordon
Paladini Albert W.
Shoop Jr. William M.
Twomey Thomas N.
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