Patent Application
20120112771
This invention relates in general to systems that include solenoids and in particular to an apparatus and method for detection of small undesired solenoid currents.
A typical prior art control circuit 10 used for multiple solenoids is shown in
The circuit 10 further includes a plurality of voltage feedback circuits which are used to detect undesired activations of a solenoid or failures of a solenoid to activate when desired. One high side voltage feedback circuit is shown to the left of
Also shown in
As described above, when both the high side FET 12 and a selected low side FET, such as T2, are switched to their conducting states, a large current flows through the associated solenoid coil L2 and an associated low feedback voltage VFB2 will be provided to the controller 14. When the low side FET T2 is switched to its non-conducting state, a small current will flow through L2 and VD2, and an associated high feedback voltage VFB2 will be provided to the controller 14. In the absence of a fault, the level of the voltage feedback VFBY and VFB2 would indicate that the FETs 12 and T2 and the coil L2 are operating correctly. A fault due to an open or shorted coil L2 or an open, leaky or shorted low side FET T2 can be detected through the voltage feedback VFB2 when the monitored voltages are not as expected, as shown in following table.
The resistors R1 through RN, that are connected in parallel with the coils L1 through LN, and the coil voltage divider circuits, VDD through VDN, allow an open coil to be distinguished from a shorted low side FET. With an open coil, the current flows through the corresponding resistor and the resulting voltage drop causes the voltage feedback to be lower than expected when the FET is off, but not so low as it would be if the low side FET is shorted. Although this allows an open coil to be detected, it may be difficult to distinguish an open coil from a leaky FET in certain ranges of current. If the leakage current is very low, there is little voltage drop across the coil and the resulting feedback voltage is close to the supply voltage and thus may not be detected. If the leakage current is higher, but still relatively low, the feedback voltage is close to the open coil voltage and becomes difficult to distinguish from an open coil.
In certain solenoid control systems, such as, for example, Anti-Lock Brake Systems (ABS) and Electronic Stability Control (ESC) systems used in vehicle electronically controlled brake systems, the solenoids are utilized to operate valves that control the flow of brake fluid during a brake system operation. An unintended leakage current through a solenoid coil could result in a partial or complete opening or a closure of a valve, which could result in an undesired flow or an undesired blockage of brake fluid. If this condition does occur, it is necessary to switch off the high side FET 12 in order to stop the leakage current flow and prevent undesirable effects on brake system performance. Since the high side FET is common to all solenoid coils, switching it off also disables all of the solenoid coils, and therefore all electronically controlled brake system functions which require solenoid operation. In the case of an open solenoid coil, there is no undesired current flowing, so it is not necessary to switch off the high side FET. The other solenoid coils which are functioning properly are not disabled and therefore the only electronically controlled brake system functions which need to be disabled are those that require proper operation of the affected solenoid coil. Accordingly, it would be desirable to provide an apparatus and method to detect such small currents to distinguish a leakage current from an open solenoid coil.
This invention relates to an apparatus and method for detection of small solenoid currents.
The present invention contemplates an apparatus for controlling current flow through a coil that includes at least one coil having a first end and a second end and a first electronic switch having an input terminal adapted to be connected to a voltage supply and an output terminal connected to the first end of the coil. A second electronic switch is connected between the second end of the coil and ground and a capacitor is connected between the first end of the coil and ground. The capacitor is charged when the first electronic switch is in a conducting state and discharged when the first electronic switch is in a non-conducting state. A feedback circuit is connected to one end of the coil and is operable to provide a discharge path for the capacitor, and to monitor the rate of decay of the resulting capacitor charge to determine if there is a fault present in the control circuit.
The present invention further contemplates a method for detecting leakage current flow through a coil that includes the steps of providing the apparatus described above. The method then places the first electronic switch in a conducting state to charge the capacitor in the feedback circuit. The method next places both the first and second electronic switches in a non-conducting sate while monitoring the rate of decay of the charge on the capacitor to determine whether an excessive leakage current is flowing though the coil.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.
The present invention is directed toward a low cost circuit that allows detection of low levels of leakage current, not detectable through existing means. Referring now to the drawings, there is illustrated in
The control circuit 20 includes a large capacitor C, which, in the preferred embodiment, has a value within the range of approximately 1 to 4.7 uF. However, the invention also may be practiced with a capacitor having a value outside of the preceding range. The capacitor C is connected between the source terminal of the high side FET 12 and ground. The capacitor C also provides a secondary filtering function of reduced conducted emissions due to solenoid operation. While one capacitor C is shown in
The circuit 20 also includes a controller 22 that typically includes a microprocessor (not shown) and an algorithm. The microprocessor is responsive to the algorithm to generate signals for controlling the circuit. In a manner similar to
The operation of the circuit shown in
A relatively large leakage current of 150 mA, which is far greater than a typical leakage current, would typically not be enough current to cause movement of a solenoid valve armature. However, this amount of leakage current would cause the voltage to decay 40 times more quickly than normal. The difference in the voltage decay rate could be observed on all N+1 of the voltage feedbacks which include N low side solenoid FET's T1 through TN and the one high side FET 12, within a few milliseconds. Since the detection can occur within one software loop, which is typically 7 ms, the check can be performed without impacting availability of solenoids in the event that they need to be activated during the next software loop.
A series of curves shown in
The curve labeled 24 represents an expected nominal Voltage (Voltage(nom)) decay without the presence of any large leakage currents. Thus, the Voltage(nom) decay curve 24 assumes nominal resistor values and no FET leakage;
The curve labeled 25 represents an expected minimum voltage decay (Voltage(min)) with the presence of a large leakage currents. The Voltage(min) decay curve 25 assumes nominal resistors and max FET leakage of about 100 uA each;
The curve labeled 26 represents an expected voltage decay (Voltage(det)) that assumes leakage current is 10 time the nominal leakage current and 6.8 times the minimum. Thus, the Voltage (det) curve 26 represents the desired detection threshold:
The curve labeled 28 represent an expected Voltage(fail) decay assumes leakage current that is much less than what could cause an undesirable response of unwanted movement by a solenoid armature, typically 150 mA: and
The remaining voltage traces labeled 24′, 25 and 28′ show the impact of a maximum software delay of 1 ms between request a check of the feedback voltages and a disabling of the controller output upon the above unprimed voltage decay curves.
It can be seen from
The method of detection described above includes an algorithm that is illustrated by the flow chart shown in
Upon reaching functional block 42, the algorithm enters a second subroutine for timing the start of the test. In functional block 42, a timing index TIME is set equal to an initial time T1, which may be selected as any value, including zero. Also, in functional block 42, the high side FET 12 is changed to a non-conducting state. This change may occur before, after, of when the initial time is set. The subroutine then advances to decision block 44 where the timing index TIME is compared to a timer threshold Tt. For the example described above in
Upon entering the third subroutine, the index N is again set to unity in functional block 48. The subroutine than advances to decision block 50 where the feedback voltage VFBN associated with the current value of the index N is compared to a voltage threshold VT. In the example described above, the voltage threshold VT was selected as approximately half of the magnitude of the supply voltage V+; however, it will be appreciated that other values may be utilized for the voltage threshold VT, such as, for example, 25 percent of the supply voltage V+ or 75 percent of the supply voltage V+. If the feedback voltage VFBN, is less than the voltage threshold VT, it is an indication of excessive leakage current through one of the FET's or due to another cause, such as, for example, a short circuit developing upon the circuit board substrate of the controller 20, and the subroutine transfers to functional block 52 where an error flag is set. The subroutine then exits through block 36.
If, in decision block 50, it is determined that the feedback voltage VFBN, is greater than or equal to the voltage threshold VT, it is an indication that any leakage current through the associated FET TN is at or below a satisfactory level and the subroutine transfers to functional block 54. In functional block 54, the subroutine increases the index N by one and then advances to decision block 56. In decision block 56 the current value of the index N is compared to the total number of FET's in the circuit 20, NMAX. If N is less than or equal to NMAX, not all of the FET leakage currents have been checked and the subroutine transfers back to decision block 50 for another iteration. If, in decision block 56, N is greater than NMAX, the leakage currents of all of the FET's have been checked and it has been determined that all are at or below a satisfactory level. Accordingly, the algorithm leaves the third subroutine by transferring to functional block 58. In functional block 58, which is optional, the circuit 20 is deemed to be operational and a corresponding flag is set. The algorithm then exits through block 36. It will be appreciated that, for the example circuit shown in
It will be understood that the algorithm illustrated in
While the preferred embodiment has been described and illustrated for FET's, it will be appreciated that the invention also may be practiced with other electronic switching devices, such as, for example, bipolar transistors. Additionally, while brake control systems typically place the solenoid coils within an Electronic Control Unit (ECU), the present invention contemplates that the capacitor C may located either within or outside of the ECU. Furthermore, it is contemplated that the invention may be utilized to detect leakage currents through any coil that is switched on and off by an electronic switch and is not limited to solenoid coils, as described above. Finally, it is contemplated that the test may be implemented to read multiple samples of each feedback voltage with the samples spaced apart by a predetermined time period (not shown). The voltage difference between the samples would then be used to determine a time rate of change of the voltage. The voltage rate of change would then be compared to a rate of change threshold as the criteria for setting an error flag.
In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.
