GB2218288A

GB2218288A - Switch bounce discriminator for electric motor

Info

Publication number: GB2218288A
Application number: GB8810627A
Authority: GB
Inventors: Jonathan Dorking
Original assignee: Delco Chassis Overseas Corp
Current assignee: Delco Chassis Overseas Corp
Priority date: 1988-05-05
Filing date: 1988-05-05
Publication date: 1989-11-08
Also published as: GB8810627D0

Abstract

A first resistor (16) is in series with a switch (12) between first and second (Vcc) supply rails. A low pass filter (18, 20) is connected between the first resistor and an input of a first inverter (22). A diode (24) connects the output of the first inverter and an input of a second inverter (30), whose output feeds the input (14) of a control circuit. A capacitor (28) and a second resistor (26) are connected in series between the first and second supply rails with their junction (D) connected to the input of the second inverter, the time constant of the capacitor and second resistor being such as to prevent unintended actuation of the second inverter and hence the electric motor by the control circuit, due to unintended opening of the switch. For windscreen wiper motors in motor vehicles to prevent unintended parking of the motor due to erroneous opening of the park switch. <IMAGE>

Description

ELECTRIC MOTOR FILTER CIRCUIT This invention relates to a filter circuit for a control circuit for an electric motor, and in particular to a filter circuit which prevents erroneous triggering of the electric motor.

The present invention has particular application for windscreen wiper motors on motor vehicles in which the control circuit includes a park switch control for the wiper motor. A problem associated with conventional wiper motor control circuits is that the parking finger and parking plate (which constitute the park switch) may fail to maintain electrical contact throughout the full wipe cycle. These intermittent, momentary interruptions can be interpreted by the rest of the control circuit as a signal to perform some function; for example, parking of the wiper motor. This problem is compounded further by control circuits which contain multiple park switches.It is believed this problem is mainly caused by the grease in the switch thinning at sub-zero temperatures, and may also be is generated by the low currents which pass through the park switch, which cause deterioration of the surface of the switch contacts, and which lead to the interruptions in electrical contact.

It is an object of the present invention to overcome the above mentioned problem.

To this end, a filter circuit in accordance with the present invention for interfacing between a switch and a control circuit for an electric motor, comprises a first resistor connectable in series with the switch between first and second voltage levels; a low pass filter connected between the first resistor and an input of a first logic inverting means; a diode connected between an output of the first logic inverting means and an input of a second logic inverting means, an output of the second logic inverting means being connectable to an input of the control circuit; and a capacitor and a second resistor connected in series between the first and second voltage levels and having a junction therebetween connected to the input of the second logic inverting means, the time constant of the capacitor and second resistor being a predetermined amount such as to substantially prevent unintended actuation of the second logic inverting means, thereby preventing unintended actuation of the electric motor by the control circuit, due to unintended opening of the switch.

Preferably the first and/or second logic inverting means is an inverter. Alternatively the first and/or second logic inverting means may be a NAND gate, or a NOR gate or an EXCLUSIVE OR gate, or another suitable alternative.

The first voltage level is preferably ground and the second voltage level is preferably the supply voltage for the control circuit.

Preferably the low pass filter comprises a second capacitor and a third resistor.

The first resistor preferably has a predetermined value which allows a current of at least 20 mA to pass through the switch when closed.

The present invention also includes a control arrangement comprising the filter circuit described herein; and an electric motor having this control arrangement.

The present invention has particular advantage when used with windscreen wiper motors for motor vehicles in that it substantially prevents unintended parking of the motor due to erroneous opening of the park switch.

The present invention is now described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a filter circuit in accordance with a first embodiment of the-present invention for interfacing between a park switch and a control circuit for a windscreen wiper electric motor; Figures 2A to 2E are plots of voltage against time at correspondingly labelled junctions in the filter circuit of Figure 1; and Figure 3 is a filter circuit in accordance with a second embodiment of the present invention.

The filter circuit 10 shown in Figure 1 interfaces between a park switch 12 and an input 14 to a control circuit (not shown) for a windscreen wiper electric motor. The park switch 12 is actuated by the electric motor. The filter circuit 10 comprises a resistor 16 positioned between the park switch 12 and earth (first voltage level). The other side of the park switch 12 is connected to an input voltage Vcc (second voltage level). The value of the resistor 16 is such as to maintain a current through the park switch 12 at a predetermined level which is high enough to keep the contacts of the park switch substantially clean.

An R-C low pass filter comprising a resistor 18 and a capacitor 20 is connected to the junction A of the park switch 12 and resistor 16.

The junction B between the resistor 18 and capacitor 20 is connected to the input of a first logic inverting means in the form of an inverting buffer 22 with a Schmitt trigger input.

The output of the inverting buffer 22, at junction C, is connected through a diode 24 to a junction D. A resistor 26 is connected between junction D and the supply voltage Vcc, and a capacitor 28 is connected between junction D and ground. Junction D is also connected to the input of a second logic inverting means in the form of an inverting gate 30 having a Schmitt trigger input.

The output of the inverting gate 30, at junction E is connected to the input 14 of the control circuit.

The diode 24, resistor 26, capacitor 28 and inverting gate 30 behave like a monostable circuit with its input (at junction C) held reset by the output of the inverting buffer 22. As the output of the inverting buffer 22 goes from a LOW logic level to a HIGH logic level, capacitor 28 begins to charge via resistor 26.

As the HIGH logic level threshold is transcended at junction D, the output from the inverting gate 30 at junction E goes from a HIGH logic level to a LOW logic level. The time for this transition to occur is dependent on the values of resistor 26 and capacitor 28. This time constant is chosen to be such that it exceeds the maximum period expected for any illegal opening of the park switch 12.

The operation of the filter circuit 10 will now be described with reference to Figure 2. The plots of voltage against time shown in Figure 2 are labelled A to E, which correspond#to the junctions A to E of the filter circuit 10 shown in Figure 1.

When the electric motor is parked, the voltages at the junctions A to E are as shown at time to with junctions A, B and E at LOW logic level, and junctions C and D at HIGH logic level. When the electric motor is started at time tl, the park switch 12 is closed, causing junction A to go to a HIGH logic level. Junction B will follow junction A to a HIGH logic level by way of the R-C low pass filter (resistor 18/capacitor 20). Inverting buffer 22 inverts the signal to a LOW logic level at junction C, allowing capacitor 28 to rapidly discharge by way of diode 24. Junction D follows junction C to a LOW logic level and the signal is inverted to a HIGH logic level at junction E at time t2 by way of inverting gate 30.Under ideal conditions, junction A will remain at its HIGH logic level throughout the operation of the electric motor, until the electric motor is stopped, at which point the park switch 12 opens.

If, during operation of the electric motor, the park switch 12 momentarily opens, and electrical connection across the contacts is temporarily broken, the signal at junction A will immediately fall to a LOW logic level and then return to a HIGH logic level as indicated at point 32 in Figure 2. Subsequently junction B will follow junction A, and junction C will be inverted to a HIGH logic level and then returned to a LOW logic level by way of inverting buffer 22 as shown at point 34. With a conventional control circuit, such signals might easily be interpreted by the control circuit as a command to park the electric motor, thus causing erroneous operation. Such erroneous operation is prevented by the monostable operation of the diode 24, resistor 26, capacitor 28 and inverting gate 30 of the present invention.

With the situation described above of signal 34 at junction C, the diode 24 will be reverse biased allowing capacitor 28 to begin charging via resistor 26. However, with the time constant of resistor 26/capacitor 28 being chosen to be greater than the maximum period expected for any illegal opening of the park switch 12, the capacitor 28 will not fully charge before the contacts of park switch 12 close again. The closing of the park switch 12 will result in junction C returning to a LOW logic level, allowing capacitor 28 to rapidly-discharge through diode 24. The signal at junction D will be as shown at point 36. That is, junction D will not reach the HIGH logic level threshold required for triggering inverting gate 30. The signal at junction E, and hence at the input 14 to the control circuit, therefore remain unchanged, preventing erroneous operation of the electric motor.

Upon completion of the operation of the electric motor, the park switch 12 is opened causing junction A to go to a LOW logic level (at time t3).

Junction B follows junction A, and inverting buffer 22 inverts the signal to a HIGH logic level at junction C. With diode 24 reverse biased, capacitor 28 charges by way of resistor 26. As the park switch 12 is genuinely open, the capacitor 28 fully charges, such that junction D rises to a HIGH logic level which is above the threshold input level for the inverting gate 30. The signal at junction E consequently goes to a LOW logic level at time t4, signalling a genuine park signal to the control circuit.

The time constant of the resistor 26/ capacitor 28 generates a time delay tD between opening of the park switch 12 and junction E going to a LOW logic level. In practice, tD can, if necessary, be compensated for by suitable adjustment of the park switch 12 parking plate.

Suitable values for the various components are Vcc = 12 volts; resistor 16 = 470 ohms; resistor 18 = 100k ohms; capacitor 20 = 0.1 microfarads; resistor 26 = 150k ohms; and capacitor 28 = 0.22 microfarads. This allows a current of approximately 26 mA to pass-through the park switch when closed; and provides a time constant of approximately 25 ms (which compares to a usual maximum of 10 ms for an erroneous opening time of the park switch 12).

Several alternative arrangements of filter circuit may be used without departing from the scope of the present invention. One example is shown in Figure 3, in which the park switch 12 is connected to ground. In this case, resistor 16 is positioned between junction A and supply voltage Vcc, the positions of resistor 26 and capacitor 28 are reversed, and diode 24 is reversed. The operation of the filter circuit is unchanged, except that the signals at junctions A to E are the reverse of those in Figure 1.

Similarly, inverting buffer 22 and inverting gate 30 can be replaced by any other devices which provide an inverting function, such as, for example, NAND gates.

Claims

1. A filter circuit for interfacing between a switch and a control circuit for an electric motor, the filter circuit comprising a first resistor connectable in series with the switch between first and second voltage levels; a low pass filter connected between the first resistor and an input of a first logic inverting means; a diode connected between an output of the first logic inverting means and an input of a second logic inverting means, an output of the second logic inverting means being connectable to an input of the control circuit; and a capacitor and a second resistor connected in series between the first and second voltage levels and having a junction therebetween connected to the input of the second logic inverting means, the time constant of the capacitor and second resistor being a predetermined amount such as to substantially prevent unintended actuation of the second logic inverting means, thereby preventing unintended actuation of the electric motor by the control circuit, due to unintended opening of the switch.

2. A filter circuit as claimed in Claim 1, wherein the first and/or second logic inverting means is an inverter.

3. A filter circuit as claimed in Claim 1, wherein the first and/or second logic inverting means is a NAND gate, or a NOR gate, or an EXCLUSIVE OR gate.

4. A filter circuit as claimed in any one of Claims 1 to 3, wherein the first voltage level is ground and the second voltage level is the supply voltage for the control circuit.

5. A filter circuit as claimed in any one of Claims 1 to 4, wherein the low pass filter comprises a second capacitor and a third resistor.

6. A filter circuit as claimed in any one of Claims 1 to 5, wherein the first resistor has a predetermined value to allow a current of at least 20 mA to pass through the switch when closed.

7. A filter circuit subsantially as hereinbefore described with reference to, and as shown in, Figures 1 and 2, or Figure 3, of the accompanying drawings.

8. A control arrangement for an electric motor comprising a filter circuit as claimed in any one of Claims 1 to 7 connected between an input to a control circuit for the electric motor and a switch associated with the input.

9. An electric motor having a control arrangement as claimed in Claim 8.

10. An electric motor as claimed in Clain 9, wherein the electric motor is a windscreen wiper motor for a motor vehicle, and the switch is a park switch actuated by the electric motor.