GB2316983A - Protection circuit inhibiting gearchage unless brake is applied - Google Patents

Abstract

A circuit for the protection of an electrically-selectable vehicle transmission comprises a signal input for receiving a brake signal representing the operation of a brake and an output which enables a gearshift when the brake signal has been maintained for a predetermined delay period. By ensuring continuous application of the brake for a predetermined period, the circuit prevents damage to the transmission caused by changing gear (e.g. from forward to reverse) at excessive speed.

Description

TITLE Transmission protection circuit DESCRIPTION The invention relates to the field of electrically selectable transmissions. It has particular application to the control of transmissions in vehicles such as fork lift trucks.

Existing transmissions are vulnerable to damage if the operator of the vehicle attempts to change gear at excessive speed. For example, a sudden change from forward to reverse gear, or vice versa, while a fork lift truck is moving can cause a shaft to snap or teeth to be stripped from the crown gear of the differential. There is a need for a device to protect transmissions from such damage.

Summary of the invention The invention provides a circuit for the protection of an electrically selectable transmission, the circuit comprising: a power supply input for connecting the circuit to a source of electrical power; a signal input for receiving a brake signal representing the operation of a brake; an output connected through a switching means to the power supply input for selectively transmitting power to the electrically selectable transmission; and control means connected to the signal input for enabling the switching means to transmit power to the output when the brake signal has been maintained for a predetermined delay period.

The circuit of the invention thus ensures that the brake is applied for a predetermined period, such as 5 seconds, before the electrically selectable transmission can be activated so that the vehicle in which the transmission is located is not moving excessively fast when the gear change takes effect. Damage may thus be avoided.

In a preferred a circuit according to the invention, the delay period is determined by the charging rate of a capacitor through a resistor, which may be a variable resistor. The voltage across the capacitor is compared with a reference voltage by a differential amplifier to generate a high or low amplifier output signal, which controls the switching of a transistor, the transistor in turn providing a control signal to the switching means.

This arrangement provides a cheap and simple device that is easy to manufacture and quick to install in new or existing transmission control circuits. The variable resistor allows a suitable delay period to be selected, for example between 5 and 15 seconds.

The invention may be embodied using alternative circuit elements such as timers to achieve the desired effect.

Preferably once the switching means has been enabled to transmit power, it can continue to transmit power after the control signal has been removed. A suitable such switching means is a thyristor. This allows the transmission, once activated, to remain activated after removal of the brake signal until there is a further gear change.

Description of a preferred embodiment The Figure is a schematic diagram of a circuit according to a preferred embodiment of the invention This circuit is suitable for controlling operation of the electrically selectable transmission of a vehicle such as a fork lift truck. The circuit illustrated has been found to solve the problems associated with the prior art, using only a small number of components. It may easily be installed in the power supply line of an existing transmission control circuit.

Power for the circuit is provided at terminals 2,3 from a power supply (not shown) such as a single-ended 12 volt supply. An input terminal 4 receives a trigger signal representing operation of the vehicle brake. An output terminal 5 is connected to the direction switches of the electrically selectable vehicle transmission.

An operational amplifier IC1 acts as an open loop differential voltage comparator. The supply terminals of the op amp IC1 are respectively connected to the circuit supply terminals, with a bypass capacitor C2 connected between them to remove fluctuations in the supply voltage.

The inverting input terminal of the op amp IC1 is held at a reference voltage equal to half the supply voltage, the reference voltage being provided by a potential divider that comprises equal resistances R1,R2 connected in series between the positive and negative supply voltage terminals.

A charging capacitor C1 has a first terminal connected to the negative supply voltage and a second terminal 6 connected through resistor R6 to the non-inverting input of the op amp IC1. The second terminal 6 of capacitor C1 is also connected through variable resistor VR1 to the input terminal 4. A first diode D1 is connected in parallel with the variable resistor VR1 and a second diode D2 is connected between the capacitor terminal 6 and the positive voltage supply.

The output of the op amp IC1 is connected through a resistor R3 and a diode D3 to the base of a transistor TR1. The collector of transistor TR1 is connected to the positive supply voltage and the emitter 7 of transistor TR1 is connected through resistor R4 to the negative supply voltage. The emitter 7 of transistor TR1 is also connected through resistor R5 and diode D4 to the gate of a thyristor TH1. The anode of the thyristor TH1 is connected to the positive supply voltage and the cathode of the thyristor TH1 is connected to the circuit output terminal 5. A light emitting diode LD2 is optionally connected between the output terminal 5 and the negative supply voltage.

The operation of the circuit will now be described.

Initially the capacitor C1 is not charged so the non-inverting input of the op amp IC1 is at a low voltage and the output of the op amp is driven to negative saturation. The transistor TR1 is therefore switched off and the gate of the thyristor TH1 is held at a low voltage so the thyristor cannot become conductive. There is no output from the circuit at the output terminal 5 and the electrically selectable transmission of the vehicle is disabled.

A positive voltage signal applied to the input terminal 4 indicates that the vehicle brake is being operated. This signal may derive, for example, from a brake light switch or from a pressure switch in the hydraulic system of the brake.

When a positive input voltage is applied to the input terminal 4, the capacitor C1 begins to charge and the voltage at its second terminal 6 and at the non-inverting input of the op amp IC1 gradually increases. The voltage at the capacitor terminal 6 is prevented from exceeding the supply voltage by the reverse-biased diode D2. After a time determined by the time constant of resistance VR1 and capacitance C1, the voltage at the non-inverting input will exceed the reference voltage at the inverting input determined by the potential divider R1,R2. The output of the op amp IC1 is then driven to positive saturation and a positive voltage is transmitted to the base of transistor TR1, switching on the transistor TRl.

When the transistor TR1 is switched on, current flows through bias resistor R4 and the voltage at the emitter 7 of the transistor TR1 becomes positive. The positive voltage is transmitted through resistor R5 and diode D4 to the gate of the thyristor TH1, switching on the thyristor to allow current to flow to the output terminal 5, thus enabling the electrically selectable transmission. The light emitting diode LD2, if installed, will light to indicate that the transmission is enabled.

When the positive input voltage at input terminal 4 is removed, the capacitor C1 discharges through diode D1 and the voltage at the non-inverting input of the op amp IC1 becomes low. The op amp output is accordingly driven low and the transistor TR1 is switched off so that the positive voltage is removed from the gate of the thyristor THl. However, the thyristor continues to conduct as long as a load is connected to the output terminal 5, drawing current. When a gear change is selected by the vehicle operator, the load is briefly removed from the output terminal 5. The thyristor TH1 switches off and will not conduct again until a positive voltage at input terminal 4 is maintained for a sufficiently long time to trigger the switching of amplifier IC1 again. Thus the gear change cannot take effect until the vehicle brake has been applied for the predetermined time.

The length of time for which the brake must be operated depends on the time constant of variable resistor VR1 and capacitor C1. The variable resistor may be chosen to give a choice of time periods between about 5 and 15 seconds.

Examples of suitable values for the other components are listed below.

Resistors Semiconductors R1 100kn 0.25W IC1 741 op amp R2 100kin 0.25W D1 IN4148 diode R3 10kQ 0.25W D2 IN4002 diode R4 100kin 0.25W D3 IN4148 diode R5 390 Q 0.25W D4 IN4002 diode R6 1MQ 0.25W TR1 TIP121 transistor TH1 BT152 thyristor Capacitors C1 47 F 50V electrolytic C2 0.01 F 100V ceramic

Claims (8)

1. CLAIMS 1. A circuit for the protection of an electrically selectable transmission, the circuit comprising: a power supply input for connecting the circuit to a source of electrical power; a signal input for receiving a brake signal representing the operation of a brake; an output connected through a switching means to the power supply input for selectively transmitting power to the electrically selectable transmission; and control means connected to the signal input for enabling the switching means to transmit power to the output when the brake signal has been maintained for a predetermined delay period.
2. 2. A circuit according to claim 1, wherein the delay period is determined by the charging rate of a capacitor through a resistor.
3. 3. A circuit according to claim 2, wherein the resistor is a variable resistor.
4. 4. A circuit according to claim 2 or claim 3, wherein the voltage across the capacitor is compared with a reference voltage by a differential amplifier to generate a high or low amplifier output signal.
5. 5. A circuit according to claim 4, wherein the amplifier output signal controls the switching of a transistor, the transistor in turn providing a control signal to the switching means.
6. 6. A circuit according to claim 5, wherein once the switching means has been enabled to transmit power, it can continue to transmit power after the control signal has been removed.
7. 7. A circuit according to any preceding claim, wherein the switching means is a thyristor.
8. 8. A circuit for the protection of an electrically selectable transmission substantially as described herein with reference to the drawing.