Tow truck with underlift control

Abstract

A vehicle includes a chassis and an underlift supported by the chassis. The underlift has a boom and a vehicle engaging device coupled to the boom. The vehicle also includes an operator control coupled to the underlift that allows the operator to command the boom to lift and tilt to various positions and a control system that restricts movement of the boom when the boom reaches a predetermined position as the boom is being lowered to a loading position. The control system allows the operator to move the boom after the predetermined position is reached such that the boom may continue movement in a downward direction.

1 . A vehicle comprising:
 a chassis;   an underlift supported by the chassis, the underlift having a boom and a vehicle engaging device coupled to the boom;   an operator control coupled to the underlift that allows an operator to command the boom to move to various positions; and   a control system that restricts movement of the boom when the boom reaches a predetermined position as the boom is being lowered to a loading position, the control system allowing the operator to move the boom after the predetermined position is reached such that the boom may continue movement in a downward direction.   
     
     
         2 . The vehicle of  claim 1 , wherein the control system allows the operator to tilt the boom downward after the predetermined positioned is reached. 
     
     
         3 . The vehicle of  claim 2 , wherein the control system allows the operator to tilt the boom downward to a negative tilt position after the predetermined positioned is reached. 
     
     
         4 . The vehicle of  claim 1 , further comprising at least one position sensor configured to detect the predetermined position. 
     
     
         5 . The vehicle of  claim 4 , wherein the at least one position sensor comprises at least one tilt position sensor and at least one lift position sensor. 
     
     
         6 . The vehicle of  claim 1 , wherein after the movement of the boom has been restricted, the control system is configured to wait to receive a further input from the operator control before allowing the operator to command the boom to move further downward. 
     
     
         7 . The vehicle of  claim 1 , wherein after the movement of the boom has been restricted, the control system is configured to wait until the operator has released the operator control that was causing a downward motion of the boom before allowing the operator to command the boom to move further downward. 
     
     
         8 . The vehicle of  claim 1 , wherein the control system comprises a time delay relay configured to restrict an actuating signal sent to the boom when the predetermined position is reached. 
     
     
         9 . The vehicle of  claim 1 , wherein the control system further comprising a position sensor that is a switch that breaks a movement actuating circuit between the operator control and the boom when the predetermined position is reached. 
     
     
         10 . The vehicle of  claim 9 , wherein the vehicle further comprises an operator bypass switch that recompletes the actuating circuit. 
     
     
         11 . The vehicle of  claim 9 , wherein the switch resets when the boom has moved back to or above the predetermined position from a position below the predetermined position. 
     
     
         12 . The vehicle of  claim 1 , further comprising at least one movement actuator configured to move the boom. 
     
     
         13 . The vehicle of  claim 12 , wherein the at least one movement actuator comprises at least one tilt cylinder configured to tilt the boom and at least one lift cylinder configured to lift the boom. 
     
     
         14 . The vehicle of  claim 13 , wherein the at least one tilt cylinder and the at least one lift cylinder are hydraulic cylinders. 
     
     
         15 . The vehicle of  claim 1 , wherein an underside surface of the boom is substantially parallel to a ground surface at the predetermined position. 
     
     
         16 . The vehicle of  claim 1 , wherein the operator control comprises at least one switch configured to be actuator by the operator. 
     
     
         17 . A vehicle underlift supportable by a vehicle chassis, the underlift comprising:
 a vehicle engaging device;   a movement actuator for moving the vehicle engaging device;   a control system configured to restrict actuation of the movement actuator when the vehicle engaging device moves to a predetermined position; and   an operator control coupled to the control system that allows the operator to selectively actuate the movement actuator after actuation of the movement actuator has been restricted.   
     
     
         18 . An underlift control circuit for controlling at least one underlift movement actuator, the control circuit comprising:
 an operator interface configured to apply a control signal to the control circuit;   an underlift position sensor configured to apply a position signal to the control circuit;   the control circuit configured to operate at least one underlift movement actuator in response to the control signal until the position signal changes in a predetermined way, operation of the underlift movement actuator being inhibited until there is a change in the control signal.   
     
     
         19 . The underlift control circuit of  claim 18 , wherein the underlift control circuit further comprises:
 a relay configured to complete or break the communicative circuit between the operator interface and the movement actuator; and   a silicon controlled rectifier configured to latch the relay in a circuit breaking state when the gate of the silicon controlled rectifier is pulsed by the position signal, the position signal pulsing when a default loading position has been reached.   
     
     
         20 . The underlift control circuit of  claim 19 , wherein the silicon controlled rectifier and relay are configured to remain latched in the circuit breaking state until an operator discontinues the underlift actuating signal coming from the operator interface.