Patent GB-1485216-A (GB)
Title: Hydrostatic transmission controls
IPC: F16H63/40
Filed: 1975-10-30
Abstract: 1485216 Hydrostatic transmission control; variable-ratio gearing GENERAL MOTORS CORP 30 Oct 1975 [22 Nov 1974] 44920/75 Headings F2W and F2D A variable-ratio hydrostatic transmission 14, 16 is controlled by a governor circuit 60 which compares a ratio request signal (from a manual control 62) with a control signal, which controls the transmission ratio, to produce an error signal which is integrated to provide the control signal so that the control signal changes with time when the error signal is present to diminish the difference between the control signal and the ratio request signal. The control signal is supplied by a line 72 to a circuit 74 which operates a servo 80 through a circuit 78 to control the displacement of pump. 14 of the transmission, the control signal also operating the motor 16 servo 94 through circuits 88 and 92. The circuits 78 and 92 receive feedback from their respective servos 80, 84 indicative of the positions of the pump and motor adjusting means. The transmission shown is hydromechanical with a two-speed planetary gear train 20, 22 in series with the hydrostatic transmission, low ratio being selected with a brake 42 applied and a clutch 30 released and high ratio being selected with the clutch 30 engaged and the brake released. As shown, with the clutch 30 engaged the gearing has two inputs, one from pump 14 and one from the motor. The governor circuit 60 (Fig. 2, not shown) in addition to performing the above function receives signals representing throttle 64 position and engine output speed to limit the minimum engine speed as a function of throttle position and also to limit the maximum engine speed by altering the transmission ratio. A pressure sensing circuit 70 supplies a signal to the governor circuit 60 so that it limits the maximum pressure in either of the transmission main lines 18 by control of the transmission ratio. The governor circuit 60 also limits the rate of change of transmission ratio. A signal on a line 120 from a logic circuit 116 (Fig. 9, not shown) prevents change of ratio in the hydrostatic transmission while the mechanical gear ratio is being changed. The governor circuit limits the transmission ratio (and thus the vehicle speed) in reverse. The forward-reverse inhibit circuit (Fig. 8, not shown) prevents a shift between forward and reverse if the output speed is high or the transmission ratio above zero by sending a reverse attained signal to the governor circuit 60 on a line 130 only when reverse has been selected at a shift tower 102 and when the speed and ratio conditions have been met. In addition a forced downshift signal is supplied to governor circuit 60 to cause the required conditions to be met. A high and low pressure logic circuit 116 (Fig. 9, not shown) generates a signal on a line 120 to the governor circuit 60 to prevent ratio change in the hydrostatic transmission while a ratio change in the mechanical gear-box is being effected. The clutch 30 and brake 44 in the planetary gear train 20, 22 are operated by an hydraulic control 110, shown in Fig. 10, which comprises a pump 600, a neutral valve 610, a shift valve 612 and first and second trip valves 614, 616. The neutral valve 610 is controlled by a solenoid valve A which, when energized, exhausts a chamber 624 to allow the valve spool 618 to move upwards. Shift valve 612 is controlled by solenoid valves B and C. With valve A deenergized the valve 610 permits fluid pressure to be supplied to valve 612, via line 632. In the condition shown the pressure is supplied to brake 44, solenoid valve B energized and valve C de-energized. Energizing valve C and deenergizing valve B moves the spool 636 up, exhausts the brake and engages the clutch 30. The trip valves 614 and 616 control the overlap and timing of the clutch and brake operation. The solenoid valves A, B and C which control valves 610 and 612 are in turn operated by a logic circuit 104 (Fig. 7, not shown) in response to operation of the shift tower 102 and in response to a signal on a line 106 which responds to pump displacement and instructs the logic circuit 104 to effect a ratio change in the planetary gearing.