Patent GB-1532790-A (GB)
Title: Device to enable co-ordinated driving of a towing vehicle and a towed vehicle
IPC: B62D59/04
Filed: 1975-12-17
Abstract: 1532790 Hydrostatic transmission control BOFORS AB 17 Dec 1975 [18 Dec 1974] 51719/75 Headings F2W and F2L [Also in Division B7] A towed vehicle has each driving wheel (two as described) driven by a respective separate hydrostatic transmission 19, 20 having a variable-displacement pump 21, 21<1>, a control circuit being provided for each transmission to maintain the driving torque at a wheel constant at a value set on a control device on the towing vehicle. In the embodiment shown in Fig. 2 the control device on the towing vehicle (not shown) is a variable potentiometer which supplies current to operate a valve 9, the control pressure from which pilots an amplifying valve 10. The pressure from valve 10 is compared with the respective transmission working pressure in double-acting piston devices 6, 6<1> (Fig. 3, not shown) which operate follow-up servo control valves 41, 41<1> to adjust the pumps 21, 21<1> to maintain the transmissions at a set pressure. The devices 6, 6<1> are provided with manual controls 44, 44<1> for self-propulsion of the towed vehicle. The variable potentiometer acts on valve 9 by way of an electronic cirucit (Fig. 6, not shown). The electronic circuit receives an input indicative of engine speed (the engine of the towed vehicle, driving the pumps 40, 40<1> through gearing 22) to operate the transmissions to reduce the load on the engine should its speed fall and it also receives an input indicative of vehicle speed to disengage the hydrostatic drive (by disengaging clutches 31 between hydraulic motors 30, 301 and the wheels 17, 18) if the vehicle speed exceeds a predetermined value for a certain distance. In an alternative embodiment the difference in pressures between the main and return lines of a transmission is converted to an electrical signal which is electronically compared with the desired value from the potentiometer and the resulting signal operates an electrohydraulic converter to provide a control pressure for adjusting the transmission (Fig. 7, not shown). In a further embodiment the controlling potentiometer controls a variable relief valve 100, this being shown in Fig. 11, which regulates the pressure in a line supplied by a feed pump (Fig. 10, not shown), the line including a throttle 102. Comparators 99<1>, 99<11> are acted upon by this control pressure at one end, 120<1>, 120<11> (Fig. 12, not shown), the comparators also being acted upon at their other ends 124<1>, 124<11> by main line pressure from a respective shuttle valve 123<1>, 123<11> and supplying pressure to a respective servo control valve 93<1>, 9311 to control the servos 90<1>, 90<11> and adjust the respective pump 85<1>, 85<11>. The servo control valves 93<1>, 93<11> are electrically controlled reversing valves (Fig. 9, not shown) and the servos 90<1>, 90<11> are spring biased towards neutral. This embodiment also has an electronic circuit for prevention of overload of the engine, selection of forward or reverse and drive disengagement when the vehicle speed rises above a set value. The towed vehicle is a heavy field artillery gun and has a reciprocating petrol engine driving the hydrostatic transmissions. Each hydrostatic transmission shown in Fig. 2 has a feed pump 23, 23<1> a flushing valve 29 and relief valves between the main and return lines and also in the feed pump output and the outlet from the flushing valve. Clutch control.-A hydraulic motor 30 is coupled to a driving wheel by planetary gearing 60 (Fig. 5), a brake 61 being hydraulically engaged on actuation of a valve 42b to hold the reaction member of the gearing to transmit drive, The valve 42b is manually operated but an electrically operated valve may be operated by an electronic circuit (Fig. 5, not shown) automatically to disengage the drive when the speed exceeds a predetermined value. Valve 42b is provided to disengage the drive when the transmission pump is to supply other services on the vehicle.