Control arrangement for a hydrostatic system

A control arrangement for a hydrostatic system having a power source drivingly connected to a variable displacement pump, a variable displacement motor fluidly connected to the variable displacement pump and operative to provide output power to a work system in response to a desired input command, and a source of pressurized pilot fluid, the variable displacement pump and motor each has a displacement controller thereon, the control arrangement comprising:

• a first speed sensor operative to produce an electrical signal representative of the input speed of the variable displacement pump;
• a second speed sensor operative to produce an electrical signal representative of the output speed of the variable displacement motor;
• a first solenoid operated proportional valve operative to selectively connect the source of pressurized pilot fluid to the displacement controller of the variable displacement pump to controllably change the displacement thereof;
• a second solenoid operated proportional valve operative to selectively connect the source of pressurized pilot fluid to the displacement controller of the variable displacement motor to controllably change the displacement thereof;
• a microprocessor operative to receive the respective electrical signals from the first and second speed sensors, process the speed signals with respect to the desired input signal and known parameters, and transmit a first control signal to the first solenoid operated proportional valve to control the displacement of the variable displacement pump between a minimum and a maximum displacement and a second control signal to the second solenoid operated proportional valve to control the displacement of the variable displacement motor between a maximum displacement and a minimum displacement to obtain the desired output speed as established by the input command, the displacement of the variable displacement motor is reduced once the displacement of the variable displacement pump reaches its maximum displacement position;
• and
• a valve mechanism which, in use, selectively directs system pressure to the displacement controller of the variable displacement motor to positively maintain the variable displacement motor in its maximum displacement position during the time that the displacement of the variable displacement pump is being controlled between its minimum and maximum displacement positions.