Machine Design

Pump Controls

To minimize wasted energy in hydraulic systems, variable-displacement pumps were developed. They reduce power loss by adjusting the relationship between pressure and flow for various power levels. Electrohydraulic control of flow and pressure directly at the pump often permits more flexibility and more efficient operation of variable-volume pumps, and reduces the number of valves in a circuit.

The basics of electronic pump control are rather straightforward. A programmable controller, PC, microprocessor, potentiometer, or other such device provides a signal to an electronic driver board, or amplifier card. This low-current signal is amplified into either a pulse-width-modulated signal sufficient to drive proportional solenoids on the pump, or an amplified current signal to drive proportional pressure controllers (PPC).

The solenoids or PPC convert current to a proportional force, providing pressure control to a piston which adjusts the ring position in a variable vane pump, or the swash plate in an axial piston pump, in turn controlling pressure and flow.

Repeatability when moving from low to intermediate pressure is typically within 1%. Linearity is generally within 3% for flow, and 4% for pressure control. Due to hysteresis when cycling up and down, pressure differential can be as much as 4%. Usually, this is not a problem, because the input signal can be adjusted to compensate for the difference. However, when precise linearity is a must, closed-loop feedback is necessary.

With closed-loop control, command inputs, amplifier card, and pump hardware remain the same. Sensors are added to the system to measure the desired parameter, along with an electronic summing card that compares input with actual output. If pressure must be controlled precisely, a pressure transducer is added to the system. For flow control, an LVDT can be connected to the cam ring or swash plate to monitor position and, thus, flow. If the pump is driving a hydraulic motor, a tachometer can be used to sense motor speed and adjust pump flow as necessary.

The benefits to using electrohydraulic control are numerous. A major factor is the tremendous energy savings -- 40 to 50% is not uncommon -- realized by applying these controls. Accuracy is enhanced, especially with closed-loop feedback control.

A circuit can be substantially simplified by moving to electronic pump control, because valving is eliminated. For a complicated circuit, the cost of pressure and flow control valves, manifolds, connectors and other plumbing, often exceeds that of the controller and other electronics. Because the circuit is generally simplified, and control and power elements are separate entities, tracking down a problem area is usually simplified. By disconnecting the electronics and actuating the pump manually, one quickly determines if the problem is a hydraulic one. If not, the problem probably lies in the electronics. The error can then be isolated by checking inputs and outputs at the controller and boards.

Programming these controls is not very complicated, because pump outputs are directly proportional to command inputs. Programmable controllers are used most often, so programming is really just a matter of setting voltage or current, and timing. Loading a new program to the controller is all that is necessary to change operation, resulting in more flexibility and substantial setup time savings.

Because of precise control over acceleration and deceleration, cycles times can be decreased without introducing detrimental impact loads. Because the pump always operates at the lowest possible pressure, pump life can be extended dramatically, and these controls are, for the most part, adapted for retrofit onto standard pumps in the field today.

One possible drawback to electrohydraulic pump control is that there is only one pressure or flow to work with at one time. For applications that require two or more simultaneous pressure or flows, some sort of valving must be used.

This method cannot be used to control stationary position of an actuator. Acceleration to a general location is performed well by electrohydraulic pumps, but stopping a specific position requires valves.

Because the pump is controlled by pressure compensation, it will not work if system pressure falls below the pump's minimum deadhead value. If control is needed at low load pressures, either an artificial load pressure must be generated at the pump outlet, or another means of control must be used. Also, controls affect only swash plate or ring position, and do not compensate for pump inefficiency or fluid compression. Closed-loop feedback is needed should such compensation be necessary.

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