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Accumulators can conserve energy, make systems easier to control, and extend a machine's useful life, making them especially important for electrohydraulic motion control systems
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One of the most important, but possibly least understood components of a hydraulic motion system is not an active component at all. It is component that saves power, makes the system easier to control, and can extend a machine's useful life — the accumulator.
For the purpose of this article we will describe an accumulator consisting of a high-strength tank with a bladder inside and ports for the tank oil and bladder gas, Figure 1. Pressurized nitrogen gas is typically used to expand the oil when under normal operating pressure. If pressure on the oil side drops, the pressurized bladder pushes oil out of the accumulator. When oil pressure increases, then oil flows into the accumulator and compresses the bladder.
Accumulators store energy
Hydraulic systems can have a big advantage over servo motors in systems with varying loads. Although each electric actuator motor in an electromechanical system must be sized for its peak load, a hydraulic power unit (motor and pump) in an electrohydraulic system can be sized for the average power required of all of the hydraulic actuators over a machine cycle. To take advantage of this power averaging, accumulators are used to store energy when the energy demands are less than average and transmit energy back into the system to satisfy peak demands. The accumulator capacity must be high enough so the pump need not respond to quick changes in the demand for oil.
Keeping supply pressure constant
An ideal power unit would be able to supply an infinite amount of oil without allowing supply pressure to drop. Keeping the supply pressure constant is important because it keeps the gain of the hydraulic system constant. The hydraulic system gain determines how fast the hydraulic actuator moves when a control valve fully opens.
Once a hydraulic system is designed, the system pressure and the load are the major performance variables. For a fixed load, the system pressure directly affects the hydraulic gain. For example, a 10% drop in system pressure will cause a 5% decrease in actuator velocity. To make up for the drop in the hydraulic system gain, the controller must be able to open the valve 5% more. However, once the valve is fully open, the controller cannot compensate for the lower hydraulic system gain. When this occurs, either the requested speeds must be reduced or changes to the system must be made for the controller to maintain precise control.
One of the easiest ways to avoid a costly major redesign of the hydraulic system is to keep the supply pressure relatively constant by designing in more accumulators.
Accumulators as shock absorbers
In some applications, the hydraulic cylinders are stationary and some load tends to retract their piston rods. An example is rollers used for crushing rock. When a large rock enters the crusher, a roller must back off very quickly to relieve the mechanical strain. In these applications, the controller can do little to relieve the pressure because the valves cannot react quickly enough to relieve the pressure. Even a pressure relief valve may not react quickly enough to avoid jamming or causing damage to the system.
In this application, the amount of oil that must be relieved is usually just a matter of a few cubic inches. An effective technique is to place a small accumulator between the valve and the cylinder. Normally, this is not a good practice — especially with servo systems — because the accumulator increases compliance of the system. However, if the accumulator is precharged to a pressure above the system pressure then it will effectively be out of the circuit and have no effect on the motion control under normal conditions.
With these shock absorbing accumulators in place, when an object pushes the cylinder's piston rod back faster than the motion controller and valve are capable of responding, the pressure will increase above the system pressure and the shock absorbing accumulators will start to fill. The action of pushing back to cushion the shock will temporarily generate an error between actual and target actuator position, which will be reduced as the pressure returns to the normal system pressure when the obstruction passes or is removed and as the motion controller acts to reestablish the actuator position to match the target.
The hydraulic cylinders typically only need to retract a few inches in order to cushion a shock, so not much oil will be flowing into the accumulators. For example, a 3.25-in. bore cylinder that is pushed back 1 in. displaces only 8.3 in.3 of volume. Even if the accumulator size is only 1/2 gal, the pressure would increase by less than 8%. This is very effective shock absorption.