Control using electrohydraulic proportional or servo valves is usually achieved by using a power unit that maintains a constant inlet pressure to the control valve. Such a design strategy offers several advantages and raises several issues:
- With a high-pressure supply, hydraulic power is available immediately when a signal is given to open the valve. That is, there is no need to wait for pump pressure to build before flow occurs and the load moves.
- Conventional servo and proportional valves can be used because nearly all have a closed-center center-spool position.
- Because the valves are closed center, a pressure-regulating supply must be used. Pressure-compensated pumps are a frequent choice for regulating pressure.
The greatest accuracy is achieved by regulating the valve supply pressure to the lowest practical tolerance. That is, supply pressure is kept constant, regardless of the variation in flow demand by the load.
A pressure-compensated pump may be able to supply hydraulic power at a relatively constant pressure in the face of flow demand changes that can suddenly increase from zero to maximum, and vice versa. Then again, it may not. Depending on the circumstances, an accumulator may be used to supplement pump flow during brief periods of peak flow demands. But often, the pressure-compensated pump by itself will suffice.
This begs the question: Do any simple rules of thumb exist to help circuit designers determine the need for an accumulator? The short answer is yes, and my immediate aim, here, is to explain how it works. I'll introduce a sample system to explain the process.