The purpose of this paper is to help reduce the uncertainty in behavior introduced when changing hydraulic oil from mineral oil (HLP) to biodegradable oil (synthetic esters - HEES) by comparing the behavior of proportional valves with HLP and with HEES at various temperatures.

The focus of this article is on classic proportional valves used in the industry. The study is based on tests and modelling with characterization of dynamic behavior in mind. The characterization is based on tests of two pressure compensated proportional valves, one with closed loop control of the spool position, and one without. The two ester types tested are one based on a saturated, fully synthetic ester and a regular fully synthetic ester.

The tests consist of steps and frequency responses. Both valves are tested at oil temperatures 20°C, 40°C and 60°C.

The adopted models are based on a third order linear model with parameters identified using frequency responses from actual valve tests.

The variation of amplitude and bias has some influence on the resulting frequency response especially at lower temperatures. But the general tendencies are unaffected by amplitude and bias.

As expected a clear tendency for both valves of increasing dampening at decreasing temperatures is seen regardless of oil type, but the increase in dampening is similar for all oil types.

The saturated ester leads to less bandwidth at lower temperatures for both valves, but the overall variations between all oil types stay within 1.66Hz of each other when tested with the same test parameters.

The investigation indicates that the difference in dynamic characteristics at 20°C caused by the different oil types can not be explained with variations in any single one of the classic liquid properties density and viscosity and more investigations are needed to identify the cause.

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