Hysteresis is a common shortcoming in applying fluid power products to industrial and mobile equipment. It is frequently unreported due to a lack of a cost-effective option to address this issue. As the hydraulics industry works to enhance precision and repeatability in machines, a better understanding of hysteresis—or the variance of output flow from the input command—and how it affects fluid power may help.
What causes hysteresis?
Hysteresis is an error based on past input, a variation caused by friction and drag of various interfaces in the control loop. For hydraulic products, these interfaces are found in the servo valve, control piston, swash plate and rotating group, mechanical feedback link and the swash feedback valve. All hydraulic piston pumps have these key features, and Eaton engineers have confirmed through testing of several similar displacement closed circuit pumps with electro-proportional controls that hysteresis is common and changes with varying operating conditions.
Hysteresis is measured as a percentage of error against the pump’s peak flow, and ranges from 4 to 11% for most hydraulic pump controls on the market today. A typical hysteresis level in pumps today is 5%, which means that a 100cc pump commanded to half displacement could produce either 50cc or 55cc, depending on the previous commanded position. This variance alters the output flow unless the input command is changed to compensate to achieve the desired flow.
Hysteresis is perceived as a relative nuisance to many operators, but the impact can be quite costly in terms of time and production efficiency. With constant variance, operators must repeatedly adjust the input control position to achieve consistent machine behavior. The operator is part of the feedback loop and must slow down to understand how fast the machine is moving because hysteresis will cause the machine speed to vary in a propel circuit.
Applying software and control solutions
Though hysteresis is a commonly accepted annoyance, there are control solutions that can help to address this issue. Eaton’s solenoid control offers a predictable output flow that adjusts pump displacement independent of hysteresis. Part of Eaton’s Dynamic Machine Control solution offering, this control utilizes an algorithm in an HFX controller that takes the input command and compares it against the current displacement of the pump provided by the swash feedback sensor. The HFX controller provides current to the solenoid valves to minimize the difference between the input command (desired position) and the existing position provided by the swash feedback sensor.
The feedback sensor reads the swashplate position directly—bypassing the sources of mechanical friction and backlash—allowing the controller to dynamically adjust the current to the solenoid valves as operating conditions change. This means that the overall control system provides an accurate and repeatable output flow from the pump. Any remaining error is primarily due to the small amount of error in the swash sensor, typically less than 1%. At this level of error, the predictable flow offered by the solenoid control makes it one of the most repeatable controls on the market today.
As more and more applications combine electrical and hydraulic power to meet the performance requirements stipulated by machine builders and end users, it’s easy to lose sight of solving these everyday nuisance problems. Why focus on something like hysteresis when new electrohydraulic capabilities can monitor systems, send alerts in the event of error, and even shut down equipment in the event of failure?
Hysteresis corrections may not be as exciting, but improving predictability and repeatability can have big returns for productivity, operational efficiency and operator comfort. Sometimes even the smallest changes can make a big difference—and those changes are made possible by the latest in electrohydraulic technology.
Vincent Duray, closed circuit piston product manager, Eaton, wrote this article for Truck & Off-Highway Engineering.Continue reading »