Don’t Go Halfway to Closed-Loop Control

Some people are still leery of closed-loop motion control when both position and pressure control are needed. Hence, they put two different valves on their system, one for position control, and one for closed-loop control of pressure/force, along with the appropriate system components to switch between the two. This results in a complicated system, requiring more plumbing and programming and maintenance than a simple one-valve closed-loop system. Closed-loop control of a single servo-quality proportional valve based on position and pressure/force is not hard to do -- if you plan ahead.

We talked about selecting valves before in previous blogs. But what about feedback sources? As any controls engineer knows, good feedback is necessary for closed-loop control. And the more precise the feedback, the more precise the control. In fluid power systems, feedback typically comes from position sensors and pressure/force sensors or load cells.

Position feedback often comes from a magnetostrictive displacement transducer (MDT), a non-contacting source of absolute positioning info (i.e., not requiring a homing step). Machine builders should mount the MDT in the hydraulic cylinder. Cylinders can be ordered so they're "MDT ready." MDTs can be ordered with Start Stop, PWM, SSI, or analog outputs (analog is not recommended for best control because of susceptibility to noise, which can significantly reduce resolution). SSI feedback (a digital serial communications protocol) is preferred, and the finer the resolution the better.

Pressure sensors should be mounted in the cylinders, in locations where fluid flow doesn't affect pressure readings. To measure the force being applied by the actuator, mount a pressure sensor at the A and B ports of the piston and compute the difference (differential pressure equals force). Sensors with response times of three to five times the loop time are also best for proper closed-loop response.

Use a suitable motion controller. Precise fluid-power control requires parameters, transducer interfaces, and algorithms not available on standard motion controllers intended for motor control. For instance, control theory says that a standard controller using a PID (proportional, integral, derivative) algorithm can't move all the closed-loop poles left, or more negative, than the natural frequency in radians times the damping factor divided by two.

In other words, dealing with the special characteristics of fluid power systems requires a motion controller that can handle the special characteristics of the fluid system. For example, some systems benefit from modeling of system behavior using a second-order equation and/or adding predictive gain terms called velocity and acceleration feed-forwards to the loop equation.

Peter Nachtwey is president of Delta Computer Systems Inc.

Peter Nachtwey has more than 30 years of experience developing hydraulic, pneumatic, electronic and vision systems for industrial applications. He graduated from Oregon State University in 1975 with a BSEE and served in the US Navy until 1980. He became president of Delta Computer Systems, Inc. in 1992. In addition to leading Delta's engineering and R&D programs, he has presented technical papers for IFPE, NFPA, FPDA and various technical conferences.

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