The main reasons for using closed-loop control are flexibility, accuracy, speed, and the ability to maintain precision with changing conditions (loads). As productivity demands increase, more and more applications and processes require
more sophisticated closed-loop controllers.

At the high end of control is a combination of continuous feedback and closed-loop control. Closed-loop control compares the feedback position from the feedback device to a desired position.
This error is multiplied by a proportional gain — a ratio that converts the error in position units to output in volts or milliamps. The greater the error, the higher the output must be to correct the error. Higher-end controllers augment the proportional (P) gain with integral (I) and differential (D) gains. Figure 1 shows how the gain factors combine to implement precise closed-loop control.

Higher end controllers can also augment proportional gain with feedforwards, which are really just open-loop gains used as predictive factors in combination with closedloop control. For example, if you know the actuator will move at 2 in./sec-V, then 4 V should be applied to go 8 in./sec. Of course, this assumes that hydraulics respond in a linear manner, which they don't. However, if the feedforward term provides an output that approximates the desired value, then the PID terms can correct for any non-linearity or changes in load not predicted by the estimate. Without feedforward terms, the PID terms would have to compensate for a larger error, resulting in an increased likelihood of system lags and instability.

The combination of PID, feedforwards, continuous feedback, and a servovalve or servo-quality proportional valve allows the controller to go to positions using velocities, accelerations, and decelerations that the user can program. The programmable accelerations and decelerations reduce the wear and tear on the hydraulics and mechanical system.

For example, with presses, you can ramp down the velocity so that when the tooling hits the work piece, the press will have just the right amount of kinetic energy to do the necessary work. Figure 2 shows a system diagram of the hydraulics used in a highend press, with position feedback provided by a magnetostrictive LDT and differential pressure (force) feedback provided by two pressure sensors mounted in the cylinder. Control is provided via a proportional servovalve.

Sometimes a combination of open loop and closed loop is best. For instance, many single-axis presses do not need precise control until the tooling gets close the work piece. In this case, the actuator can be commanded to move down at a high speed using openloop control. Once the actuator gets close to contacting the material, control can be switched to closed-loop position control. Upon contact with the material, a logical decision can be made to seamlessly transition from closed-loop position control to closed-loop pressure or force control by using pressure feedback sensors.

Communications
A good controller should have good networking support in order to fully take advantage of its control capabilities. It makes no sense for a controller to be able to change set points on the fly if these set points can't be updated from some external source quickly and easily. Also, a closed-loop motion controller is not very flexible if it has only a handful of inputs that can tell it to go to only a small number of pre-programmed positions. More-advanced controllers have Ethernet, PROFIbus, or other fieldbus networks that make interfacing to other devices easier.

This allows parts manufacturing machines to take advantage of the flexibility of the controller so new set points can be down-loaded when making new part types. In some cases, new set points can be downloadedfor each unique workpiece.

Another valuable advantage of motion controllers with fast communications is diagnostics. The best can graph the desired and actual motion profiles. This greatly eases tuning and troubleshooting of the entire motion system.

Choosing between closed-loop and open-loop control ultimately depends on the requirements of the specific application. In some cases, using continuous feedback and proportional valves with open-loop control may be reasonable answer. Whichever route you choose, go with an appropriate motion controller that can be optimized for hydraulic motion control.

For more information, contact the author at peter@deltamotion.com or visit www.deltamotion.coms