Deciding whether to use discrete (on/off) control or closed-loop control requires consideration of several factors. In actuality, the choice between discrete control (sometimes called bang-bang control because of the noise that an actuator makes when it stops abruptly) and closed-loop control is not an "apples-to-apples" comparison because discrete control only specifies the output, whereas closed-loop (servo) control implies continuous feedback and continuous output control.

Factors to consider
You can use discrete valves with no feedback, with discrete limit switches, or photo eyes to provide coarse position information. Discrete valves can also be used with continuous position or velocity feedback. Continuous feedback can come from an analog feedback device, an encoder, or a linear displacement transducer (LDT). Even if you are using a proportional or servovalve, using only open-loop control with the same feedback options as with a discrete valve will suffice for many applications. Finally, high-end systems use an encoder or LDT for feedback, closing the loop with a servovalve or servo-quality proportional valve.

The available choices can be arranged with the feedback options on one axis and the control options on another (see table). Two combinations in the table do not apply because closed-loop control cannot be accomplished without continuous feedback — limit switches or photo eyes are not enough.

Feedback options
Obviously, the least expensive feedback method is to use none at all. This no-feedback control often is augmented by human operators that close the loop using their vision, reflexes, and hand-eye coordination. For automation, this method provides limited flexibility because the only positions that can be achieved with certainty are when the cylinder is fully extended or retracted. The actuator will also travel at varying speeds depending on changes in load, pressure, and even how warm the fluid is. However, this method is adequate for many applications.

Feedback and control combinations
  Control type
Feedback On-off Open-loop servo Closed-loop-servo
None Least precise Limited speed control  
Discrete Stop at position Stop at position with ramping  
Continuous Flexible stop at position Ramp to position Precise control

Discrete feedback (limit switches and photo eyes) is the next step up. This option uses either discrete valves or proportional valves to drive the hydraulics. Although it is possible to hard-wire limit switches and photo eyes directly to the valves or use mechanical relays, the tradeoff is a very inflexible system. Changing the wiring can be expensive, and the locations where the limit switches and photo eyes can be mounted may change with different manufacturing needs. Also, it can be difficult to mount limit switches and photo eyes where they need to be. Therefore, to provide the needed flexibility, computers or programmable controllers normally are placed between the detectors and the valves. Because of this, a large cost that isn't immediately obvious — but which must be considered — is the cost of programming.

The most flexible form of feedback is one that provides continuous position feedback. Magnetostrictive LDTs mounted inside or alongside cylinders provide excellent position feedback. With continuous feedback, velocity can be determined by detecting how fast position changes. This information is critical in applications where motion must be accurate and repeatable.

Output control options
Open-loop control with on/off valves — Using discrete valves is the simplest, but most limited way to control hydraulic flow. The ability to control or synchronize motion is difficult. However, you can use multiple discrete valves for a rough control of speed.

For years, systems have used high-and low-flow valves mounted in parallel. Decreasing the flow rate of oil to decrease the speed of an actuator required shutting off the high-flow valve as the actuator approached the set point. This left the low-flow valve on, causing the actuator to creep close to the set point when it, too, was shut off.

This high-speed/low-speed valve approach achieves relatively good accuracy with little complication except for the feedback device. On/off valves can be used with limit switches or continuous feedback devices depending on the requirements.

Open-loop control with proportional valves — Accuracy and throughput can be improved by replacing high-and low-speed discrete valves with a proportional valve. Even simple systems can run in open loop until a ramping zone is reached. This occurs when the actuator nears a target set point, so output tapers down as a function of the distance remaining. This is not strictly closed-loop control because the error (the distance between actual and target positions) is not being used as feedback. However, it can be an effective means of control if loads are relatively constant.

Proportional or servovalves can be used in either open-or closedloop mode. Open-loop control sometimes is all that is required — for example, if a process is repeatable enough, you can be fairly sure a given output will result in a desired speed. This is easy to implement because you can use a simpler controller or PLC with an analog output.

An analog output from the PLC or computer ranging from 10 to +10 V can cause a valve spool to shift continuously as the voltage to the valve's solenoid changes. This allows rough control of the flow and, therefore, the speed — as long as variables such as the pressure across the valve and the load remain constant.

A slightly more sophisticated variation is to use feedback not to close the loop, but to change the output as a function of distance. Ramping of the control output as a function of distance is usually accomplished using a PLC or a basic motion controller. This form of control works well in applications where accuracy is not as important as simply ramping down the output to accomplish smooth speed reduction. Generally, no precise tuning of the motion is required in these systems.