The answer to that question all depends on the customer’s application. Is the system more complicated than just turning on and off a simple solenoid valve? Does system control need to be variable depending on the situation or scenario? Does the customer need to vary velocity on the fly? Does the customer need to position a load precisely on a dime (or tighter tolerance than that)? Does the system need to provide a constant force to finish making the workpiece? These are just a few of the questions that a hydraulic system designer must ask himself when deciding on what technology is needed to achieve the perfect solution for his customer. If the hydraulic system designer answered, “yes,” to any of the questions above, there are two technologies that could save the day: open loop and closed loop hydraulics.
In the simplest form, an open loop system has an input that is not dependent on the output. For example, the motion controller/PLC sends a command signal to a proportional directional control valve and this command signal does not change based on whatever is happening downstream in the hydraulic circuit. There is no feedback coming back to the motion controller/PLC. Open loop systems are typically used for velocity type applications. One could almost compare it to using the accelerator pedal in a car. The harder the pedal is pushed, the faster the car goes. The less the pedal is pushed, the slower the car goes. The spool in a proportional directional control valve can proportionally slide open to go faster and proportionally slide closed to go slower, depending on the command signal to the valve from the motion controller/PLC.
On the contrary, a closed loop system has an input that is dependent on the output. In a system closing the loop around position, this type of system requires feedback to be able to know where the actuator is and what it is doing. The motion controller/PLC compares the commanded position to the actual position of the actuator and sends a command signal to the proportional directional control valve. The larger the difference between commanded position and actual position, the higher the error, and the more the proportional directional control valve’s spool is shifted. Once the difference between the commanded position and actual position starts to decrease, the proportional directional control valve then begins to shift the spool back towards neutral and slows the actuator. Once the actuator is in position, the error is low and the motion controller/PLC only commands the proportional directional control valve enough to keep the actuator in place.
Closed loop systems can be used in more than just positioning applications. A hydraulic systems engineer may use a closed loop system for force control and pressure control. Force control is frequently used on a hydraulic press. When the press closes on the die, force begins to build pushing on the workpiece. Too much force may crush the workpiece, but too little force may not develop the workpiece enough. The system would get feedback from a load cell in the system to determine actual force and the proportional directional control valve would then be commanded accordingly. Pressure control is similar to force control, but the system uses pressure transducers to close the loop. The motion controller/PLC monitors the pressures in the actuators and commands the proportional directional control valve accordingly. A pressure control system may be used where constant tension is required on a product and a certain amount of pressure is required in the actuators at all times to maintain this tension.
By using open and closed loop hydraulic systems, the control is greatly improved, but at the price of efficiency. There are many other design considerations that have to be accounted for with open and closed loop hydraulic systems, such as system pressure, max velocity, hose and tubing lengths, and increased system heat. Each application has its own characteristics to be reviewed. For more information on how you can utilize open and closed loop hydraulic systems in your applications, please contact us here to receive a personalized contact by an IFP Application Engineer: