Closed Loop Hydraulics

Position and pressure control of a hydraulic cylinder is a common application, but precise control of these systems has traditionally presented significant challenges because of the high speeds and pressures. When machine builder Euro Electronics designed a closed-loop hydraulic cylinder control system for a die-casting press machine that included press moves anywhere from 0 to 10 msec, they knew it would require a high-speed control system.

"In our application, the cylinder moves over a software-defined trajectory with specific velocity and acceleration profiles that must guarantee accuracy and repeatability up to a maximum speed of 10 msec," says Paolo Catterina, an engineer for Euro Electronics. "For reliable control of cylinder braking and acceleration, the closing of the loop must perform at 1 kHz processing rates."

To meet the challenge, Euro Electronics developed a system using the LabVIEW FPGA module and CompactRIO hardware from National Instruments. Using the integrated FPGA on the controller, the company developed a system capable of low-level customization using commercially available tools. They also implemented a highly optimized encoder interface in the FPGA to measure the cylinder position while programming the system entirely in LabVIEW.

Essentially, the system controls the cylinder position to track the velocity and acceleration trajectory values the operator enters. The operator can control the cylinder movement through a PID algorithm using pressure sensor feedback that is on the order of a few milliseconds.

Catterina says the first thing to consider when choosing an acquisition system is the quality of sensors required for pressure measurement and position. In this application, they utilized linear magnetic stripe sensors for position measurement because the signal processing interface for the sensor signals must be fast and rugged.

He says they were able to fully implement the encoder function for cylinder position control using just two high-speed digital inputs. Using a FPGA, the signals for encoding positions were handled directly from the sensors. "We didn't need an intermediate processing or amplification device, and we achieved an evident reduction of noise and a correspondent increase in processing speed," he says.

A key to the system is the cylinder movement precisely following the position, speed and acceleration profiles determined by the supervisory software. In a process cycle faster than 1 msec, the valve position is measured, the speed is calculated, both are compared to the set point and the movement is corrected using a PID algorithm. To keep the hydraulic circuit balanced, pressure values in the front and back of the cylinder are simultaneously controlled in order to avoid instantaneous peaks.

The ability to effectively close the loop over the hydraulic servo valve is possible only if the processing cycle time is absolutely deterministic and the hydraulic circuit is responding quickly, precisely and repetitively. In this case, the hydraulic servo valve was controlled through an analog output signal.

Precise tuning of the PID algorithm was achieved through calculations using a linearization "table" of the response values for the valve, which has unique nonlinear characteristics. Using this PID gain scheduling method, Euro Electronics obtained very accurate responses both at low velocities (from 0.05 to 0.30 msec during the start movement of the cylinder) and at high velocities (the real maximum speed reached is 7.5 msec).

Using feed-forward and smoothing techniques on the command signal, they calibrated the PID algorithm so that in rapid commutation points movement instability was eliminated.