High-Speed Control of Hydraulic Die-Casting Machine Using NI CompactRIO and LabVIEW FPGA
"With this project, we were able to go from the prototyping phase to the final setup of the machine in only three weeks. "
- Paolo Catterina, EUROelectronics, srl
Developing a closed-loop control system for a hydraulic cylinder that is both cost-effective and highly reliable.
Using the high-speed performance of NI CompactRIO to develop a rugged system architecture that meets the rigorous requirements of our control system.
Paolo Catterina - EUROelectronics, srl
EUROelectronics is a machine builder that was asked to design a closed-loop hydraulic cylinder control system for a die-casting press machine. The high-speed press moves anywhere from 0 to 10 m/s and therefore requires a high-speed control system. To meet this challenge, we relied on the NI LabVIEW FPGA Module and CompactRIO hardware. With the integrated FPGA on the CompactRIO controller, we could develop a system capable of low-level customization using commercially available tools. To meet the unique requirements of the application, we 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 proportional, integral, and derivative (PID) algorithm. The pressure sensor feedback is in the order of a few milliseconds, which was fully matched by the processing speed of CompactRIO.
The motion profile and other parameters are shared via Ethernet, with a PC running a complete supervisory software application for the machine. We implemented this operator interface with NI LabWindows™/CVI software. This application also provides diagnostic and machine monitoring features by measuring motion quality and repeatability, performing industrial data acquisition measurements of the process variables (positions, speeds, pressures, temperatures), and reporting statistics for quality certification.
Position and pressure control of a hydraulic cylinder is a common application in the industrial automation field, but the precision control of such systems has traditionally presented significant challenges because of their high speeds and pressures.
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 m/s. For reliable control of cylinder braking and acceleration, the closing of the loop must perform at 1 kHz processing rates.
The first thing to consider when choosing an acquisition system is the quality of sensors required for pressure measurement and position. In this case, we used linear magnetic stripe sensors for position measurement. Sensors must guarantee precision and reliability measurements, and the signal processing interface for the sensor signals must be fast and rugged.
The analog input modules available with CompactRIO have excellent accuracy and precision, and we were able to fully implement the encoder function for cylinder position control using just two high-speed digital inputs. Because of the FPGA, the signals for encoding positions have been treated 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.
The cylinder movement must precisely follow the position, speed, and acceleration profiles that the supervisory software predetermined. In a process cycle faster than 1 ms, 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 at the back of the cylinder are simultaneously controlled in order to avoid instantaneous peaks. The complete interface with the machine is constantly and perfectly held by a CompactRIO real-time program, serving in the place of a traditional programmable logic controller.
The ability in this application to effectively close the loop over the hydraulic servo valve is possible only if the processing cycle time is absolutely deterministic. Likewise, the hydraulic circuit must respond quickly, precisely, and repetitively. In this case, the hydraulic servo valve was controlled through an analog output signal.
The precise tuning of the PID algorithm was obtained through the calculation of a linearization “table” of the response values for the valve, which has unique nonlinear behaviour. With this PID gain scheduling method, we could obtain very accurate responses both at low velocities (from 0.05 to 0.30 m/s during the start movement of the cylinder) and at high velocities (the real maximum speed reached is 7.5 m/s).
Using feed-forward and smoothing techniques on the command signal, we also were able to calibrate the PID algorithm so that in rapid commutation points (the positions at which the cylinder must increase or decrease as quickly as possible) the risk for movement instability was solved.
With the CompactRIO Ethernet port, the embedded LabVIEW system is able to communicate with the supervising application developed using LabWindows/CVI. An operator can define the injection profile for the cylinder in two different ways with the supervisory software: 1) by inputting numerical values, or 2) by drawing the profiles interactively using a graphical procedure. The operator also can set various parameters required to perform the machine cycle, including position, velocity, pressure, and time.
Using a National Instruments PCI-6025E data acquisition board, we also added a number of diagnostic signals to the supervisory software such as the position, pressure, and temperature profiles for each injection. The machine-monitoring software provides plots of the machine operation and calculates various control values such as position of speed commutation, mean and peak velocities, times, pressures, and temperatures.
Programming CompactRIO with LabVIEW FPGA made it easy to implement the most critical parts of the motion control system with very high-speed processing. We could implement command and feedback control without resorting to low-level development tools, custom circuitry, or programming languages other than LabVIEW. The performance of CompactRIO gave us the ability to encapsulate the system into a small, complete, and rugged device. We were able to achieve significant improvements in process yield thanks to the advanced control capabilities that system enabled us to implement.
Project and development time required for this application were significantly reduced thanks to this integrated approach for homogeneous development using National Instruments software and hardware. With this project, we were able to go from the prototyping phase to the final setup of the machine in only three weeks.
The mark LabWindows is used under a license from Microsoft Corporation.
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