Tuning hydraulic systems can be a frustrating trial-and-error process. It is not always clear how changing one control parameter will affect the function of the system. Often, tweaks to tuning parameters can make things worse rather than better.
Most controllers employ standardized PID (proportional integral derivative) control loop gains that relate the current state of the system to the next desired state. Engineers must set the appropriate gains in order to get the system to move correctly.
Recently, controllers have added "feed-forward" parameters, which are predictive terms in the control equation. In theory, these feed-forward gains should be set to do most of the work in causing the motion. The PID "feedback" gains are then used to fine-tune the motion to compensate for system environmental conditions, changes in load, or the effects of wear and tear, that can be different from day to day or cycle to cycle. Unfortunately, the reality is that there can still be a lot of trial and error involved in getting a system up and running.
To speed up the process while setting up a robotic manipulator used for electric coating, RoboVic (Princeville, Quebec) recently employed the Tuning Wizard from Delta Computer Systems Inc. (Vancouver, WA), part of the software toolset that is provided with Delta's RMC100 motion controller family. The Tuning Wizard helps engineers match up a system's actual motion profile with the ideal target profile.
The application is one in which a large anodizing machine dips metal components into a succession of different vats. The total weight of the parts and fixturing for the machine is about 1,000 pounds. A system of hydraulic actuators needs to position the parts being coated to within a tolerance of 0.25 inch.
The Tuning Wizard starts out with graphs of both the actual and target motion profiles for an initial parameter value set. It then analyzes the differences between the two and automatically explores how different system mathematical models can reduce the error. Next, a best model is selected and optimal parameter values for P, I, D, and the acceleration and velocity feed-forwards are automatically calculated. A perfectly tuned system will have overlapping actual and target motion curves.
According to RoboVic system integrator Yannick Bertrand, using the program not only reduced the programming time to a fraction of what it had been in the past, it simplified the process to the point where he plans on having a less experienced technician do the job in the future.
"When I plugged in the parameter values that the Tuning Wizard generated and re-ran the axis and graphed the motion, to my amazement the actual and target values completely overlapped," Bertrand says. "With a single trial and one parameter adjustment, my job was done...I continued tuning until all the motion axes in the system were adjusted. For all 10 axes, it took just 15 minutes."
For more information on motion control software, call 360-254-8688, visit www.deltamotion.com or eInquiry 4.
For more information on design and systems integration, call 819-364-2022, visit www.robovic.com or eInquiry 5.
