June 2019

www.hpmag.co.uk HYDRAULICS & PNEUMATICS June 2019 41 solved the problem by increasing the gain of the acceleration component of the control loop algorithm (the ‘D Diff’ gain in Figure 3), which gave increased fluid flow to the cylinder to provide extra energy to the motion. In this case, the second derivative gain is applied to the error between the target and actual cylinder acceleration values, overcoming the effects of stick-slip and compensating for the too-small cylinders. Another technique for improving the performance of lagging hydraulic motion systems is to use feed forwards, predictive terms that are incorporated in the closed-loop control algorithm (the ‘FF’ gains in Figure 3). Feed forwards for velocity and acceleration can ‘look forward’ and predict the required oil flow needed to achieve the desired velocity or would occur if the PID factors were used alone. Using the PID terms in this manner focuses that part of the algorithm to respond to environmental conditions such as temperature changes that can vary from production cycle to production cycle. But what happens if fast-changing feedback levels cause the motion controller to change its outputs too quickly, a phenomenon that could be a result of noisy transducer connections. Figure 3 shows the inclusion of an output filter between the output of the control loop and the connection to the valve. In the case of Delta Computer Systems’ motion controllers, support for output filtering is provided as a standard programming function. Good tools help with diagnosing and fixing problems When selecting a motion controller, designers should look for a product that is supported with tools to simplify the development and tuning of motion systems, as well as diagnostic tools that can help isolate system problems. Delta Computer Systems’ Plot Manager, a feature of the company’s RMCTools support package, can be used to monitor how the actual motion differs from target motion during an operational cycle. It was by using the Plot Manager that the engineers identified a problem with tuning the motion of the gang saw wiggle box described above. Another Delta tool is the Tuning Wizard, which can be used to automatically generate a set of control parameter gains that will reduce the error between the target and actual motion profile (see Figure 4). Further tweaking is sometimes necessary, but the Tuning Wizard will accelerate the tuning process. In the case of the wiggle box problem, the actuator tuned up easily after changing the tuning to acceleration control using the Delta’s Tuning Wizard. The tuning process took less than 10 minutes. By choosing the correct system components, including the right motion controller and automated tools such as these, hydraulic system designers can avoid some difficult problems and gain the full potential of hydraulic power. www.deltacomputersystems.com Figure 4. Delta Computer Systems’ Tuning Wizard generates control loop gains that reduce the error between actual and target motion profiles. “A big advantage to using linear magnetostrictive displacement transducers (LMDTs) is that they always know the cylinder’s position; no homing step is required at system initiation or reset.” acceleration, respectively. These terms anticipate where the system needs to be, and reduce the load on the P, I, & D gains to match the targets. Because the effectiveness of a closed-loop control system depends on the system’s response to the error between the actual and target measurements of the system, it may take a long time for an actuator to start from a stopped state. This is because it may take a while for the P and I terms in the control algorithm to generate a strong enough output signal to cause the actuator to move. If quick motion is necessary, a feedforward component can be added that anticipates the drive required to start the motion. A good design practice is to use feedforwards to produce the bulk of the motion drive signal, with the P, I and D terms doing fine- tuning of the motion in response to smaller error factors than