April/May 2019

www.hpmag.co.uk HYDRAULICS & PNEUMATICS April/May 2019 33 desired velocity and acceleration in each direction of motion. As the motion controller moves the target position from one point to another, it also generates a target velocity and acceleration. These values are then multiplied by their respective feed forward gains to generate the required output at the current velocity and acceleration. In theory, if the predictive gains are computed correctly, there should be no error. But real-world systems are seldom exactly linear and the loads on many systems change from cycle to cycle. This causes errors that the PID can correct. In general, one should be able to predict the required output within 5% of the desired goal using feed forwards, so the PID component of the closed-loop control algorithm only needs to correct the last 5%. This is much better than forcing the PID gains to do all the work. Since the controller multiplies the instantaneous velocity and acceleration by the feed forward gains to determine the feed forward contribution to the output, these values should change smoothly without discontinuity or the control output will also change in steps. Ideally, motion profiles with simple linear ramps (trapezoidal moves - see Figure 1a) should only use velocity feed forwards, since there are step changes in acceleration. Step changes in the acceleration feed forward will cause step changes in the output, which can excite higher frequencies in the system, resulting in oscillation and following errors. In order to use acceleration feed forwards to the best advantage, S-curves or some other acceleration limiting technique must be used (Figure 1b). Feed forwards should be used whenever axes must be tightly synchronised, or when precise gearing or profile tracking is needed, including applications such as flying cutoff saws and flying shears. I-PD is another form of PID Sometimes the target position is not generated by the motion controller. Instead, it may be generated by a joystick or the outer loop of another PID, or some other external source (see Figure 1c). In these cases, the target position is not guaranteed to move smoothly from one point to the next one. A PID control algorithm will try to follow the ‘noisy’ target resulting in ‘noisy’ actuator motion. In order to smooth the output to the actuator, the target position or the error can be filtered. Another technique is to use a form of PID called the I-PD. This form of the PID uses the error only for calculating the integrator term. The P and D terms use only the negative feedback from the actual position. Since the P and D terms do not depend on the error between the target position and actual position, the controller will not generate large changes in the control output in response to ‘noisy’ target signals or when there are step changes in the target position (Figure 1c and 1d). Another way of describing the result in terms familiar to control system experts is Figure 1a. Simple trapezoidal motion profiles allow the use of PID algorithms and velocity feed forwards. The abrupt edges limit the use of acceleration feed forwards, however. Figure 1d. If a motion controller must work with external target signals with step changes, filtering or I- PD algorithms should be used. Figure 1c. The same applies to external target signals with abrupt changes or ‘noise’. Filtering or I-PD algorithms are needed. Figure 1b. Adding S- curves to the trapezoidal motion profiles allows the full use of acceleration feed forwards in the control algorithm. This waveform provides smoother motion and does not provoke oscillations.