April/May 2019

APPLICATIONS spring) and a high static to dynamic friction ratio. In these applications, the force builds up across the piston until the static friction force is overcome and the piston starts to move. When the piston moves, the force across the piston falls below the dynamic friction force and the piston stops. Figure 2 contains the motion plot of an actively damped system running an I-PD control algorithm. Any system that suffers from this stiction or chatter when running in open-loop mode will also do the same in closed-loop mode. Borderline systems also tend to exhibit the same stiction action when in closed-loop mode. The solution is to limit the acceleration, or the rate-of-change of the force on the piston. This is problematic as calculating the instantaneous acceleration from position transducers is usually not feasible. Instead, some method of obtaining to say that a well-tuned I-PD system effectively turns the controller, actuator and load into a multiple-pole low pass filter. Of course, since the target position is not followed precisely when using filters or the I-PD algorithm, it limits the precision of profile tracking or synchronisation. So, while this algorithm is not suitable for every application, I-PD should be considered when the target positions, velocities or accelerations are not smooth. Active damping Active damping includes several methods of using feedback and a controller to electronically remove unwanted motion or oscillations. Active damping is normally required on systems that have a low natural frequency (i.e. they can be modeled like a mass on the end of a acceleration feedback is needed. The most direct way to do this is to attach an accelerometer to the carriage or actuator that is moving the load, but this can put the accelerometer in a nasty environment, which may not be practical. Alternatively, the differential force across the piston can be used to estimate the acceleration. This requires a controller with the necessary analogue inputs to connect to the two pressure sensors and the ability to calculate the differential force on-the-fly. This technique is not as accurate as the accelerometer approach but is commonly used and very effective at solving the stiction or chattering problem described previously. Active damping reduces the rate of force buildup across the piston and works best where the objective is to get from one point to another as smoothly as possible. Of course, active damping also limits the maximum acceleration and deceleration, which in turn limits the ability to follow a motion profile. If this is the primary objective, then larger diameter cylinders should be considered in order to increase the natural frequency, which reduces the effects of any stiction. Another method of setting up an active damping control algorithm is to use a model-based control system that can internally estimate accurate accelerations. The advantage is that no extra hardware is required. The disadvantage is that the system must be relatively linear so that an accurate model can be developed. This explains why non-linear valve spools and pneumatic systems do not model well. For best performance, it is better to design a hydraulic motion axis to have a high natural frequency and a linear response. But when extremely large masses must be positioned, at some point the extra expense of making the system hydraulically ‘stiff’ may become too great and electronic means are required to dampen the system. Pressure/force control Since fluid power is so well suited for applying pressure, a mention of pressure/force control is valuable. Today, pressure/force control (P/F) and dual- loop position-pressure/force control (P- P/F) algorithms are often used. Other systems may only need closed-loop for P/F and use open-loop for position. In some pressure applications, position PIDs can be used for pressure/force control. Other applications may need special features for combining open-loop position and single-loop P/F PID. Dual-loop P-P/F algorithms offer more flexibility than single-loop algorithms. Since a controller cannot simultaneously fully control position and pressure, two PIDs are used, a position PID and a P/F 34 HYDRAULICS & PNEUMATICS April/May 2019 www.hpmag.co.uk Figure 2. This plot shows an actively damped system. Figure 3. Plot of a system where velocity is being controlled during a move operation until a pressure limit is reached.