Process and Control Today | Feed forward in motion control improves positioning accuracy

23/01/2023 Clever Movement Management (INMOCO Ltd)

Movement purposes usually require excessive accuracy all through the loop. Nonetheless, even when the PID (proportional, integral, spinoff) settings that management the servo loop are optimized, further tuning is commonly helpful to maximise management. In a movement system setup, all kinds of influencing forces come into play, however identification adopted by feed-forward counter-action can enhance machine efficiency.

INMOCO movement specialist engineer Gerard Bush explains the significance of feedforward.

Feed ahead can improve machine efficiency by including management components exterior of the servo compensation loop, offering a smoother system with higher accuracy. The extra we all know concerning the variables within the mechanics of the machine, its load and its forces within the surroundings, the higher the potential for utilizing feed ahead and optimizing the management loop.

The management loop consists of the place PID loop, which is commonly utilized in point-to-point positioning programs, or a velocity loop that features a PI (proportional, integral) filter used to manage many controls. rotating and pumping purposes. Increased efficiency place management purposes can use each. These loops then output a present loop that ensures that the precise present by the motor coils is the same as that of the command.

Why is feedforward essential?

Nonetheless, a movement management system additionally should observe the specified profile on the load. Sometimes, there’s a delay through the acceleration section and an overshoot through the deceleration section. It’s because the actual world system should Steadiness excessive accuracy with different load necessities and the necessity for steady, oscillating movement throughout desired profiles. Additionally, the essential suggestions management construction of a PID loop signifies that it has no direct course of data and subsequently operates as a response, which reduces accuracy in the actual sense.

A feed ahead enter might be added to the movement controller to cut back these place errors. This may be achieved by including a compensating worth to the PID place loop output that modifies the present (torque) loop command or velocity command. Nonetheless, the contributing elements to the feedforward phrases are exterior the servo loop and rely on the data of how the system will reply to modifications within the desired movement profile. The nearer the feedforward values ​​are to predicting the torque ranges the motor will expertise because it strikes, the decrease the demand on the servo loop and the extra correct and responsive the ensuing machine efficiency.

Load, acceleration and velocity

The best feedforward setting is to interact a continuing offset within the desired torque command, which is often vital for a gravity-affected vertical axis. A big sufficient torque command raise whereas carrying the load, the axis eliminates the necessity for a PID loop for added compensation. Compensation for friction will also be made. This may be achieved by participating the rate or acceleration proportional feedforward, multiplying the values ​​appropriately by a programmable scale issue. If servo lag is famous, applicable feedforward achieve settings might be checked till the issue is minimized. For essentially the most correct outcomes, it’s helpful to run a number of cycles of the axis and choose the typical.

Robotic configurations corresponding to six-axis articulated robotic arms have extra advanced mirrored torques in comparison with X- and Y-axis programs. and likewise It consists of elements corresponding to centripetal pressure. Because the robotic arms rotate, mirrored forces that had been beforehand fixed, corresponding to gravity, may change, so robotic buildings require further feed-forward calculations. Nonetheless, with sufficient information, these forces will also be compensated utilizing the identical rules to calculate load, velocity and acceleration feedforward.

Compatibility and friction

Compensation should even be made for the machine, together with forces performing on elements corresponding to gears, bearings, hyperlinks and different inline mechanisms. Feedforward for compatibility pertains to the impact of flexibility, i.e. balancing could also be required for a versatile arm attachment by way of a element that’s inherently appropriate, corresponding to a rubber shock absorber. Through the use of a element whose habits is thought for sure working situations, we are able to compensate for the management loop calculations to enhance dynamic servo monitoring of the particular load.

In the meantime, adhesion or preliminary friction is the tendency of the axis to want a breaking pressure to provoke motion, which is hindered by elements corresponding to rust, chemical adhesion or different micro-mechanical adhesion forces. Creating a compensation plan might be tough as a result of these forces can change over time or with environmental situations corresponding to humidity. Even so, most friction compensating schemes use direct feedforward by making use of a brief pulse when a stationary axis is about to start movement. A trial and error strategy is a typical approach for adjusting the energy and length of the influence pressure.

For instance, positional distortions from the motor itself might also require correction if it doesn’t present an ideal conversion of present move to mechanical torque. Equally for stepper motors, pulses passing by every coil immortality creates precisely equal movement increments. The motor compensation scheme requires a lookup desk that information compensation values ​​primarily based on common turns utilizing totally different motors. By mapping the torque profile of a motor over one full cycle, it’s doable to know the stabilizer motor command required.

Feed ahead in apply

Though feedforward methods can enhance machine efficiency, attaining good compensation might be tough. In the actual world, variables associated to settings, forces, machine and motor habits might be too many and complicated to exactly outline, measure, and stability.

At INMOCO, we often advise design engineers on establish and measure compensation necessities and apply feedforward. Contemplating the influence these elements have on the broader system design, our engineers often interface between an OEM and a movement management vendor corresponding to Efficiency Movement Gadgets (PMD) to acquire optimum profiles and efficiency for his or her particular software and situations.