It’s the time of year where some of us take a break from the plant to get outside and go hunting. Did you know that there may be hunting already happening inside the plant? If your machine has a servo motor, then the servo could be hunting. Do not expect it to bring home Thanksgiving dinner though. It’s more likely to cause you not to be able to take your hunting vacation.

Servo hunting is one of the most common motion-control problems encountered in industrial automation. It appears as a continuous oscillation around a commanded position or speed, even after the motor should have settled. Hunting can reduce positioning accuracy, increase wear on mechanical components, generate excessive noise and vibration and eventually lead to machine faults.

Servo hunting occurs when a closed-loop servo system repeatedly overshoots and corrects itself without reaching a stable equilibrium.

What kind of symptoms should you look for? Motor "buzzing" at standstill is an indication that the position has not quite been reached and that the encoder is encouraging the system to search. This is indicative of a constant position correction and a repeated overshoot around a target. That buzzing could be audible vibration or an unstable velocity response. The servo will respond with increased following error and poor repeatability of reaching the target when commanded. A hunting servo is typically a sign that something is wrong in the control loop, feedback system or mechanical structure.

There are three nested loops found in most industrial servo drives: current (torque) loop, velocity loop and position loop. Instability in any of these loops can cause hunting behavior in the overall system.

Proportional gain increases system stiffness and responsiveness. When set too high, the servo overreacts to small errors. The corrections become too aggressive, and oscillation develops around the setpoint. The same thing happens when young pilots come in too fast on landing and then overcorrect the angle of attack and get into an argument with gravity. The plane oscillates or porpoises. Signal oscillation can cause high-frequency vibration or a buzzing motor. Another indication is oscillation that starts immediately after a response to a move command.

Troubleshooting steps are:

1. Reduce proportional gain by 10-20%.
2. Observe position tracking.
3. Compare settling time and overshoot. Does the overshoot decrease with the adjustment of the reduction in gain?

The key is that a high gain does not equal faster target acquisition. Beyond a certain point, additional gain reduces stability, rather than improving performance. If we recall the pilot example, when a pilot induces oscillation on the landing, the pilot overcorrects, and the oscillation increases. This is similar to adding gain to get an “expected response” more quickly. The energy in the plane is met with an equal reaction, and it causes a larger oscillation in the opposite direction. It’s kind of like added gain caused by the pilot overcorrection. The overcorrection to the “error” is much like an accumulated integral response.

An excessive integral gain will cause the system to overshoot. In response to this, the slow oscillatory hunting develops. This causes low-frequency oscillation with a repeated correction occurring after reaching target position. If the system finds the target, eventually it will settle. Reduction of integral gains and checking for integral windup settings in relationship to the application will decrease the probability of low-frequency oscillations.

These responses can be shown on the bench. If engineers will set up a servo and an actuator and then adjust the proportional gains and integral gains in two separate exercises, then the servo response can be observed. Doing so would allow the controls engineer to see the envelope that is best for the servo to run at and could set restrictions on the proportional integral loops to keep the motor and encoder from hunting.

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However, there are times that natural frequencies may cause mechanical resonance. Every machine has natural frequencies. When servo bandwidth approaches a structural resonance frequency, then mechanical vibration is amplified and feedback sensors interpret vibration as position error. Hunting could occur due to natural resonances. How do you discern this compared to the other two examples? Oscillation happens only at certain speeds and instability occurs despite reasonable gain settings.

Reducing servo bandwidth and applying a notch filter could be appropriate. Adding machine rigidity with physical support could be necessary. Mechanical resonance is among the most overlooked causes of servo instability.

Physical problems can cause poor feedback. A broken cable can also cause poor feedback. Noise in the encoder signal creates false position errors and the controller continually attempts to correct those errors, causing hunting. The result is a random oscillation at standstill and position jitter. Check wiring, shielding and grounding, as well as connectors and plugs.

Mechanical backlash is another physical phenomenon that will cause servo hunting. Backlash introduces a dead zone between motion commands and actual movement. The controller repeatedly attempts to compensate, resulting in oscillation around the commanded position. This would be indicative of worn ball screws, gearboxes, rack-and-pinion systems, loose couplings and the like. See how controls people need a systems point of view to understand the electromechanical responses? Troubleshoot it by checking for mechanical play and tighten/replace worn, broken or misaligned components.

Another physical problem that causes servos to hunt is inertia caused by a motor inertia and load inertia mismatch. Large mismatches produce excess overshoot and inability to tune. Hunting during acceleration and deceleration occurs causing instability during movement. Review drive sizing and consider gearbox modifications.

Over-aggressive motion profiles will cause instability in servo systems, as well. Considering how the machine will function best over time in a repeatable state is sometimes best in order to reduce jerky movements and hunting and pecking. Understanding that a sweet spot in the system may not be with an extreme acceleration or abrupt deceleration and that looking at the application in context can help define the motion profile is important. This also relates back to the sizing and the inertia required for movements. What is the end goal of the application response?

In general, does the mechanical setup match the profile and does the electrical feedback provide an expected response? Can it be repeated? This is how to go about troubleshooting and/or designing a servo system that won’t hunt. Servo hunting is rarely caused by one factor alone. While excessive PID gains are often blamed, the root cause may actually be mechanical resonance, encoder noise, backlash, load mismatch or an aggressive motion profile. The most effective troubleshooting approach is systematic: verify mechanics first, confirm feedback integrity, evaluate machine dynamics, and then fine-tune the control loops. Engineers who diagnose hunting from a systems perspective typically resolve the problem much faster than those who simply lower gains and hope for improvement. After all, hunting should be for vacation in the fall, and not for servos on the plant floor.