Robots and vibration

Reducing vibration in robots is an often overlooked but essential consideration when designing and installing robot work cells. Vibration can not only reduce the performance and repeatability of your robot, but it can also significantly reduce the lifespan of the robot.

How robots move

Robots don’t simply navigate from waypoint to waypoint in perfectly straight lines, stopping precisely and quickly. Each robot motor must index at varying rates of speed and acceleration so that the Tool Center Point (TCP) of the robot not only arrive at its destination precisely, but that the path also be smooth and tight (referred to as path linearity). For trajectories to be smooth and swift, each motor must be sampled against a PID loop while in motion. Vibration throughout the mounting structure can throw off this loop sampling and cause the robot to adjust eratically on the fly in an attempt to maintain path linearity specifications.

When the TCP arrives at its destination, the robot is controlling for settling and position again based on a PID loop sampled against each motor. There is overshoot involved, and ultimately a settling time before the robot is confident it has arrived at its desired pose. Algorithms that control for settling and servo precision require a stable structure to perform at their best.

What’s happening when my mounting structure is resonating or vibrating?

Mechanical oscillation in your mounting structure can introduce inertia and positional inconsistency during path trajectory movement and waypoint settling. The servo motors in each joint will fight hard to maintain tight performance specs, but the commutation of the mechanical vibration can cause a repeated over-correction and under-correction of each servo motor during operation. Not only does this heat up your robot motors excessively, but it grinds and damages internal gearing structures attached to each robot joint. These high-stress loads on gearboxes are sometimes called “shock loads”. Shock loads typically occur during rapid start-stop conditions and often exceed normal operating loads. When your robot is attempting to control for precise TCP orientation or trajectory manipulation, several motors can intermittently experience shock loads. This is especially damaging for strain-wave gearboxes, like the kind found in most collaborative robot axes or on the final axes of some larger traditional industrial robots.

Are there guidelines for minimum vibration exposure to robot hardware?

Sometimes. Guidelines for vibration in robots and servo assemblies is a gray area. Some manufacturers of servo motors, or gearboxes, or robot arms will publish maximum amplitude or frequency standards for their equipment. Because manufacturers of robot arms like to keep their internal components confidential, it can be hard to find specific manfuacturer specifications without knowing brands and model types for servo motors and gearbox assemblies.

Universal Robots has recently begun publishing guidelines on minimum vibration resonance frequencies that mounting pedestals can exhibit. Lower frequencies tend to have higher amplitudes and take longer to settle out, so it would make since to have a “minimum threshold” for frequency in this case. Universal Robots, as of the time of this writing, has offered guidance in their new UR series robots that pedestals do not exhibit a resonance frequency below 45 Hz. Even still, there are not any hard-and-fast guidelines on amplitude or wave energy. Ask any of the design engineers behind this technology and I’m sure they’d tell you something along the lines of “rule of thumb: reduce as much as physically possible”.

Unifi pedestals and vibration

Here at Unifi, we have a couple of internal standards for products that we design and test. Reducing vibration is our very first design consideration, and we design our products to dampen quasi-static waveforms to within 10% of their initial amplitude in less than 1/5th of a second. Why? 1/5th of a second tends to be the settling time for high-performing robots and seemed like a good benchmark. The resonance frequencies of our pedestals are also very high - in the hundreds and even thousands of Hz. We want high frequency resonance because these wavelengths tend to exhibit smaller amplitudes and dilute quicker as they propogate through the mounting structure. Whether it’s a standard product or a custom design for your application, Unifi engineers perform finite element analysis and empirical, physical vibrational studies on finished parts before they ship to your facility. Robots mounted on Unifi hardware are happy robots.