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Improving move and settle times with a machine or payload that shakes

Improving move and settle times with a machine or payload that shakes

With industry’s desire to make smaller, more accurate parts at a faster rate, there is also a parallel demand to inspect these systems at high speeds. In both cases these higher process speeds requires the use of high accelerations. There is a risk that theses accelerations will inject motion energy into a machine that may create a system and or payload resonance.

This oscillation can delay a camera or sensor from taking data because it may, or the payload may not have settled to within a window of stability. If the motion system could cancel the vibration during its move, then the time for the system to be within tolerance could be greatly negated. 

Let’s consider, then, some practical applications where users want faster move and settle times, but experience shaking of the chuck or camera or tool or bracket. Normally in these cases, you would try to modify the acceleration shape or rate to reduce the energy that is injected into the system. Obvious downsides of this would be that the torque/force capacity of the motor and drive would be under-utilised, and that the time to carry out the move would lead to the throughput of the system being compromised. And the vibration will still remain, just at a smaller amplitude. 

In cases where point-to-point movement are the main criteria for the motion system, the use of a feedforward technique can be highly effective in reducing vibrations resulting from structure resonances. These can be used where issues like camera shake, laser head mounting vibrations and chuck motions or machine construction affect the final process time.

Aerotech controllers have the ability to use a feedforward technique named “Command Shaping” –  found in the “Dynamic Controls Toolbox” feature of Aerotech’s motion controllers. This feature is applicable to Aerotech stages and motors, but also to third party motors, stages and systems constructed from component level equipment. Command shaping uses the technique of superposition, which overlays several delayed commands which make up the final required move.

Because command shaping is an open loop technique not only can it be extremely effective, it does not affect servo stability and can be use in conjunction with other techniques. However being open loop it won’t compensate for non-predicted events such as non-motion generated vibrations. The actual initial move time is lengthened so gains in settling times must outweigh any increased move time. These gains can be potentially improved by increasing the rate of acceleration.

The plots opposite highlight below a move on an axis called Y. The top plots show the machine before command shaping. After a move it creates an oscillation in the machine that can be clearly seen on another axis (X) feedback device. The measured frequency is approximately 15Hz. The axis which initiated the motion cannot pull itself into an acceptable position in a timely manner. The Aerotech scope has a feature where it will change its plot line colour on the position error from blue to red, once it has met its imposition position error. We can see that position error bounces in and out of this stability criteria. 

This is important as in many cases the motion controller may have reported that the motor has finished its move and the system has settled within criteria, where actually the system has oscillated out of position. In insertion processes this could be a major problem.

In the lower plots, the motion controller has been ‘informed’ of the 15Hz issue and will generate motion taking this problem resonance into account. Not only does the motion axis now make its stability criteria, the other axis now exhibits a much smaller oscillation. Notice the change in the velocity commanded profile and its increased period. This is typical of how command shaping affects the profile. However the previous motion did not meet the stability requirement within recorded window. The user can now also increase the acceleration of the system to make up additional time and also utilise more of the motor’s capacity.

Even with higher end mechanics, command shaping can help system builders increase their machine performance by relatively simple techniques, hence offering their customers higher throughput without the need to increase the cost of the machine structure or other mechanical components.

Consider a real world high precision planer air bearing stage built directly into a granite base. In this case the  customer had requested that a 30mm move will settle to a tolerance of within 0.2µm. With command shaping implemented this was achieved in 200ms, outperforming the 300ms plus of the same mechanical system.

However, an oscillation that occurs on a machine structure may not be picked up by the feedback device on part of the motion system, or the system uses a motor type that does not incorporate a feedback type. Accelerometers can be used in servo systems to add additional continuous information to the motion controller about the resonances that are occurring in real time. Such accelerometers like the Aerotech enhanced tracking module (ETM) are great for compensating for errors generated by base motion.

As command shaping is an “open loop” method it does not relay on feedback or servo feedback; therefore it can be used to adjust for oscillations that occur elsewhere on the machine that are regions of interest, or with open loop motors such as steppers. Accelerometers may be used during system characterisation to measure the frequency of the oscillation, this data can be used to ‘shape’ the motion command using the tool to reduce such oscillations.

We can see, then, that command shaping algorithms can be an effective technique for reducing residual payload disturbances using feedforward techniques – where an additional sensor is impractical (or impossible). Throughput on machine can be improved if a structural resonance causes the time to settle to exceed expectations, and a reduction in acceleration is required to damp such an oscillation. 

Being an “open loop” technique, servo stability is not affected and a servo loop is not required making it applicable to both closed loop and open loop motors.

The use of accelerometers can be used to determine motion induced oscillations. This information can be used to provide resonance data to command shaping tool, thus reducing the effect of vibrations.

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