Seven considerations for applying servo drives, motion controllers and PLCs

July 11, 2013
Technology advances in servo drives have made them the workhorses of motion control. Here are some recommendations for implementing servo drives:
1. Don't fear servos. There are many different levels of positioning and motor controls now. Some basic variable frequency drives can even be used to do what could have been done only with coordinated drive systems in the past. So don't be wary of servo technology. They are now more cost competitive with auto-tuning, which makes them straightforward to implement.

2. Quick changeovers. Machine setup and changeovers lead to lost production time and inefficient production. Changeovers can vary from minutes to hours. Consider using low voltage servo drives with built-in encoder, controller or removable memory units, specifically designed for auto-format settings and quick changeovers. Using a customer friendly servo drive for quick changeovers and machine-set up is an investment that can quickly pay for itself, typically for users who run multiple format sizes on the same production line.

3. Why use motion controllers? Everyone knows some PLCs can do lots of things, and that includes motion control. However, separate motion controllers persist because they perform combined servo and stepper motion control and are properly coordinated as a high performance system with the same servo drive supplier. Companies that produce motion controllers design them specifically to increase a machine's output with improved accuracy. Trying to use a PLC to address registration or robot-type control, plus handle recipe data, may require additional PLC programming capability and time versus some motion controllers. Past experience on such applications says it's best to use the optimal tool for the job. And with the latest motion/machine controllers with built-in IEC 61131-3 functionality or other straightforward programming capability, you can have the best of both worlds.

4. Over-sizing wastes energy, cost and panel space. Some of the biggest energy wasters on a machine are frequently overlooked. Over-sized servo drive/motor systems, for example, cause machines to consume more energy than necessary-something that can easily be avoided through proper design. End-users often under-estimate the returns from energy-efficiency investments, since it costs more up front and may take a few years to achieve payback. As a result, they often inadvertently build in extra long-term, ongoing costs by overlooking details when sizing machine components. When sizing a machine, the entire motion profile is important, not just speed and load. Having a detailed and accurate profile of needed motion can pay dividends. Typically, a less precise profile will lead to an over-sized servomotor. This means that the energy consumption of the system will be higher than needed. The key to getting the motion profile right is properly calculating velocity, continuous versus peak torques, acceleration, and matching load and motor inertias. Further, refine the profile by accounting for cycle times. How long does the system have to move from one point to the next, and how long can it take to complete the entire trajectory?

5. Document requirements. When building dedicated automation with motion control, whether you source it out or not, it is extremely important to understand the system you are building and the functionality that you intend it to have. There are different levels of motion control, especially if using a PLC as the primary logic controller. For example, on an indexing assembly station that used a servo drive to index large assembly pallets from one station to the next, the controls company used an incremental encoding servo drive system rather than an absolute encoding system. This error crept in because the original rationale for using an absolute system was not documented. Consequently, in an emergency stop situation, the indexing system would stop and then want to do a complete index, starting from its current point. Had absolute encoding been used as originally planned, recovery from an e-stop would have been very simple. If the expected functionality had been properly documented, additional complexity would not have been added to what should have been a simple solution.

6. Minimize vibrations. The latest vibration suppression algorithms in some servo drives can actually minimize vibrations from occurring on overhung loads and in the machine base without additional sensors. These vibration suppression algorithms, combined with auto-tuning and filtering, can allow for high performance motion without complicated mechanical damping or heavy bracing.

7. Reduce wiring, space. By using common DC and/or multi-axis servo drives, OEMs can reduce wiring, energy consumption and panel space. These systems use regenerative energy to power other axes, rather than wasting this energy as heat in the electrical enclosure. Panel space is reduced by up to 30 percent, while wiring is reduced by up to 50 percent, compared to a traditional single axis servo system architecture.

Load to motor inertia ratios - improve response time

Calculating an inertia ratio is often overlooked by newcomers to servo sizing, but is arguably the most important factor in determining the performance of a servo system. Inertia ratio is calculated by dividing the load inertia by the motor inertia. Lower load to motor ratios improve response times, reduce mechanical resonance and minimize power dissipation.

An inertia mismatch of greater than 10:1 can produce oscillations and extended settling times. To prevent overshooting and oscillations with very large mismatches, the control gain may have to be reduced. A motion system with a load to motor inertia less than 10:1 can reach a set speed or move into position in less time than one with a ratio greater than 10:1. Large inertia mismatches require higher current to drive the motor, thus they dissipate more power. For example, a mismatch of only 5:1 will dissipate six times more power, and it gets worse as the load inertia increases.

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