All mechatronic systems, electric, pneumatic and hydraulic, can be evaluated according to thei rdynamics. The aspect of dynamic performance that is most significant is the bandwidth, the inverse of time or 1/t. Its not obvious, but once considered, can make the technology decision a lot easier.

1/t can also be expressed as Hertz, the number of cycles per second that a system behavior is changing. The most familiar domain for Hertz is, of course, sound. But most mechanical systems have resonant behaviors that can indicate a variety of conditons that are important in that system.

Beyond the mechanics of vibration, the most important issue for mechatronics purposes is the variation in the load. If a load has a cyclical variation, that is a fundamental property that must be considered to make an appropriate technology selection. The higher the system throughput, the smaller 1/t becomes tougher to deal with.

To some extent the technology continuum can be considered as the slowest responding to the fastest responding. Hydraulics tend to be slower, pneumatics faster and electrical actuators the fastest. This would be an intuitive assertion just considering the dynamics of the medium, hydraulic fluid power is more viscous than air and takes more time to propagate. Pneumatics are faster because air is a much lighter fluid. Electronics are the fastest because electrons travel at almost the speed of light, although it is fascinating to consider how long it takes an AC motor to reach full speed.

And for every assertion that we make about one technology or the other, there are workarounds. Fluid power systems can use accumulators and servo valves (with electronic controls) to improve the responsiveness of the system, both in terms of time and in terms of precision.

In the field of AC drives the responsiveness is the key determinant of what type of drive to use. Take a simple conveyor application. If the system has to move 50 pound bags of dog food to a palletizer, things are pretty easy. Throughput might only be a few bags a minute, lets say 6 a minute. So every ten seconds a bag gets dropped off changing the load condition on the conveyor by a small fraction. No big challenge.

But if you are loading cases of beer on a pallet, 10 cases of six packs of glass bottles per pallet layer every six seconds, then there are 100 cases a minute flying through the system and the speeds at which a diverter must sweep to move each case into the correct lane are pretty challenging. At 30 pounds per case, a 10 case pallet layer is 300 pounds. 8 pallet layers is 2400 pounds of beer bottles. And when you have to lower the pallet to a precise height to make the next layer slide on smoothly there is a speed issue which is compounded by changing load. Every 6 seconds the load increases by 300 pounds. And when the pallet is empty the load change is disproportionately large, when the seond layer is added the change is 100%, and when the next layer is added its 50%, and so on.

So while there is a frequency consideration to the rate at which conditions are changing, there is also a load component. Factor in the true dynamics of performance in your next project to insure success.

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