A student type question.

[Rick Densing]

One more note. As CPU power is rising, a lot of servo manufacturers are adding PLC like features. I understand the the latest Bosch (Indramat) drives have a PLC.

 

[Peter Nachtwey]

With the indramat motors they have turn counts in the millions. The accuracy of the screw is usually the limiting factor.

We handle counts up to 4MHZ on our quadreature encoders. We could handle faster rates, but it is very hard to convince people to buy 10K count encoders and spin at 100 revs/sec. Some of our competitor go up to 10 MHZ, but I have never seen the need.

There are MANY threads that ask about how to calculate RPM. It is easy to calculate RPM when the sample rate is regular and you have a 10000 line encoder that generates 40K counts per rev. When going slower we can also count the time between pulses. At slow speed this can increase the speed accuracy by about 25%. You would think the results would be much better. Unfortunately the encoders phases are not all exactly 90 degrees apart so the results are less than ideal.

 

[Terry Woods]

Ron,

There are two primary facets to motion control...
1. Number Cruching (calculating).
2. Speed of the system being controlled.

Number crunching, or calculating, is nothing more than grinding your way through the various mathematical algorithms to obtain the required result (gee, that was easy to say!).

Any particular algorithm takes a certain number of steps (number crunching is not time-dependent per say, rather, it is step-dependent, which is then time-dependent... sort of) to obtain a result. Some algorithms have many steps, some have few. If a processor is available to do nothing but crunch through these algorithms then the speed of the result is dependent solely on the speed of the processor and the number of steps.

However, if the processor is loaded down with extra responsibilities and the processor can only do number crunching on a time-share basis then the speed of the result also depends on the extent of that extra load as well.

If a processor can only provide 10% of its' time to crunching numbers, then it will take ten times longer to crunch the numbers than a dedicated processor would take.

Ultimately, the result will be available.

However, the question becomes... "Are the results available in a timely manner?"

That is... Are the results available soon enough to provide adequate control over the driven device?

If a motion system is operating at speeds and times such that results must be available in such-n-such time, then the processor must ALWAYS be able to produce those results in no more than that time!

The results of the calculation must available soon enough to keep the motion on-track as expected.

In the final analysis, the ability of any particular controller to control any particular motion system depends on the ability of the processor to provide results as they are needed by that system.

For one particular application, one motion control system might not be good enough to handle the job because it is too slow. The driven device moves faster than expected and thus produces greater errors. In that case a faster control system is needed. It depends both on the application and the speed of the controller.

In another application, a particular motion control system might be so fast that the controller finds itself recalculating the same parameters, over and over, before it notices a detectable change in the process. In this case, the driven device moves much slower than the processor expects. This not to say that the processor can't handle it but the particular processor is simply much more than is required by the process.

Speed is expensive. The question becomes... "Is that speed worth the expense?"

To simply say "I feel the need for speed" is to say "I want to blow some bucks!" Do you really need that speed? Can you afford those bucks?

Depending on the particular situation, a small PLC dedicated to nothing but motion control can be quite effective.

Speed of the process, resolution required, and processor speed determines the final solution.

(427) Wow! Chevy!

 

[Rod]

5000 RPM and beyond

The 5000Rpm was shipped to a customer in Mex. I did the install/training.

It's only a 4ftX by 3ftY with a rotary turret punch with a 4th rotary tool. (Amada30) Still scary though.

The problem was keeping the 3' x 6' 16Ga sheet from 'flying' when moving more than 3ft. We had to back it off to 4000RPM/IPM.

Yes, the ball screw has to have SERIOUS end bearings with adequate preloading. The ballnut has to be 'tuned' also (preload/backlash) otherwise the machine will eat itself.

To spin a ballscrew at 4000+RPMs you need at least a 1 1/2" to 2" diameter screw at maybe a max length of 8 foot. Sizing the motor to the intertial mass of the screw + the carriage and the material's mass = motor. I would recommend Kollmorgen Gold motors series and their servos.

As Peter said the encoders may not be 50/50% on pulses (not counts),
so we have used 1 million count encoders and scaled the inputs (1/1000 error) and the $ diff is minimal.

Technician tip #N10:4 (Encoder test)
Using a dual trace scope, channel lead A to encoder channel A and B to B. Select ADD on the scope. Spin encoder - it will be obvious if the encoder channels are not properly phased. Smack the head of the repair tech who sent you out with an otherwise 'repaired' encoder.


Rod (The CNC dude)

 

[dandrade]

Deepening

But really which are the features, that can decide for bigger earned between the two equipment? Both capable saints of reading of encoders.which are the logics capacities additional?

-It recognize forms geometrics: Memory objects 3D fix or dynamic in the position and form
-Decide to new form geometrics object, after displacement of an axis.
-Capable of it foresees the position of the objects above avoiding collision between axes.
-It foresees the time of movement and ideal speed.
-Capable of they trajetory defined by measures standards: I.E: Perform the trajetory internal or external of sphere.
Trace perimetre of a cone.
-It decide way of quick journeys, contouring objects of standstill collision or in movement.
-It draw figures geometrics.

This definitely, for me, consider a controler of movement. They are capacities that can be achieved by equipment of movement in the market???