In our last posting we raised some questions about how the “position make up” made up for the steps that were removed.
Position make-up will compensate for position errors due to a
disturbance during a move by reinserting missed steps into a motion profile
as conditions allow. A mode selection parameter defines how that compensation
occurs.
Aggressive move profiles may present no opportunities to make up missed steps during the move profile. The missed steps would be inserted at the end of move which can affect the overall move time.
To me, the first thing is that the “Hybrid” system can’t make up for is a motor that has been under sized.
You still need to do your homework in choosing the proper size.
An example of this is a customer that needed to do an aggressive move from point A to point B and back in a specified time. They tested a motor and it worked. They went into production only to find that some motors were a bit slower than others and that messed up the machine’s timing. When you use hundreds of motors you’re going to come across some that produce more torque and some that produce less than the published value.
Remember that the speed-torque curves are for a “typical” motor, not the weakest. This customer’s application weeded out the weaker ones. Thus, the move time of the “Hybrid” system worked fine until a weaker motor appeared. The weaker motor generated less torque and took longer to move the same distance. It didn’t stall, it just took longer to get to its position, so be careful when the machine design timing requires it to be within a few milliseconds every time in order for it to work properly.
The second thing that jumps out at me is if the “motion path is critical,” as in multi-axes of coordinated motion with marginal motor torque that requires “position make up” If it requires “position make up” then the path isn’t where you want it to be and that’s not a good thing. I hate it when the welder is not following the proper helix path.
The third thing that jumps out at me is if you have an aggressive-overhauling move that goes beyond the target position and then backs up to get to the proper position as a function of the “position make up”
If the application can allow the “overshoot” fine, if it can’t, then we’re in trouble.
This is similar to “the motion path is critical” as noted above, but in a single axis application. I remember a “non-Hybrid” application that had an encoder on a grinding bed and the motion control system moved the bed and the work piece into a grinding wheel. There was a lot of backlash between the motor shaft and the bed. The motor kept moving the work piece into the grinding wheel at a nice slow pace until the proper encoder count was reached. Then the move profile stopped only to have the encoder count change, so the motor hunted a bit before it was happy with its final position.
All this “hunting” removed too much material from the work piece. It’s tough to put material back on an expensive work piece once it has been taken off.
The bottom line is that the “Hybrid” motor technology can be used in applications that were “reserved” for other motor technologies. It can be a simpler more cost effective solution by just eliminating the PID tuning requirements costs in addition to the hardware cost.
The integrated control-mounted-to-the-motor design brings another cost benefit because you’re mounting the motor and control at the same time. Eliminating or at least reducing the need for controller/amplifier enclosure space.
Next time we’ll take a look at how you go about choosing the right motor size.
Bob
