Last month, I discussed what I consider to be 10 important considerations when starting to design a motion control system. I covered the first three of them:
1. Is the required motion linear or rotary?
2. Do you need a stepper, servo- or induction motor?
3. Do you need a dumb drive or a smart drive?
Let's continue with the remaining seven considerations on the list.
4. What kind of controller will you need? The answer to that depends on the kind of motion control you need, and if there already is some kind of overall machine controller (typically a PLC). For demanding motion control requirements, it's best to let a dedicated motion controller handle the motion control for the machine. Using various forms of communications, you can pass commands to the motion controller from your machine controller. If your machine's primary function is motion control, you can let the motion controller become the machine controller. If your motion control requirements are not demanding, then it's possible to have the PLC handle these functions.
5. How will you interface to your motion controller? You somehow have to give commands to the motion controller. This can be something as simple as a push button or as complex as a machine network. If you need to change settings and monitor conditions, then perhaps a typical touchscreen HMI will suffice. If you need to log data and keep track of events, maybe you'll need a PC.
6. What electrical supply is available? If you only have 120 Vac, then you'll need to know that up-front. Your drive choices will be limited, and your motion system requirements could need more power. Higher power systems typically will need a three-phase power source. Sometimes, three-phase, 480-Vac power will be required.
7. Who will do the motion integration? If you're a seasoned controls engineer with motion control experience, you probably can handle this with little outside help. If you have little motion control experience, you'll need quite a bit of help. Does your motion
8. Does your motor-sizing software know what it's doing? The short answer is “maybe.” Motor sizing is very important. Even if torque requirements are met, the inertias could be mismatched. Even if your motion control system is properly modeled in the sizing software, it could select the wrong motor for you. Look at the motor requirements, then select the proper motor after looking at the speed and torque curves.
Also Read: Three Top Tips When Designing Motion Controllers, Part 1
9. Does auto-tuning work? Sometimes. Most auto-tuning routines work very well for velocity control. Some work well for position control. There are none that work for very demanding positioning applications. Depending on your system dynamics, auto-tuning might result in acceptable performance. Auto-tuning will frequently get you close to where you need to be, but then you'll have to do some fine tuning. At this point, it's more of an art than a science. Even if the final positioning is accurate, how long did it take to settle at the final position? Long settling times kill productivity. If someone tells you that tuning parameters can be completely derived mathematically, don't believe it. If that were the case, auto-tuning would work perfectly.
10. Who will support the machine in the future? If the person who programmed the motion control won't be around for troubleshooting, then this task usually falls to maintenance personnel, who often have little or no motion control experience. An experienced motion control programmer will bring out any faults and alarm codes to the HMI to make troubleshooting easier.
The important theme to take from all this is that motion control system design is more involved than one might initially think. It's best to consult with an expert in the field to make sure your system will do what you need it to do.
[Part I of Brian's discussion is at bit.ly/1jVHgZS and in the June 2014 issue of Control Design.]
Brian Beal is president of Highland Controls, an integrator and distributor of industrial motion control systems and related products. Learn more at www.highlandcontrols.com.