IAS Blog

This little servo is going to run my machine?

Posted on 08/24/2009 at 02:22 pm by IAS
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Recently I had the opportunity to specify a complete automation solution. Servo's, PLC parts and an Operator Display. This was a 4 axis machine with two relatively large robotic arms. The robotic arms are supported by an overhead rail. The two arms independently move along the horizontal rail. The also extend and retract vertically.

I used Manufacturer provided software to size servo's for this application. After plugging in all the mechanical data the software said that 400 Watt servo with a 20:1 gearbox was sufficient for the vertical motion. This combination of servo and gearbox met the lifting, inertia matching, and speed requirements.

Servo motors, depending on the design, they can have very high torques for there size. But great care must be taken to insure there is not too much inertia. Multiply the small servo's torque by 20 because of the gearbox and you have a lot of lifting power.

Now picture the scene. I am sitting at a conference room table discussing another application with a programmer. In walks the owner with a 400 watt servo motor in his hand. Its dimensions are ~2.5" * 2.5" * 4". He firmly places the motor in front of me and says, "You mean to tell me this little motor is going to run my machine". As I look at the little servo I start to question myself. Did I enter the data correctly? Did I get good information to start with? Let me just say that I was concerned. I simply replied that the software said it will work but lets connect it as soon as possible for a test.

I returned later for our test. Just a few minutes to connect power to a PLC on the bench. Connect the cables and write a small program. Start off moving axis slowly up and down figuring out speeds and distances. In no time at all the system was running and using less then 40% of its rated torque. I explained to the customer that servo's can produce ~300% of there rated torque for instantaneous accel / decel. The customer grabbed the vertical axis and tried to prevent it from moving up. Our "little motor" was able to lift the customer off the floor!