Servo noob

[theripley]

Hi.

Basically servo motors have 3 control mode - positioning, speed & torque. How will I know which among the 3 I need for an application? What is the basic difference between them?

 

[Diameter157]

Tell us what you are using the servos for and then we can tell you which of the 3 you need.


If the servos are making something move in a straight line such as a linear drive you will want to use position mode (you will also be setting a speed and make acceleration).


If you have the motor spinning and want it to spin at a constant speed then you use speed mode.

 

[shooter]

On a locomotive positioning is not important, but torque is. and when speed is reached the speed takes over.
on a crane only speed is imortant, however the torque is a safety.
on a elevator all three are needed, etc.

 

[jdbrandt]

On a locomotive positioning is not important, but torque is. and when speed is reached the speed takes over.
on a crane only speed is imortant, however the torque is a safety.
on a elevator all three are needed, etc.

Wow. An excellent explanation.

 

[Peter Nachtwey]

On a locomotive positioning is not important, but torque is. and when speed is reached the speed takes over.

This isn't quite right. When at constant velocity the torque applied by the motor must match the opposing torque whether that is caused by the load, friction, air resistance or any thing else.

on a crane only speed is imortant, however the torque is a safety.

Aren't you ever concerned with positioning something with the crane?
I have algorithms that will nullify the swinging action of the load. Position is important.

on a elevator all three are needed, etc.

Yes. Elevators probably use 7th order motion profiles so the jerk at the start and the stop will be 0. Position, velocity, acceleration and jerk are all important.

 

[ndzied1]

Yes. Elevators probably use 7th order motion profiles so the jerk at the start and the stop will be 0. Position, velocity, acceleration and jerk are all important.

We are working on a project with a guy who has been in the elevator business for over 30 years. Maybe new system do this but I'd guess the older stuff just uses a very low acceleration.

Something else I learned is that most elevator accidents occur with the car traveling up and crashing at the top, not with an uncontrolled fall.

 

[kamenges]

While Peter's response is correct I don't think it answers the original question. Peter is indicating physical properties you need to be concerned with for a given application type but he really doesn't get into what drive class you would want to use for each of them. And in reality that is a very application specific decision with no real clear-cut answer. Much of the decision has to do with relative performance level of other components in the system.

For example, with the components available to him and their relative capability distribution, Peter would very often use torque amplifiers for his applications. His motion controller is the most capable component in most systems so it makes sense to consolidate as many motion functions into it as possible. If the drive being used in that same system had a very high bandwidth velocity loop and had some good dynamic tuning tools Peter may opt for velocity mode operation.

As shooter alluded to, not all applications require all physical control capabilities. Not all applications require positioning so the positioning function is not required. If your application doesn't need positioning your drive certainly doesn't need the positioning function. However, even if positioning is required, it doesn't mean you necessarily want to use a "positioning" drive. Be EXTREMELY careful with "positioning" drives, by the way. The difference in capability across the available choices is truly staggering. If you have an application that requires torque/force limiting then you need a drive that has torque control capability. However, where that torque/force limiting function resides is, again, a separate design decision. The function could reside in the drive or in an external controller. If torque limiting resides in the external controller a drive that accepts a torque command would be required. If the torque limiting is performed in the drive then the drive could accept either a velocity or a torque reference.,

As you can see, this is not necessarily a component level decision. It is a system level decision. Some of the choice lies in your personal decision about who's hands you choose to lay your chances of success in.

Keith