ball screw feedback

Hi,
we have a servo motor driving a ball screw, we have a position feedback from the motor by means of resolver, but we intend to have a feedback from the ball screw to monitor the mechanical motion, kindly suggest the feedback device and how we can mount on the ball screw to get proper feedback during motion.

kindly suggest me some ways

thanks

Ravi

Most motion controllers use the feedback device on the motor for both position and velocity feedback and do not require a separate feedback device on the driven load.

I think he is wanting to do some backlash compensation.

Peter,

You're probably correct. If that's the case, the feedback needs to be mounted so as to measure the motion of the ballnut, and not the rotation of the ballscrew.

Peter Nachtwey said:

I think he is wanting to do some backlash compensation.

Click to expand...

ur absolutely right, i want to monitor the backlash, in that can u suggest me what i should use and how i should mechanically mount in the ball nut

thanks

I've seen systems that have a feedback (resolver/tach or encoder) mounted on both the motor AND the ballscrew.
But, there is still the backlash as Peter mentioned.

Perhaps the poster will give us more information.

Rod (The CNC Dude)

I think you will need some type of linear scale mounted directly to the device that is being moved. This will give you the actual movement of the load. The signal from this scale can then be read for the commanded move verses the actual distance. In theory, the actual distance can be fed to the control for position , and the ballscrew feedback can be used for the velocty portion of the loop. This set-up is called "Dual Loop", and is very accurate, but I have read that it is tough to tune properly .

One option, depending on your application, is to mount a precision indicator to the load(it must be exactly paralell to the axis of travel), command a move (e.g. +10mm), then command back to the original position(e.g. -10mm) and then compare the commanded move to the actual move. this will tell the actual backlash of the system at that point.

Most servo drives(and CNCs) I have worked with have a parameter to input the backlash. The software then will correct on moves in the opposing direction.

Hope this helps.

Many motion controllers have a backlash compensation parameter. In the simplest form this is just a number of pulses that the motion controller ingores every time the motor reverses direction. More sophisticated controllers may be able to apply the backlash compensation whenever the load reverses. You may be able to use a ballnut with a high enough preload so that you don't need to apply backlash compensation. The downside to higher preload is higher frictional losses.

Adding a second feedback device to the control loop adds additional complexity both in hardware and in tuning the controller. If you can achieve your positioning tolerances without having to use the additional feedback, the people who have to commission and maintain your design will thank you.

linear scale

Hi
As it's mentioned above you should go for linear scales which is widley used for posioin feed back in CNC machines to feedback the actual position to the CNC controller.
i know two manufaturer for this scaled the beat one all over the world is Heidenhain from germany and the second is Fagor
Best regards

Also, dont forget to factor in the thrust bearing that support the ballscrew. In my limited eperience, they are usually more prone to cause backlash than the ballscrew.

The linear position feedback devices I have seen are always far less accurate that the resolver off the motor. This is fine when you don't need that much accuracy, but is difficult when you want to do something fiddly like get rid of your backlash.
The main solutions that I have seen is to compensate for the backlash somehow.
First you have to decide what is acceptable for your operation. If you can somehow get the backlash down to a level where it doesn't matter, that would be best.
So, here are my ideas:
1, The best ball screws have little backlash between the screw and the nut. Therefore, you may be able to just run an encoder directly off the screw as some other posters have suggested. If this is not good enough, then it can be combined with other methods.
2, Drive your load in one direction only. If you need to drive the load in the opposite direction, overshoot by a distance in excess of the backlash, then drive it back in that direction. It is best if you load has a bit of resistance to movement of some sort of damping, to prevent it from ending up in a random location within the backlash band. This compensates for the backlash by ensuring that it applies in a determanistic manner.
3, Use backlash compensation calculations. Some servo drives have these built in, or if not you will have to create these yourself. You must calculate the magnitude of the backlash, and feed it into your calculations. Note with this method that mechanical wear will increase backlash over time.

Renishaw and Heidenheim make very good linear feedback devices Renishaw less expensively, Heidenheim higher accuracy. If possible use a sine encoder with a high level multiplier to get the best resolution. If you are in a highly contaminated environment you can use a sealed encoder from Baluff. If you need to get extreme accuracy you can use a laser interferometer and conceivably get down to the Angstrom level if you have all the right conditions.

Ravisundar12, is not possible in your application to achieve targets always by moving in the same direction?