Analog vs pulse direction... Which is preferable and why?

Hello all,

I am in a bit of a conundrum with my machine upfit and would like some advice from those who have experience and knowledge beyond my own to help guide a decision I'm now faced with.

I purchased AC Servo motors and analog drives from a company who sent pulse direction drives out incorrectly. I did not notice due to the project being on hiatus for a long time but recently discovered it when trying to get the machine to move via linuxcnc and a 7i77. No movement obviously as the incorrect drive would not recognize the analog signal.

My dilemma is the drive company is reluctant to replace these drives and even if they do I would be paying the shipping cost overseas to do so which would be more expensive than going with a 7i76 and running in pulse direction mode.


My question is which method is better in your experience and if you could explain any short comings of either it would be most appreciated! My current understanding is that analog would be the preferred choice because the feedback loop is closed in the controller and the movements are being coordinated in real-time based on that loop. I am unfamiliar with servos in pulse direction mode but believe the loop is closed in the drive. I am not sure if the use of an encoder board to accompany the 7i76 would do anything more than display a realtime position but have no effect on actual movement.

This would lead me to believe that analog would handle a sinusoidal acceleration curve much better than pulse direction which would be using a trapezoidal linear acceleration curve ( not as smooth of a movement). Also would there be much of a difference in speed of the motors between the two control options? The current drives have a 500khz pulse read speed.

So if you have a moment to help me understand if this is correct or even important in choosing between the two control options that would be fantastic!


Thanks so much in advance!!!

Chris

Anyone?

I'm not an expert on the details of what you are asking - here is my opinion.
The main differences here is:
- the fact that the actual position is not positively known if you use a stepper based drive.
- tuning of the drive must be correct and done outside of linuxcnc.

Linuxcnc uses the same motion planner regardless (but your stepper drive may use something else in it's loop.)
Drive bandwidth must be faster the linuxcnc's bandwidth (which is usually 1 KHz)
As for speed if you are using a Mesa card it can output steps faster then you are going to need for common milling speeds.

High performance step driven servos can work very well - I believe Major CNC OEM builders have used them.
The trick is to have the drives/motors well matched to the machine.

Assuming the motors are the right power and the drives are a good design and well tunned you are going to be fine.
as for using feedback to linuxcnc with step drives, this is possible but uncommomly tried - I doubt it would be worth it.

Hopefully the guys who have more technical knowledge will chime in too.
Chris M

cmorley wrote:

I'm not an expert on the details of what you are asking - here is my opinion.
The main differences here is:
- the fact that the actual position is not positively known if you use a stepper based drive.
- tuning of the drive must be correct and done outside of linuxcnc.

Linuxcnc uses the same motion planner regardless (but your stepper drive may use something else in it's loop.)
Drive bandwidth must be faster the linuxcnc's bandwidth (which is usually 1 KHz)
As for speed if you are using a Mesa card it can output steps faster then you are going to need for common milling speeds.

High performance step driven servos can work very well - I believe Major CNC OEM builders have used them.
The trick is to have the drives/motors well matched to the machine.

Assuming the motors are the right power and the drives are a good design and well tunned you are going to be fine.
as for using feedback to linuxcnc with step drives, this is possible but uncommomly tried - I doubt it would be worth it.

Hopefully the guys who have more technical knowledge will chime in too.
Chris M

Chris M.

Thanks for the reply!! The drives are industrial servo drives and they and the motors are matched to the Machine by the manufacturer. They are 1.8kw servos and matched drives. It states 500khz pulse frequency. Not sure if that's good bad or average..lol. This is my first rodeo with servos so everything is alot to absorb it would seem to me. The machine is a full sized VMC with linear rail ways and 6mm ballscrews, so I know it's not going to move like a dmgmori, but was hoping for rapids sound 700ipm or so.

Do you think pulse would above this or would analog control be required?

Thanks again for your input it is highly value and appreciated!

Chris

I take it the drives 500khz is the maximum pulse input. Without the other specs (lead screw pitch, gearing, encoder PPR) it's hard to be specific but to put that in perspective:

If your drives system needed 10000 steps to move 1 inch the drive could move the machine 50 inches per _second_
700 ipm is about 12 inches per second

I believe Mesa cards can pulse even higher then the drive ( I'm guessing Mhz)

Chris M

One thing to consider with step/dir drives is how to do homing.
If the encoder signals come back to LinuxCNC you can home to index using
LinuxCNCs built in homing logic, if not you either cannot do accurate homing
or you must somehow integrate the motor drives built in homing procedure with LinuxCNC

cmorley wrote:

I take it the drives 500khz is the maximum pulse input. Without the other specs (lead screw pitch, gearing, encoder PPR) it's hard to be specific but to put that in perspective:

If your drives system needed 10000 steps to move 1 inch the drive could move the machine 50 inches per _second_
700 ipm is about 12 inches per second

I believe Mesa cards can pulse even higher then the drive ( I'm guessing Mhz)

Chris M

Chris M,

The machine has 6mm lead ballscrews direct coupled to the motors 1:1, encoders are 5000 line encoders, I'm not super Keen on what that means but it was higher than 2500 so I figured that was better, they are incremental encoders.


Chris

PCW wrote:

One thing to consider with step/dir drives is how to do homing.
If the encoder signals come back to LinuxCNC you can home to index using
LinuxCNCs built in homing logic, if not you either cannot do accurate homing
or you must somehow integrate the motor drives built in homing procedure with LinuxCNC

PCW,

The drives to have an internal home limit switch functionality with digital output signals for in position ready. But I suppose it would be better for Linux cnc to know where everything was during the home process.. certainly something I had not considered. You mentioned in another thread I posted in yesterday (was an old thread) that pulse and feed back position control was possible, this would put Linux CNC back in charge of the homing I would imagine correct? Would position control with pulse be attainable with a 6i25, 7i76 and 7i77?

Thanks in advance!

Chris

Lcvette wrote:

PCW wrote:

One thing to consider with step/dir drives is how to do homing.
If the encoder signals come back to LinuxCNC you can home to index using
LinuxCNCs built in homing logic, if not you either cannot do accurate homing
or you must somehow integrate the motor drives built in homing procedure with LinuxCNC

PCW,

The drives do have an internal home limit switch functionality with digital output signals for in position ready. But I suppose it would be better for Linux cnc to know where everything was during the home process.. certainly something I had not considered. You mentioned in another thread I posted in yesterday (was an old thread) that pulse and feed back position control was possible, this would put Linux CNC back in charge of the homing I would imagine correct? Would position control with pulse be attainable with a 6i25, 7i76 and 7i77?

Thanks in advance!

Chris