Question about XY Stage Stats

[dcharlot]

Hello my is David Charlot and I'm trying to use a NI 7344 motion controller and a MID-7604 motor driver to control a microscope XY stage. I want to make sure I am controlling the stage correctly. The manufacturer told me that the stage has a 2mm pitch lead screw and has a linear encoder resolution of 2nm/pulse. I don't know how that translates to MAX's  steps/revolution and encoder counts per revolution. Any help would be most appreciated. Thanks.


David C.

[Brian_Beal]

That is an unconventional way to state that.  A 2mm pitch screw means that your stage moves 2mm/rev.  That means the encoder will put out 1,000,000 pulses per revolution.  That doesn't seem right, as it is quite high.  You are still missing something.  It appears you are using stepper motors to drive this.  I have never used the MID 7604, but there will be a way to set the microstepping resolution.  Match that with the steps/revolution setting.

[biller]

The breakdown of these stages goes something like this:

The stepper is most likely a 1.8 deg/step standard type.  The encoder will have XXX pulses/revolution and is connected to a second shaft coming directly from the motor.  While the stage has a 2mm pitch (2mm travel/revolution of screw), this gives XXX pulses pre 2mm of travel.  There is probably a gearing ratio, most likely a worm gear.  Worms ratios are usaully in the range of 20:1 to 100:1, giving a XXX*100pulses/(2mm travel).  With the 2nm/ pulse reported by the stage supplier,  and assumin a 100:1 gear ratio, the encoder has XXX=10^4 pulses/rev.  This is incredibly high.  I have never heard of anything higher than 2000 pulses/rev.

I suggest contacting the supplier and asking for specs on the encoder itself, which will be reported in pulses/count ( or counts/rev), not in length/pulse.  This is the number you specify in MAX.

For the steps/rev:

MAX will read the microstep setting set on the 7604 and will assume a standard 1.8 deg stepper, which is 200step/rev.  So, if you set the microstep on the 7604 to 10x, the 7344 controller will read 10*200=2000 steps/rev.  Now, if you want to include gear ratios, you must manually multiply it in MAX.  Normally, this is not done, since max velocities are specifed at the motor.  If you use a rotation stage, then you might want to multiply it in.  It's a matter of taste.