Homing repeatability of ASI stages

[makarov]

Apologize if this is off-topic. I've noticed several people are using Applied Scientific Instrumentation translation stages.

Dows anybody know what is homing repeatability of MS-2000 and LS-50 stages (i.e., repeatability of the absolute positon when it hits the limiting switch used for homing)? The manufacturer does not list this parameter for these models.

These stages with an OEM control unit are available cheaply on Ebay, without the linear encoder option. I am considering them for a non-microscope application that would require repeated setting of the same absolute position over weeks, when the setup can repeatedly be powered down, stages jerked perhaps rotating the lead screws a bit, then powered up, re-homed and moved to a previously recorded position. ASI does not seem to be very keen to answer my questions because they are not selling these OEM setups. However, it seems the stages in them are their standard ones.

makarov wrote

Apologize if this is off-topic. I've noticed several people are using Applied Scientific Instrumentation translation stages.

Dows anybody know what is homing repeatability of MS-2000 and LS-50 stages (i.e., repeatability of the absolute positon when it hits the limiting switch used for homing)? The manufacturer does not list this parameter for these models.

These stages with an OEM control unit are available cheaply on Ebay, without the linear encoder option. I am considering them for a non-microscope application that would require repeated setting of the same absolute position over weeks, when the setup can repeatedly be powered down, stages jerked perhaps rotating the lead screws a bit, then powered up, re-homed and moved to a previously recorded position. ASI does not seem to be very keen to answer my questions because they are not selling these OEM setups. However, it seems the stages in them are their standard ones.

Hi there,

I don't know how much help I can be directly, but using microswitches for home/limit switches on small CNC machines is commonplace and the repeatability is generally very good. It is possible to test this yourself by writing a loop that will repeatedly reverse off the switch a known number of steps/known distance then drive the stage back into it, recording the position at which the switch trips each time.

I suspect if you set up your homing procedure well, you would be able to get a very good result. The key is to do the final homing slowly so that the electrical switch bounce is orders of magnitude shorter than a single step/position measurement cycle of the motors/servos/controller hardware. You can opt for a faster coarse homing procedure, then back off the switches and do it again slower.

If your application requires better repeatability than that of the existing switches, either opt for the encoded versions or fit your own home sensors if this is possible. Most microswitches whilst not identical across many units, are very repeatable at their own switch tripping point and the degree of the system's overall repeatability is directly proportional to the speed of the homing procedure.

Alternatively, you could optically align the setup with a mark on the stage and a software and image based homing routine.

I hope that helps,
Best,
Ed
--
[e-mail]
www.esimaging.co.uk

[makarov]

Thanks. These stages use Hall switches and magnets (Z comes with fixed magnets, while the XY stage allows the user to reposition the magnets). We have tested coarse homing with a micrometer to read the position: it gives roughly 20 um range homing scatter on all three stages. We haven't implemented yet the slow homing part, but may do it if this becomes important.

An alternative for us is to buy standard translation stages with actuators that guarantee better homing repeatability out of the box (e.g., most Newport stuff), but they are more expensive than this relatively cheap used microscope stage.

[Gary Rondeau]

Our stages with linear encoders can be homed "to the encoder count" using the index on the linear encoder scale.  This is accurate to the 10nm encoder count.
For stages with just the rotary encoder, users can use the limit magnets as a position reference.  It is recommended that the limit be approached at a slow speed for best repeatability.  Typically you can get <~ 10um accuracy from the limit magnets.
The MS2000 controller will remember stage positions through power cycle.  On power down, motor power is cut and encoder positions saved.  The reverse happens on power up.  As long as the stage is not physically moved when the controller is not powered up, the stage will correctly remember its position through a power cycle.

Gary


> Thanks. These stages use Hall switches and magnets (Z comes with fixed
magnets, while the XY stage allows the user to reposition the magnets). We
have tested coarse homing with a micrometer to read the position: it gives
roughly 20 um range homing scatter on all three stages. We haven't
implemented yet the slow homing part, but may do it if this becomes
important.

An alternative for us is to buy standard translation stages with actuators
that guarantee better homing repeatability out of the box (e.g., most
Newport stuff), but they are more expensive than this relatively cheap used
microscope stage.

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