Hello everybody.
Tonight I was reviewing the CogniSys Stackshot Owner's Manual (v1.5). At the top of page 16 the manual states that there will be a ticking noise if the carriage of the CogniSys rail assembly is driven into the end stops. Is this a feature of the rail, the motor, or the motor coupling? In my setup I will be driving a microscope focus block with a timing belt. Will I hear this ticking sound if I drive the focus block to it's hard stop? Will I damage anything? Will the belt just pop out of the pulley's groves?
Thanks.
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rjlittlefield
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Short answer: you will not hear the ticking sound, you may or may not damage something, and it's very unlikely that the belt will pop out of the pulley grooves.
Longer explanation...
The Cognisys StackShot rail is mechanically quite robust and does not have a huge mechanical advantage. It runs about 1.6 mm per turn of the motor (16 threads per inch of the screw). When the motor runs the carriage into a stop, the carriage simply stops moving, the screw stops turning, and the magnetic field inside the motor serves as a rather high torque clutch that slips when the rotor is blocked from rotating beyond a certain point. As the motor tries to advance, the phasing on the windings periodically causes the rotor to move away from its stop point, then back to it again, then away, then back, and so on. It is that ratcheting of the blocked rotor that causes the "ticking" noise.
When driving a microscope focus block, the fine focus shaft has a lot higher mechanical advantage, typically about 8X higher (0.2 mm per turn) or 16X higher (0.1 mm per turn) than the screw on the rail. In addition, the gears inside the focus block are quite a lot more fragile than the robust screw and nut used by the rail.
As a result, when the focus block reaches its stop, there's pretty much zero chance that the fine focus shaft will stop turning. Instead, depending on the particular block that you have, either (1) some part of the focus block mechanism will act as a mechanical clutch and prevent damage, or (2) there will effectively be no clutch and something in the focus block will break.
With all the focus blocks that I have, the outcome will be option (1). The tension control on the coarse focus knob will slip, and the coarse focus knob will turn slowly backward, just fast enough to keep the carriage stopped while the fine focus shaft continues to turn forward. However, with other focus blocks the outcome will be option (2), and something will break. There is unfortunately no way to tell which is which without trying it, and that's an experiment that should be done quite carefully and by hand, not under motor drive.
I hope this helps.
--Rik
Longer explanation...
The Cognisys StackShot rail is mechanically quite robust and does not have a huge mechanical advantage. It runs about 1.6 mm per turn of the motor (16 threads per inch of the screw). When the motor runs the carriage into a stop, the carriage simply stops moving, the screw stops turning, and the magnetic field inside the motor serves as a rather high torque clutch that slips when the rotor is blocked from rotating beyond a certain point. As the motor tries to advance, the phasing on the windings periodically causes the rotor to move away from its stop point, then back to it again, then away, then back, and so on. It is that ratcheting of the blocked rotor that causes the "ticking" noise.
When driving a microscope focus block, the fine focus shaft has a lot higher mechanical advantage, typically about 8X higher (0.2 mm per turn) or 16X higher (0.1 mm per turn) than the screw on the rail. In addition, the gears inside the focus block are quite a lot more fragile than the robust screw and nut used by the rail.
As a result, when the focus block reaches its stop, there's pretty much zero chance that the fine focus shaft will stop turning. Instead, depending on the particular block that you have, either (1) some part of the focus block mechanism will act as a mechanical clutch and prevent damage, or (2) there will effectively be no clutch and something in the focus block will break.
With all the focus blocks that I have, the outcome will be option (1). The tension control on the coarse focus knob will slip, and the coarse focus knob will turn slowly backward, just fast enough to keep the carriage stopped while the fine focus shaft continues to turn forward. However, with other focus blocks the outcome will be option (2), and something will break. There is unfortunately no way to tell which is which without trying it, and that's an experiment that should be done quite carefully and by hand, not under motor drive.
I hope this helps.
--Rik
