Prior Scientific ProScan III Manual page 83

ProScan® III Manual
The Prior ProScan stages and focus drives use high precision step motors. Generally, the
step motors used in Prior stages products are either 1.8 degree (200 steps per revolution)
motors or 0.9 degree (400 steps per revolution). The ProScan controllers provide a
bipolar chopper drive to the motors which allow for maximum torque, stabilization,
smoothness, and performance. As a simple example the motors receive pulses from the
controller which in turn causes them to rotate. Theoretically if the motors are operated
in a "full step" mode, one pulse from the controller will rotate the motor 1.8 degrees or
1/200 of a revolution (1/400 for the 0.9 degree motors). The example below uses the 0.9
degree motor; it is applicable to both motors. The ProScan controller microstep the
motors, this is a technique whereby the coil current in the motor is precisely controlled
to sub-divide the fundamental step angle (0.9 degrees) of the motor into a series of
smaller sub-steps called microsteps or pulses. The ProScan controller is capable of
creating 250 microsteps per full step of the motor.
Thus, for a focus motor (1.8 degree motor) attached to a microscope that has a fine focus
mechanism with 100 µm per revolution of the fine focus shaft, the system can achieve the
following resolution: (200 step/rev) x (250 micro-steps/step) = 50,000 micro-steps/rev
Hence, (100 µm/rev) / (50,000 micro-steps/rev) = 0.002 µm/micro-step Therefore, the
theoretical resolution of the focus drive motor is 0.002 µm/pulse.
For a typical stage (0.9 degree motor) with a 1mm pitch screw (1mm per rev or 1000 µm
per rev), the stage has a resolution as shown below: (400 step/rev) x (250 micro-
steps/step) = 100,000 micro-steps/rev Hence, (1000 µm/rev) / (100,000 micro-steps/rev)
= 0.01 µm/micro-step
For a typical stage (1.8 degree motor) with a 2mm pitch screw (2mm per rev or 2000 µm
per rev), the stage has a resolution as shown below: (200 step/rev) x (250 micro-
steps/step) = 50,000 micro-steps/rev Hence, (2000 µm/rev) / (50,000 micro-steps/rev) =
0.04 µm/micro-step
For a typical stage (1.8 degree motor) with a 5mm pitch screw (5mm per rev or 5000 µm
per rev), the stage has a resolution as shown below: (200 step/rev x
(250 micro steps/step) =50,000 micro-steps/rev Hence, (5000 µm/rev) /
(50,000 micro-steps/rev) = 0.1 µm/micro-step
© Prior Scientific Instruments Ltd Version 1.12 June 2017 Page 81