Fig. 5.8Spin stand Operation (a) shows an image of a typical spins stand. The motor is mounted
vertically and the camera is mounted above it. A fibre-guided strobe light is mounted on a movable
arm. In this case the camera is mounted on an electrical positioning stage (for lateral movement)
and a manual positioning stage (to move closer to or further from the disc) (b) is a schematic of
how a low-cost photodiode and LED can be used to generate one digital pulse per disc rotation.
This option is suitable if the motor is not fitted with an encoder that can generate a pulse per
revolution (c) the typical signal that is generated at one pulse per revolution. Note that in this
approach the entire disc must be imaged and images are only acquired at a single angular position
(d) the typical signal generated by a two channel encoder. The‘home’channel (red) shows one
pulse per revolution while the second channel (green) shows four pulses per revolution. A filter
box, with a dial to input value‘m’, permits the angular position where the disc is imaged to be
changed. Where m¼0 the offset is 0while m¼1 results in 90offset. In a typical encoder the
channel resolution will be 1000 pulses/revolution and thus the angular resolution will be 0.36
128 B. Henderson et al.