5.1.2 Positional Triggering
If the motor generates just one digital pulse, at the home position, per rotation then
the disc can only be images in at one particular angular rotation. This is also the case
if a LED/photodiode based triggering system is used. In this configuration, the
entire disc should be imaged. The images acquired can be post-processed (rotated
and cropped) using commonly available software.
However, in some cases, particularly if imaging particles or cells, imaging the
entire disc is not feasible. In this case, the angular location of the digital pulse must
be modified so that the location of interest is imaged during each rotation.
Some motors have software based controls which allow the angular location of
the trigger pulse to be varied. An alternative approach is to purchase a motor with
an integrated dual-channel encoder. These can also be purchased and fitted sepa-
rately. These encoders generate one pulse per revolution in Channel 1 and a set
number of pulses in Channel 2 can generate a single pulse. A hardware based filter
can be applied which counts a specific number of pulses (in Channel 2) after the
single pulse in Channel 1 before passing a single pulse to the camera/strobe. Thus, if
four pulses are generated per rotation in Channel 2 the system can be rotated in
steps of 90while if 360 pulses are generated the system has a resolution of 1.
A third approach, when only a single channel is available, is to use a low-cost
microcontroller to measure the time between pulses and interpolate between them.
However, this approach is less reliable than hardware based timing and is particu-
larly susceptible to jittery imaging during disc acceleration and deceleration.
5.1.3 Filtering the Camera Signal
With the motor signal now filtered to one digital signal per revolution this must now
be used to trigger a camera. However, the number of digital signals generated per
second is equal to the disc spin rate (up to 100 Hz) while typically low-cost cameras
will only acquire ~5 images per second. Therefore the additional signals must be
filtered out.
The easiest method is to ensure purchase of a camera with a‘camera ready’
output. This digital output from the camera provide information regarding if the
camera is ready (or not) to acquire a new frame. This input can be used to filter the
incoming digital signals so that a trigger signal only reaches the camera if the
camera is ready to receive it. Alternatively, a time delay can be generated so that
only one trigger signal reaches the camera in a defined time period; for example if
the camera can acquire images at 5 Hz then one trigger signal can be passed to the
camera every 200 ms.
5 The Centrifugal Microfluidic: Lab-on-a-Disc Platform 129