Concept of Operation

This section gives an overview of how the autotester works.

The autotester has several motors, but only one sensor.  That sensor is the camera and it performs various roles.  Unlike 3D printers which often use limit switches to determine the home position of the device, the autotester uses the camera.

When the autotester starts up, the first thing it will look for is two orientation dots.  These dots are part of the mixer stand assembly.  Due to manufacturing tolerances, two cameras off the same assembly line may have slightly offset images due to for example positioning of the CCD, alignment of the lens, etc.

Everything the autotester does is in relation to the orientation dots.  This removes the element of camera variability.  Once it uses the dots to determine its frame of reference, the autotester knows where to look to find the reagents, or the carousel letters, or the mixing tube, etc.

As was mentioned, the autotester has no limit switches.  It uses visual inspection to find the position of the mechanical components.  To figure out the rotational position of the carousel, it looks at the letters associated with each slot.  The plunger is outside of the camera field of view, however the stoppers are linked to the plunger, so by observing the movement of the stoppers, it can home the plunger.

The following actuators are used in the autotester.

  1. The mixer valve.  This opens and closes the bottom of the mixing tube.  It is normally open, but is closed when filling the mixing tube.
  2. The pump.  It pumps water from the source into the mixing tube.  The camera observes the tube and uses the pump and valve to get the right amount of water in the tube.  The pump also pumps in water to flush the mixing tube.
  3. The carousel stepper.  The purpose of the carousel stepper is to rotate the correct reagent into position over the mixing tube.
  4. The bottom electromagnet.  Changing the polarity of the bottom electromagnet causes a small disc magnet in the mixing tube to spin.  This agitates the solution.
  5. The top electromagnet.  This electromagnet is behind the frontmost reagent slot.  When needed, it can agitate whichever reagent is foremost.  Note that only reagents that need to be agitated should have a disc magnet inside.
  6. The plunger stepper.  The plunger lifts and lowers a threaded screw.  It has two functions.  As the plunger screw lowers, it pushes the piston on the foremost reagent, dispensing liquid.  As the plunger raises, it eventually pushes against the stopper assembly causing it to lift.  At the peak lift of the threaded screw, the stoppers are pushed against the reagents sealing the tubes.

In addition, to the camera, the unit has LEDs that are used to illuminate the unit. This allows a controlled source of light to use in measuring the color of the test results

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