Eyepiece - absolute position sensing using optical detectors.

There are several ways of locating the relative positions of two mechanical components:

• Having a datum position (micro-switch, optical interrupter etc.) and calculating the position by measuring the number of pulses (opto-interrupter, motor steps etc.) taken to get somewhere.

• Using an ultrasonic measuring device (sonar)

• Using a laser measuring device (laser rangefinder)

• Using laser interferometry

• Using an optical scale and a series of sensors.

The first method is simple for the fine focus mechanism on the M55 microscope as this doesn't drift at all.

For the macro focus mechanism, using a datum and calculation doesn't really allow for the possibility of disturbing the set up, either while changing optics, swapping samples or drift due to the heavy sample stage that has to be braked.

Sonar and laser rangefinders are both relatively inaccurate and are both expensive and challenging to implement satisfactorily.

Laser interferometry is an excellent solution for extremely precise positioning, but is unnecessarily complex for this project. Interpolation between the 1mm marks of the scale will be done by counting stepper motor steps.

An optical scale can easily be designed and sensors may be recovered from such devices as old computer mice. This is the route I have settled upon.

Being something of a pack rat, I had half a dozen defunct mice of various types - most of which contained one or more optical interrupters (quadrature encoders for mouse ball position etc). These interrupters comprise an Infra Red LED and a dual Infra Red sensing Phototransistor - as two discrete components.

Unsoldering these is a simple task, yielding about 20 detector pairs.

How does an optical scale work?

Firstly, there are two geometries available - reflective and transmission.

The reflective detector shines a light (usually Infra Red) on a scale and the light level between white (reflective) and black (absorptive) represents a binary one or zero.

The transmissive system simply shines the light through a sheet of paper or plastic. Where the material is opaque, the light is interrupted and where transparent, the light is transmitted - representing the binary value as before.

By printing a grid pattern of black and white representing binary values from, for example, |000000| to |111111| (six bits), a position scale of zero to sixty three positions may be measured. 8 bits give a range of zero to 255. The rectangles on the scale (left) are 0.8mm high on a 1mm pitch, while the ladder on the far left has rectangles 0.25mm high at 1mm intervals - this represents the millimetre datum, and should ensure that the position detection is accurate to ±0.25mm.

Arranging for reliable reflective detection of these small panels is beyond most domestic electronics workshops. On the other hand, transmissive detection is as simple as positioning LEDs and detectors on opposite sides of a piece of paper and getting everything properly aligned. In theory, it should be possible, with the salvaged detectors, to measure down to 0.25mm, though this is rather more precise than the application actually calls for.

Precision - I may test out using a thinner mm scale ladder, or even go to 0.5mm resolution



The Electronics:

Testing the concept (as well as the individual devices) requires a simple circuit to be built. Because the sensitivity may need to be tweaked, I built this using breadboard with a suitable 5V power supply.

Optical position test circuit

The two phototransistors in the package are connected in parallel, giving a degree of redundancy.

The IR LED (D1) illuminates the phototransistors (PH1.1 & PH1.2).

When illuminated, they begin to conduct through R2, pulling the input of U1.n low. U1 is a hex inverter with Schmitt inputs (74LS14), which removes the uncertainty of when the device switches between .true. and .false.

The LED (D2) provides visual confirmation of the output state of U1.n.

The value of R2 alters the sensitivity of the input - values between 82kΩ ind 180kΩ will adjust between insensitivity and extreme sensitivity respectively. 82kΩ is about right for the test IR LED that I was using.

Bit(n) represents the output to the computer.

Choice of material:
Printing the scale using an ink jet printer resulted in a much finer line, but an almost unusable scale due to the relatively low opacity of the ink. Doing the same using a laser printer resulted in an excellent dark/light sensitivity. Both scales were printed on standard photocopier paper.

Colour Scheme:

The scale is actually designed as a white panel = binary zero. This is because a generally dark scale produces less glare (crosstalk) between channels. Because of the way the detector and Schmitt input operate, this translates correctly, resulting in the output being a binary number of millimetres from the datum (maximum elevation of the microscope stage).

Mechanical support:

A paper scale, while easy to produce, is rather too flimsy for this application, thus I will, in the near future, try the transmissive detector using a scale that has been put through a laminator, otherwise, supporting it with a clear acrylic panel will be necessary.

Computer Input:

I have a Quick2Wire interface that will connect to the Raspberry Pi, and, using the port expander, will allow a number of additional interfaces to be connected while providing some protection for the GPIO pins of the Pi.