COMMUTATOR BASED DIRECTION INDICATOR

I built a 'Anemometer and Wind-Vane Wind-Speed and Direction Logger' as part of a RoadTest on the Texas Instruments MSP-EXP430FR4133 LaunchPad development kit and the ADS7042 Ultra-Low Power Data Acquisition BoosterPack.

While implementing a method that required a potentiometer to measure wind-direction and rotation of a DC-motor to measure wind-speed I came across a critical requirement that the shafts of both sensors needed to be co-axial.

I present here a novel method of creating a one-wire potentiometer based on reuse of a small universal-motor commutator-brush assembly. This approach has the important advantage that the central core of the commutator-shaft can be hollow permitting an additional shaft to be co-axially located within this hollow core. However, because of the limited number of commutator segments it provides indication of only a fixed number of discrete angles.

This method is based on separating only the commutator and shaft from a motor assembly and then soldering fixed resistors between commutator segments forming a ring of resistors. If one end of this ring is grounded and a commutator brush used as the other contact we get a resistance variation with rotation angle. This angle can be measured using suitable electronic circuitry.

I purchased a burnt out electric-Mixie motor assembly for INR 75 ( ~ 1$). The main criteria I used in selecting a suitable assembly was to look for something compact, with a large number of commutator-segments and a fairly undamaged commutator surface and brush assembly.

The assembly had 24 commutator-segments which would permit indicating wind direction in angles of 360/24 or 15 degree steps.

The shaft was around 8 mm (5/16 ") which would permit drilling of a 5 mm (3/116") hole through the central core.

I first dis-assembled the Mixie and separated out the armature-commutator-shaft portion. Then while holding one end of the shaft I used hand-tools available with me to separate out the commutator and shaft.

After rough cleaning-up I used a multi-meter to check that there was no short between commutator segments and no connection from any commutator to the shaft.

The parts were now ready for drilling out the central core from the shaft.

I used an ordinary bench-vice fixed with a c-clamp onto my drilling machine base and created a vertical jig for drilling out the central core of the shaft.

Starting from one end I used two drill bits in sequence to carefully drill first a 3 mm hole and then enlarge it to 5 mm. The depth was approximately 75 mm ( 3"). The results were not perfect as the hole was slightly off-center at the exit end.

Finally I used only one of the bearings of the original assembly holding down the assembly using a suitably drilled aluminium plate.

I fixed the commutator shaft on my drill machine and cleaned out the commutator using a strip of emery paper.

Soldering 1 kilo Ohm SMD resistors in 0805 package was quite tricky as the copper commutator segments quickly conducted the heat away from the soldering iron tip without allowing it to melt the solder.

I overcame this by placing a higher-wattage soldering on the main shaft to preheat the whole mass and then soldered the 1 kilo Ohm resistors between segments with a second lower-wattage soldering iron.

I soldered the resistors at the top end of the commutator so as to leave the rest of the portion for making contact with the brush assembly.

The resistors are soldered in the form of a ring with no-contact at the 24th segment and on-end connected to the shaft. This forms a 0 to 22 kilo Ohms variable resistor between the shaft and the commutator segment under the brush-contact.

I found that the original brush-assembly based on a providing good-contact using a small coil-spring was creating excessive friction while rotating the shaft. I modified the brush assembly to a leaf spring type using a contact spring from an old relay.

The final assembly permits another co-axial shaft through the central core of the commutator shaft which is used to rotate a small DC-motor reused from a CDROM drive.

The commutator-brush assembly with the 1 kilo Ohm resistors soldered between segments acts as a 0 to 22 kilo Ohm variable resistor which is effectively a potentiometer.

The brush contact is engineered to form a make before arrangement between two commutator segments.

So as to minimize the connectivity to this potentiometer I used a one-wire scheme.

A current source formed by transistor Q1 and IC1A inject a current of 0.1 mA through the brush-contact and selected resistors in series to ground. The voltage hence varies in steps of 100 mV from 0 to 2.2 V.

This voltage is measured by the ADC of a micro-controller and converted into angle information for display and logging.