Difficulty level : 9. Mainly because of the required mechanical setup and precision. Especially the detection wheel unit is difficult to make. The underlying mathematics are not easy either.

This is the realization of a planimeter, according to the Planimeter theory.

The planimeter is digital in the sense that the surface area (in mm2) is indicated on 6 seven segment displays + one flat LED, representing a minus sign. I use a commercially available encoder from AVAGO.

The planimeter is equiped with a stilus that is illuminated with two white LEDs. The stilus is used to move around the area to be measured. The planimeter only integrates the path of the stilus while the stilus is pushed down. I also added a red Count active LED. Since this LED is not indispensable, it is not mentioned in the electrical scheme. It is connected with its cathode to pin DSUB4 and the anode is connected via a 470 ohms resistor to DSUB5. This means that the LED turns on if the stilus is pushed down, that is, when the counter should count. Releasing the stilus allows for a break while moving around area to be measured. The pen-down switch actually is a reed relay that is actuated by a magnet attached to the center part of the stilus.

The planimeter has an on-off switch and a reset switch, to reset the display to 0. The planimeter receives its power from a battery or via a connector by means of which an external power supply can be used.

The accuracy of this planimeter is better than 2%. You should perform at least two measurements to verify the repetitiveness (which is not the same as the accuracy, but it already gives you a hint). Use a square area to verify the accuracy.

Scheme

The dimensions of L, k and s are important and should be realized as accurately as possible : 0.1 mm. In fact, I provided for a mechanical adjustment possibility for the ratio of s and k. (See pictures below).

The external power supply is a DC power supply providing between 9 and 15 volts. I did not measure the current, but 500mA will do.

From the planimeter theory to the practical implementation requires some extra calculus. This is done in the document containing the scheme (see below).

PIC code

You can copy this PIC code and paste it into the PIC controller using the free MPLAB software of Microchip and e.g. a Pickit2 unit.

Pictures

Here is how it actually looks like.

It is very important that the wheel really rolls in the direction perpendicular to its axis and that it remains still when moving in the direction of the axis. Therefore, the underground of the wheel is important. I found out that cork or a rubber mat is a good underground for the aluminium transfer wheel (alloy 7075). The wheel should not slip with respect to the 5mm axis of the encoder. This is acomplished by pushing the upper part of the detection wheel unit down onto the transfer wheel, by means of a spring. Good, ungreased ball bearings are a must ! I used a piece of a 5mm drill for the axis of the encoder. In that way, this 5mm axis does not wear (HSS steel) and the accuracy of its diameter is 0.01mm. The diameter of the transfer wheel does not matter because if it rolls e.g. 10mm on the rubber mat, then also the perimeter of the 5mm shaft moves 10mm.

Provide anti-slip at the bottom of the main unit, so that the fixed point is really fixed.

The stilus has a central part that can move upwards and downwards. It is pushed out of the stilus case by means of a spring. The central part contains a magnet to activate the reed relais as the central part is moved into the stilus case. The reed relay is attached to the stilus case by means of a transparant shrinking hose. The Count active LED illuminates if the stilus is pressed down. The counter only counts when this LED is on. The reed relay is in fact the "pen down" switch in the electrical scheme.

This is the detection wheel unit. This unit should be constructed very carefully and accurately. The transfer wheel should turn effortlessly. The encoder readout electronics should be placed very accurately with respect to the Encoder wheel. Encoder wheel and encoder readout electronics come together (AVAGO).

Finally the electronics part with the PIC controller and the six 7-segment displays. The battery compartment contains velcro at its connector end. With this velcro, it is kept inside the case. The velcro allows the battery compartment to be pulled out of the case, to replace the batteries.