Strain Gage – Tuning

Offset 50Ω Pot

Horizontal Dial

Red Arrow

3600⁰

(10 turns)

Clockwise:

Lowers baseline

Gain 500kΩ Pots

Vertical Dials

Green Arrows

270⁰ Clockwise:

Raises gain

Potentiometers are included to allow for tuning of the analog circuitry. The 50Ω potentiometer is included for balancing the bridge to adjust for variations in room temperature and the resistance of different wire lengths connecting the strain gage to the circuit. This is identified by the red arrow in the picture. The two 500kΩ pots together make up the signal amplification gain (green arrows). They need to be set to the same value for stable readings, so it is recommended that they be either maximized or minimized.

7

If it has been a couple days (or years) since MantisBot was last used, and strain gage readings are needed, the circuits should be re-tuned. This is done using the Arduino example sketch:   “AnalogReadSerial.” This sketch is in the “Basics” folder.  *****THERE IS AN ERROR IN THIS SKETCH THAT MUST BE FIXED.***** In line 19:

analogRead(A0)

^^^THIS IS INCORRECT^^^ and needs to be changed to:

analogRead(0)

The 0 can be replaced with whichever analog input (0-5 for MantisBot) is being tuned.

Ensure the gages are wired up appropriately (see wiring section) and upload this sketch to the MantisBot control board. Once uploaded, the serial monitor should be opened. For whatever reason, about ½ the time this fails and the serial monitor outputs horizontally scrolling jibberish. The other ½ of the time, this works, and readings from the gage (0-1024) will appear and scroll vertically. When the leg is loaded, these readings should increase. The setup is tuned by balancing the bridge via the 50Ω resistor such that a reading of 0 is achieved when the leg is not under load. It is important to note that a reading of 0 (or really anything under 10) is possible even if the bridge is not balanced, which represents a negative voltage (lower than ground) being read by the board. The best way to balance the bridge is to place the baseline well above 0, then slowly tune it down until it just reaches 0.