The Strain Guage

Humanoid robots uses pressure sensors on the feet to balance themselves.
The following video shows an example of using pressure sensor to balance a robot.



A robot dog also uses pressure sensors to balance.



The horse shoe project may need a compact load cell with a strain gauge inside to measure the pressure of a horse stride to diagnose leg injuries.

Here is the example that uses the same strain gauge and load cells I have in the lab. Apparently, one cell has two strain gauges. You will need to add two 1k resistors to form a half wheatestone bridge.



A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.







Here is a video shows that the resistance change is small and the resistance changes with temperatures. So the quater bridge strain gauge circuit is not a good idea.



The resistance change is small so you need the Wheatstone bridge and an amplifier to make it readable. The following one is the module we are going to use in the lab.



Here is a snapshot from the datasheet that shows the connection.




Tasks:
1. Use ESP32, HX711, and two 1k resistors to build a pressure sensor (unit in kg). Read the data into the serial monitor. (50 points)
2. Send the data over to a second ESP32. (50 points)



Reference:
ESP32 with HX711: https://randomnerdtutorials.com/esp32-load-cell-hx711/