SpotMicro: New leg with pressure sensor

The pressure sensor at the end of the leg will allow you to know:

If it hits the ground (Boolean True / False)
The force exerted on it
And at an instant "t" the direction of the force exerted (pressure // release)

Wiring

The wiring is very simple, the pressure is read on an analog port:

LegPressureSensor Library

Written for VSCode, just copy it to the ./Lib folder of your Plateform-IO project.

The Constants

const uint16_t CALIBRATE_NB = 500;
const uint8_t TRIGGER_PRESSURE = 10;
const uint8_t TRIGGER_DIRECTION = 5;

CALIBRATE_NB : Number of measurements when calculating the calibration value.
TRIGGER_PRESSURE : Deviation from the calibration validating a change of state.
TRIGGER_PRESSURE : Deviation from the calibration validating a change of direction.

Public Methods (functions)

//Constructor
LegPressureSensor(const uint8_t pin);

// Calculation of the calibration
void calibrate();   

// Pressure reading and state update
void read();

// Return the last measured pressure
uint16_t pressureValue();

// Return the calculated released pressure
uint16_t calibrateValue();

// Return the current max pressure
uint16_t pressureMax();

// Return the direction of movement
int8_t   direction();

// Returns True / False if pressed
bool isPressed();

LegPressureSensor (const uint8_t pin): The Constructor with the analog pin as a parameter, returns an object corresponding to this class.
calibrate() : carry out CALIBRATE_NB measurement afterwards then calculate the average. This method must be called with the paw raised thus allowing to have a reference.
read() : reading of the measure, UPDATE of the maximum value and the state of the tab, returns nothing.
pressureValue() : returns the value of the measurement performed by the read() method.
calibrateValue() : returns the calibration value calculated by the calibrate() method.
pressureMax()) : returns the maximum value exerted since the start of the pressure, it is reset to zero when the tab is released.
direction(): allows to know at a moment * 't' * if the movement continues towards a pressure (+1) a release (-1) or is stationary (0).
isPressed() : returns True if the pressure exerted is greater than u equal to the calibration + TRIGGER_PRESSURE, otherwise returns false.

Creation of "pear"

Just like the leg of Unitree's A1 robot, I set out to create a flexible "Pear".

First by 3d printing in filament ** FLEX ** (TPU), but the exercise is not easy because it is flexible and it is difficult to have a constant flow at the output extruder. After tests of different thicknesses, it remains too rigid and it tears very quickly:

I have not tested the TPE filament which is even more flexible because the experience with the FLEX was already complicated.

New Option : Silicone

The principle is to 3D print molds in order to "model" Silicone of bathroom joints:

The Silicone is first kneaded in water saturated with dishwashing liquid:

A few hours of drying and our part is ready, several thickness tests have been carried out:

Pressure sensor

Having no idea of the pressure differential in the "pear" between a pressed or released state, I tested 2 sensors with different accuracy.

I chose analog sensors because all of the I2Cs found had a fixed address, therefore incompatible with the SportMicro which has 4 legs, therefore 4 pressure sensors.

MPX4250AP

With an accuracy of 20 mV / kPa, this sensor is not sensitive enough, even when I blow into it to the maximum, the value varies only very slightly.

MPX5010DP

With an accuracy of 450mv / kPa, this sensor is great, I capture light pressures as well as large crushes:

DataSheet MPX5010DP

Modification of the leg

Work in progress

nao974 / spotmicro_new-leg_with_pressure-sensor Goto Github PK