Sensors for no load condition Trade-Off Study
By Jose Alcantar, Electronics and Controls
A set of different types of sensors were considered for detecting no load conditions on the Pathfinder, these include:
Current Sensors: These type of sensors allow the user to monitor the current draw on each of the motors.
Flex Sensors: This type of sensor measures the deflection caused by bending of the sensor.
Pressure Sensor: This sensor typically measures the pressure of gases.
Micro switch: This type is a switch that is actuated by physical force
Each of these types of sensors presented both pros and cons when deciding on which to implement on the pathfinder. Starting with the flex sensor, the proposed idea was that as the pathfinder was traveling the suspension system would move and bend as the rover drove along. As soon as one of the wheels came off the ground, the flex sensor would detect this and shut off power to the motor. The biggest problem in implementing this idea is that the rocker bogie system does not bend while it is driving. This makes the sensor in the field unreliable.
The idea with the pressure sensor was that the pressure in the tires would be measured. This would be beneficial if the rover had air-filled tires, which would detect when the tires would come off the ground. With the new design of the rover, this would become an issue due to the use of foam-filled tires.
Similar with the flex sensor, the micro switch would detect any force on the sensor if the suspension lifted a tire off the ground. The problem with this is with the design of the rover, there would be no suitable areas to mount the sensor on the pathfinder.
The best option for the rover were the current sensors. Due to the current draw of each of the motors, a current sensor can be wired in series with the motor to measure the current. Of the different types of current sensors, two were selected as the best options. The two options were the use of a 0.51Ω current sensing resistor and the Adafruit INA219.
When considering the two options the main differentiator between the two was the cost. The Adafruit INA219 has a total cost of $9.95 and multiplying by six (one for each motor) the total comes out to about $60. The shunt resistor has a cost of about .56 cents each, the total coming out to being about $3.50. Another benefit of using the shunt resistor is the fact that the motor shield being used has a pin output specifically for the use of current sensing resistors. Ultimately, the shunt resistor appears to be the best option due to the cost, the easy implementation, and least use of pins.