Spring 2016: 3DoT board Motor Current Safety Test

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BY: kent Hayes (Electronics engineer)

Introduction:

The recent purchase of geared motors has brought to our teams attention that the current they output under maximum conditions might exceed the limit of 450mA. In order to conduct a test to prove this to be true or false, we just needed both motors to be run at maximum voltage (5V) under the most extreme condition (motor stall) and measure the current going into the TB6612FNG dual motor driver. Prior to doing this we thought it would be a good idea to measure the current from each motor under normal conditions(gear load), no load, and stall currents at different voltages.

Requirement:

  • The provide a comprehensive test of the robot’s motor current draw that demonstrates that the current will never exceed 450 mA (both motors operating) under all conditions (including stall).

Tables of Results:

  • PWM = signal being fed into the motor driver. Higher PWM means more voltage for the motor
  • Motor terminal voltage= voltage output at the motor terminal

  • No Load current = Current going into the motor driver when there is no load

  • Gear Load current = Current going into the motor driver with the gears in place

  • Stall current = Current going into the motor driver when the gears cause the motors to stall

 

Motor 1 Test Results

 

PWM Motor terminal

voltage

 No Load

current

 Gear Load

current

 Stall

current
255 4.93V 25mA 36mA 192mA
200 3.87V 22.1mA 33.8mA 145mA
150 2.90V 19.5mA 31.2mA 124mA
100 1.92V 16.5mA 29mA 108mA
50 0.95V 13.4mA 26.3mA 97mA

Motor 2 Test Results

 

PWM Motor terminal

voltage

 No Load

current

 Gear Load

current

 Stall

current
255 4.93V 21.4mA 34mA 183mA
200 3.87V 20.5mA 32mA 137mA
150 2.90V 18.9mA 29mA 111mA
100 1.92V 15.1mA 27.5mA 90mA
50 0.95V 12.8mA 20mA 86mA

 

Motor 1 and 2 Max Characteristics

 

PWM Gear load

current

 Stall

current
255 49mA 420mA
200 60mA 300mA
150 69mA 150mA
100 61mA 125mA

 

Conclusion:

Looking at the last table, you will see that while both motors are on and max PWM signal is being applied, the current  going into the motor driver increased to 420mA. While under normal operating conditions, the current being supplied to the motor driver was 49mA.