Coasting vs Power braking

Two means of braking are available for the AEV – coasting and power braking. Coasting refers to switching off the motor so that the AEV gradually comes to rest whereas power braking refers to reversing the direction of the motors so that the AEV comes to rest due to the opposing force of the motors. There are two factors that will be taken into account when testing the two methods:

  • Energy consumption – the greater amount of energy that power braking consumes relative to coasting
  • Distance in which the AEV comes to rest

The test needs to determine whether the extra control over the AEV’s motion afforded by power braking is validated by its energy consumption. The testing procedure involves accelerating the AEV from rest to 35% power for 4 seconds at which point, the code diverges for coasting and power braking.

  • Coasting

// Run all motors at 35% power for 4 seconds.
motorSpeed(4,35);
goFor(4);

// Brake all motors.
brake(4);

  • Power braking

// Run all motors at 35% power for 4 seconds.
motorSpeed(4,35);
goFor(4);

// Reverse polarity of all motors

reverse(4);

// Run all motors at 44% power for 2 seconds.

motorSpeed(4,44);

goFor(2);

// Brake all motors.
brake(4);

Test the AEV for both braking methods until results become consistent. Two graphs were plotted using the data collected from the AEV during testing – Power (Watts) vs. Time (seconds) and Power (Watts) vs. Distance (meters).

Graph 1. Power (Watts) vs Time (seconds)

The area under the curve represents the work done by the motor (J). Hence, the work done during power braking is significantly greater than that done during coasting. It can be concluded that power braking is less efficient than coasting.

Graph 2. Power (Watts) vs. Distance (meters)

The slope of the graph represents impulse or the change in linear momentum. There is a sudden change in impulse at 2.67m away from the initial position when the AEV reverses the direction of its motors in the case of power braking and stops the motors altogether in the case of coasting. AEV comes to rest in a shorter distance (approx. 1.5 meters) in the case of power braking as compared to coasting (approx. 2.6 meters). Hence, power braking allows the AEV to come to rest faster.

 

Therefore, power braking is favored over coasting due to the fact that it affords greater control over the AEV. Even though energy consumption is greater in the case of power braking, the control that it provides over the AEV’s motion is imperative in reducing the time taken for the final run especially when the weight of the carriage is taken into consideration. Furthermore, alternate means of propulsion are being investigated that only require one motor thereby reducing the energy cost and compensating for the extra energy lost during power braking.