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An Open Source Motor Controller for everyone

An Open Source Motor Controller for everyone
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By analyzing table  it is possible to understand the different operating modes more in depth, depending on how we steer the two inputs: IN1 and IN2. In the first case (IN1=H and IN2=H) both outputs will be in a high impedance state; in such a situation it is as if the motor was disconnected from the driver, and free to rotate. If on the other hand we consider the fourth case (IN1=L and IN2=L), it is as if the motor was in a short circuit state; what does it change between the two cases, in practice? If we use the motors to advance a robot that is moving forward at a high speed, in the first case, by depriving the motors of the power, we will obtain a gradual stop of the robot, that will halt itself in a space depending from its inertia. The stop will depend from the various frictions within the mechanical units and from those between the robot and the surface it is moving on. To understand what will happen in the second hypothesis, we need to consider that a direct current motor is a reversible machine, that is to say, if we supply power to it its motor will rotate, but if it is us to rotate the motor it will supply power, thus acting as an electric generator; in this second case, due to the armature reaction, the effort required to rotate the axle will depend on how much power is drawn from the ends of the motor’s windings.

By transferring this idea to our case, we may say that when the robot is moving we take away voltage from the motor, and the latter will continue to turn because of inertia, but if the bridge short-circuits its terminals, the current originated from it and within the winding will in turn cause an electromagnetic field, so as to oppose the cause that generated it; and since this cause is the axle rotation, the latter will be slowed down until it stops, while if lacking the current, the braking action will stop as well. If you try both cases, even only by making a wheel move by means of your hand, you will notice the effect in practice. In many commercial shields these two functions cannot be selected, while for our circuit we enabled the possibility to select (via software) the mode with which to use the outputs.



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The two remaining cases that have been described in previous table, on the other hand, are easier to understand: if we set the IN1 input at a low level and command the IN2 input by means of a PWM signal, we have the possibility to command the motor to go in the forward direction, with a power that is directly proportional to the duty-cycle value, which allows to regulate the robot’s speed (when the duty-cycle reaches zero, the robot will be stopped with the braking effect we previously described). If on the other hand we applied the PWM signal to the IN2 input, but we set the IN1 input at a high level, when the duty-cycle reaches its maximum value we find ourselves in the case of the high impedance outputs, and the robot will stop without any braking effect.

By inverting IN1 with IN2 we will have the same functions, but with the motor rotating in the opposite direction. You will see then how, by operating on the inputs, we have the possibility to choose the motor’s direction and speed, and to activate or not the braking effect.

The IN1 and IN2 inputs, that are the driver’s inputs for the first motor, and the IN1 and IN2 inputs for the second motor, are connected to four ATmega32U4’s PWM outputs, that can be easily manage by means of the analogWrite instruction that is available with Arduino’s IDE. The CN2 connector has been designed for the motors’ connection and for the power supply: on it both a male or female strip can be welded, plus a connector having screw clamps. We also allowed the possibility to read the VM supply voltage by means of a simple voltage divider; to make the motors’ supply voltage level compatible to the reading one of Arduino’s analog inputs, the voltage is reduced by 11 and thus made available at Arduino’s A0 input. Knowing the voltage allows, in the case of battery power being used, to avoid a complete discharge, that is to say, to avoid the over-discharge (which is always detrimental) when loading. Further Arduino pins depend on the CN2 connector, that is to say the remaining digital pins that have not been used by the driver, and three analog pins. In practice, it is as if we had an Arduino Leonardo board having a motor driver onboard.

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