Servo - August 2012

Virtual Sensors Virtual Sensors

Part 2 - Virtual Motor Interfaces

www.servomagazine.com/index.php?/magazine/article/august2012_Blankenship by John Blankenship and Samuel Mishal

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Some robot controllers have an integrated motor controller capable of powering small DC motors. When projects require servomotors or large DC motors though, most people assume the onboard controller is a waste — as you will see, it does not have to be.

Last month, we examined how the virtual sensor techniques we developed for our RobotBASIC Robot Operating System (RROS) can be utilized in on-RobotBASIC projects. This month, we want to do the same for virtual motor interfaces. When we began development of our RROS, we wanted it to be able to manage the robot’s drive system no matter what type of motors were used. Unfortunately, the hardware being used did not have enough I/O pins to handle all three of the motor options we wished to support. We also knew that any solution we came up with had to make each of the motor interfaces indistinguishable from each other to other modules in the RROS. We hope a summary of how we resolved our problems can benefit others in similar situations. Some robot controllers — such as Pololu’s Baby Orangutan — contain integrated hardware to directly control low current DC motors. When your robot is small enough to use such motors, this feature can be very cost-effective. Hobbyists that wish to use larger DC motors or even continuous-rotation servomotors to power their robot generally must use other I/O pins to create a serial interface to drive an external controller for larger DC motors, or to pulse the control lines of their servomotors. It would be great if the pins associated with the onboard motor controller could be used for FIGURE 1.

these purposes, but usually the pins interfaced with the controller are dedicated and not made available to the user.

Since the onboard motor controller used on most robot controllers is typically a commercial IC, most hobbyists assume that it can only be used for its designed purpose. Our RROS needed every available I/O pin, so we had to find ways to use the output from a DC motor controller to control servomotors and to communicate with external motor controllers.

We will use Pololu’s Baby Orangutan to illustrate our points. It uses an ATmega328P as its core processor and a TB6612FNG motor controller, enabling small DC motors to be powered without additional hardware. In general, a motor controller is just four transistors acting as switches that are connected in an H-bridge as shown in Figure 1. If we turn on switches (transistors) S1 and S4, the motor turns one direction. Closing S2 and S3 reverses the current through the motor and turns the motor in the opposite direction. Both motor leads are shorted to ground if S3 and S4 are closed, creating a brake condition due to the motor’s back EMF. If all switches are open, the motor will coast because both motor leads are effectively unconnected. The nice thing about using a controller such as the TB6612FNG is that it is easy to select the various states as described above. For purposes of this article, we will assume we have a Motor() function that can produce all of these states by passing it one of these parameters: FORWARD, REVERSE, BRAKE, or COAST.