So I started experimenting with wheels and motors, which led to Hawi: a holonomic drive platform, which instead of using omni-wheels, simply rotates all four wheels completely independently.

This is achieved by mounting each wheel on a servo.

Of course Hawi is a work in progress, I expect to upgrade different parts as needed, just like I did with my robot arm.

Here is a video showing Hawi in action:

Main components

An Asus Eee PC Netbook is the main brains. It runs the software that commands the microcontrollers and servos. It’s pretty basic for the moment as just uses the keyboard for commands. It’s not very pratical to chase Hawi and type on his keyboard while it’s moving. So I use VNC and control it from another laptop.

Under the belly:

• 2 Arduino Duemilanove controllers are used to drive the h-bridges and handle back-emf. Each Arduino handles 2 DC-motors.
• A PhidgetAdvancedServo 8-Motor is used to handle the 4 servos that control wheel direction.
• 4 L298 H-bridges from Solarbotics: one per DC-motor. Although each L298 can handle 2 DC-motors, the way I wired back-emf only allows me to control a single DC-motor, as explained here.
• Power is provided by a 12V sealed lead-acid battery.

Wheels, motors and gearbox are all from banebots:

• BaneBots Wheel, 2-7/8″ x 0.8″
• 64:1, 36mm Planetary Gearmotor
• RS-540 Motor

The wheels are mounted to the chassis through a 3″ lazy susan, and you can almost see the HS-322HD servo under the wheel.

Going forward

Mechanically Hawi can handle pretty heavy weights. The wheels are mounted to the platform through 3″ lazy susan bearings, and both ends of my motor shafts are supported by bearing blocks.

I’m turning the wheels using cheapo HS-322HDs, they work for now and were even able to turn the wheels on carpet. I may have to move to something with more torque in the future though, or maybe get rid of the servos entirely (but that’s a whole other subject).

As far as electronics are concerned, my h-bridges are going to be my first limiting factor. I’m currently using L298s, which are limited to 2 amps. This is far below the stall current of my motors. So far I’ve been able to get away with it by adding heat sinks. The good part is that they are cheap to replace if I smoke them

But for now software is main area that needs to progress. Using VNC to remote login into the netbook was a quick but limited solution. Hawi’s software needs to have its own client-server code.

And of course Hawi needs to become more autonomous, by receiving high-level tasks and figuring out how to accomplish them, instead of the forward/backward commands it handles now.

First, what is back-emf exactly?

When you apply voltage to a DC motor, it turns. But also when the DC motor is turning, voltage appears on its pins, like a dynamo. This voltage is the back-emf I’m using here.

The back-emf voltage is proportional to the speed at which the motor turns, so it can be used to measure the motor speed. But this can only be measured when the pins are not already supplied with voltage from the h-bridge. So we need to “cut power” for short time intervals to do the measurement. For more details you can visit this site.

The controller I used is an Arduino Duemilanove and an L298 based H-bridge from solarbotics.

Here is a schematic of the circuit I used:

You’ll notice the h-bridge has one enable pin (E1/E2) for each input pin (I1/I2). I need separate enable pins when I measure the back-emf, because one output has to be set to ground, while the other is on hi-Z.

On most h-bridges I1 and I2 share the enable pin, to get around this limitation, I use only separate half  h-bridges. This way each half has its own enable pin. The L298 has 2 separate h-bridges, so I can use both of them to control a single DC motor.

Both motor wires are connected to analog input 0 through identical resistors. But since one of those 2 wires will always be set to the ground (exactly which one depends on motor direction), the other wire will receive the back-emf voltage from the motor. So effectively the three resistors are a voltage divider by 3. Since the theoretical maximum back-emf voltage is 12V, the analog would see a maximum of 4V, which is fine for its 5V range.

When the motor is running, the arduino will run the motor for 100ms, by setting one output to the ground and the other to a PWM signal.

When the 100ms are up, the PWM output will switch to hi-Z for 4ms: first 2ms to allow transient currents to flow out, then another 2ms to measure. The average voltage taken during those last 2ms is then compared to our target voltage. Then PWM signal driving the motor speed is increased or decreased depending on the difference between that average and the target voltage.

Arduino code I used for this project is available at http://code.google.com/p/robidouille/

There is room for improvement in the algorithm.

• the 2ms wait for transient voltage to dissipate is a maximum value. If we autodetected the end of the transient spike, we could start sampling the input voltage sooner
• the 2ms measuring and averaging voltages could also be shortened
• shortening the 4ms during which the motor is not driven, but back-emf voltage is instead measured would allow us to measure more frequently, and react more quickly to changing conditions.

While these 3 points are valid, I’ll wait for the motors to be mounted on an actual platform to make tweaks that are really necessary.

Here is the video showing back-emf in action.

Visit the Robidouille website for the latest updates to this project

Why not take an existing platform? Well for one, it takes half the fun away! But also it allows me to build something tailored specifically to my needs, or that can be upgraded should those needs change.

As a first step, it seems logical to start with the wheels. To keep things as upgradable as possible, I built separate modules for the motor and the wheel. This allows me to upgrade one module while affecting the others as little as possible.

Here is the motor module:

Motor module

And the wheel module:

Wheel module

They are designed to be assembled together like this:

Motor assembly

As you can see, I don’t have any encoders. I’ll use back EMF, to control the rotation speed of the wheels, like this:

I’ll post more details on back EMF next time.

For more details on my projects visit: http://www.robidouille.com