I just watched the Global Physics Department of Matt Greenwolfe showing his use of Scribbler 2 robots as physics education tools. See also Matt Greenwolfe’s blog, starting with The Robot Lab (formerly known as the Buggy Lab). It may be easier to get the content from the Matt’s blog posts than from the Global Physics Department recording, which had a lot of technical difficulties.
He did some modifications to the code for the Scribbler 2 to provide precise control of the robots (1 mm/sec velocity and 1 mm/sec/sec acceleration accuracy) and made some nice graphical interfaces for students to control the robots with x vs. t, v vs. t, and a vs.t plots (plus one interface that does 2D motion).
One big problem with the Scribbler 2 was the limitation to about 18.5 cm/s velocity, which is pretty slow. The cool thing about them is that they have wheel encoders that allow 0.491 mm resolution with 507.4 counts per revolution. One limitation that is a complete deal killer for me is that the Scribbler 2 library is only available for Windows machines, so porting to a Mac platform would be a major effort.
I was looking to see whether one could easily build such a robot from easily available parts. One cool new part is an integrated wheel, motor, controller, and shaft encoder called the HUB-ee (available for $35 from SparkFun):
The HUB-ee is a type of robot servo but designed for wheels, in fact it is a wheel, but it is also a motor, a sensor and a motor controller. What’s that? Did we just blow your mind?
When you want to add wheels to your robot you would normally start with a whole collection of parts: The motor and gearbox, a motor driver board, and maybe some sensors for measuring wheel speed and a controller to count revolutions or provide closed loop speed control. Well, the folks over at Creative Robotics thought it would be handy if you could just buy a wheel that had all of those things built in, so they designed HUB-ee – just bolt it onto a chassis, apply power and away you go!
The HUB-ee is easy to mount, too! There are two threaded inserts for M3 bolts built in, there’s also a right angle bracket included for situations when you can’t go horizontal into the chassis. The mounting holes are even LEGO® lug compatible!! HUB-ee uses Micro-MaTch connectors to keep electrical connections tight and easily changed, check out the related items for mating connectors.
The HUB-ee has a resolution of 128 counts per revolution of the wheel (1.473 mm resolution, 3× the step size of the Scribbler 2). The HUB-ee runs at 120rpm no load at 7v, which would be 37.7 cm/s, about twice the speed Matt reports for the Scribbler 2.
Although Matt reports 18.5 cm/s, the Parallax spec for the Scribbler 2 claims up to 80RPM, which would be 33.2 cm/sec, but that is probably with a 12V power source, rather than 6AA batteries. I suspect that Matt’s need for very precise control and operation with batteries limited the top speed he could use. He does say that he would have liked a voltage controller (which would have added a $3–15 part cost to the robot, so a $7–40 increase in retail cost, based on the designs by TI’s WebBench tool or the PTN78000W module from TI) in order to have better speed control without having to worry so much about keeping the batteries fully charged.
The Hub-EE takes up several pins on a microcontroller (1 PWM pin, 2 output pins to control direction, 2 input pins for the quadrature encoder feedback) in addition to power and ground. Two HUB-ee wheels would cost $70 and use 10 pins on a microcontroller—doable with an Arduino, but not leaving a lot of pins for other functions like sensor inputs. There aren’t enough interrupt pins on the standard Arduinos to have all 4 wheel encoder pins triggering interrupts (which would be the highest-precision way to use the feedback information to get precise motor control).
Internally, the HUB-ee wheels use a Toshiba TB6593FNG motor controller, an H-bridge designed to work with 1.0A average current, with an on-resistance of about 0.35Ω for the output low. The Toshiba data sheet doesn’t give the on-resistance of for high voltages directly, but if I’m interpreting their “Vsat” parameter directly, the on-resistance for each leg is about 0.5Ω, about a sixth that of the popular L293D H-bridges. At under $3 a H-bridge (in single units), the TB6593FNG does not look like a bad choice for a small H-bridge.
Of course, to use the HUB-ee, one would have to build the rest of the robot (chassis, microcontroller, battery, … ). The Hub-EE is designed to mount to Lego beams, which could make chassis building easy, at least for prototyping.
I wonder whether, in a couple of years, we’ll be seeing integrated wheel units like the HUB-ee with an SPI interface, with registers that say how many steps to make, with specified velocity and acceleration curves. That would provide very simple interfacing with fewer wires and could allow much tighter servo loops, at the price of putting a microcontroller at each wheel (probably adding $10 to the retail price of the wheels).