CHRPbot materials list
- 1 - 1/2" MDF base
- 2 - 6V DC motors
- 1 - 3/32" dia. X 7.2" music wire axle
- 2 - 1" diameter dowel wheels, 1/4" wide
- 2 - 7/8" I.D. o-ring tires
- 4 - 4-40 nylon nuts
- 2 - 1" pieces of double-sided foam tape
- 2 - plastic cable ties
- 1 - Magic Slider™ chair glide
- 7 - #4 X 1/2" tapping screws
- 1 - 4-AA battery holder (BC4AAW)
This base is sized for the CHRP boards. It's easy to scale the design up or down for larger or smaller controller boards.
CHRPbot - A simpler robotics platform
You've probably seen robot bases made using expensive gear motors or complex servo motors. While these robot bases are good solutions for many applications, the CHRP robot base has been designed for what matters most in a classroom: simplicity, low cost, and easy assembly by students. Using this platform, your students will spend more time learning about electronics and programming, and less time building and tweaking mechanical things.
How did we get here?
We tried a lot of things. Gears are expensive. Making your own gear train requires really precise manufacturing. Servos need pulsed drive signals. All these things meant students spent more time trying to get their robots to roll, rather than programming them to do neat things while they are rolling.
What's so good about this robot platform?
First, it uses simple, commonly available DC motors (we tend to buy lots from surplus electronics stores), and readily available or manufacturable parts so it's inexpensive to build.
The CHRP boards contain the L298D quad half-bridge driver, which makes it really easy to make the DC motors go — just set a PIC I/O port with the value you want and your motor is running. This is much simpler than timed pulses for servos, and walking pulse-trains for steppers.
The friction contact between the motor and o-ring wheel provides both gear reduction and, similar to a clutch, the ability to 'break free' when robots hit obstacles. No broken gears, and the elegant design relies on a certain amount slop (unlike the precision required to make gear trains) so it's really easy to build.
Making a CHRPbot
Getting started - Gather the parts
All the parts needed to make a CHRP bot base are laid out, ready for assembly.
The CHRP bot MDF base has had an axle slot sawn across it using a table saw.
A typical saw kerf (the width of the saw blade, and, the slot) of 1/8" will leave enough room for the 3/32" axle to float in the slot. Cut the axle slot to a depth just over half the thickness of the MDF. The opening of the axle slot will be on the bottom of the MDF base.
Mark the hole locations before drilling
Flip the MDF platform over so that the axle slot is at the bottom. Position a CHRP board near the front (farthest from the axle slot) and mark the locations of the corner mounting holes, floor LED, and phototransistors.
Centre the battery holder at the back of the board, lined up with the rear edge, and mark the locations of its mounting holes.
Line up a motor over the axle slot and mark the locations of two holes, one on either side of the motor, roughly centred along the motors length.
Mark a hole for the Magic Slider about 2cm back from the LED and phototransistor holes.
Drill all the mounting holes
Drill the CHRPbot base with the hole sizes required to mount the circuit parts and motors.
Use a 1/16" drill bit for the circuit board, battery holder and magic slider mounting holes.
Use a 1/4" drill bit for the motor mounting holes, and for the floor LED.
Use a 3/16" drill bit for the phototransistor silos. Be sure not to connect the phototransistor and floor LED holes.
Use a 7/64" (or 1/8") drill bit to drill the dowel wheels.
Mount the motors using double-sided tape
Double-sided foam tape will prevent the motors form sliding when they are fastened to the MDF base using cable ties.
Apply double-sided tape to one motor and centre the motor over the axle slot.
Fasten the motors with cable ties
Insert a cable tie from the top (motor) side of the board, so the cable tie fastener is above the motor.
Make sure to tighten the cable tie tightly so that it lies flat across the bottom of the base and there is no bulge sticking out.
Once the cable ties are tight, you can cut the excess sticking out of the fastener off, or leave it for decoration!
Attach the Magic Slider™ and prepare the axle
Screw the Magic Slider to the bottom of the platform.
Round off both ends of the axle using a file to remove any sharp corners. The wheel nuts grip the axle through friction, but sharp axle ends will damage the interior of the nylon nuts.
Before assembling the axle, you may want to stretch the o-rings over the dowel wheels. Be careful not to twist them or roll them on, as they'll just pop off once the robot starts rolling!
Once the axle ends are smooth and round, install the first wheel by pushing the axle through a nylon nut. Next slide a wheel over the axle, and use a second nylon nut to keep the wheel from coming off. Insert the axle into the axle slot of the base, and attach a wheel to the other side the same way.
Close-up axle detail
Here's a close-up view of the axle and wheel assembly.
Left to right, there's the robot base, the inside wheel nut, the dowel wheel with o-ring tire, and the outside wheel nut.
Notice that the plastic cable tie is pulled tight, and doesn't extend far down below the robot. You can see the slider touching the table up front.
When assembling the CHRPbot, make sure there is about a millimeter of gap between the robot base and the inside nut, as well as a millimeter of play for between the wheel and wheel nuts. The axle should float up and down in the axle slot, and each wheel should turn freely.
Mount the battery holder and CHRP board
Screw the battery holder on to the back of the CHRPbot base, and mount the CHRP board to the front.
An important thing to consider is the tightness of the screws holding the CHRP board down on the front. If they are screwed down too tighly, the CHRP board will warp since the solder connections extend below the board. Tighten the CHRP board mounting screws only until they touch the circuit board.
Once the CHRP board is attached, install the floor LED and phototransistors from the bottom (if you're making a line-following robot) so that just the curved semi-circle of the end of the casing extends through the drilled holes. Solder their connections on top.
An almost-complete CHRPbot
The only thing that's left to do is solder wires onto the motor terminals and attach them to the CHRP board.