Crawler Brainz

OK, got the Brainz installed.  The processor I used on this one is a PIC18F1788.  Its mission is speed control, and telemetry.  I need to be able to send it commands, and receive data from its sensor.  The speed control function is Pulse Width Modulation.  The PIC has a Capture, and Compare Module in it.  With the Capture, and Control Module you can setup a Pulse Width Modulation scheme to vary the Voltage that the motor gets.  Then to tell the Modulator what to do there is a potentiometer analog input.  This lets you send voltage information to the PIC, which it uses to vary the Duty Cycle of the Modulator, in turn changing the Voltage to the Motor, and changing the speed of The Crawler.

Crawler Drivetrain

Here is a closer shot of The Crawler's drive train.  I'm already looking at a motor with more power, LOL!  The trouble with toy motor is they are toy motors.  I need to move toward industrial grade, but that is usually a trade off with weight (bigger motor), and power (bigger battery), so I need to find the right one (Balance).  I'm going to move the power switch, install a potentiometer, and mount the PIC18F1788 Microcontroller board today.  Might even get it wired...

The Crawler

I spent a little more time working on The Crawler this weekend.  I managed, despite my effort at precision, to get the two shafts out of parallel.  The primary driven shaft is pretty straight, but the secondary shaft was out of parallel.  This means I needed to move one of the bearing mounting points to bring the secondary shaft into parallel with the primary shaft.  To do this I cut one of the frame rails, slotted the mounting holes in the top sheet metal, and then shortened the frame rail by 0.050 inches.  The modification is barely noticeable.

Also I found a battery holder that I can tuck under the frame rail that was modified.  This gets the battery off the top of The Crawler so there is more space for the microcontroller board, and test equipment.  Another thing that needed to be modified was the motor shaft.  It has a notch in the 1/4" output shaft, and the Pulley is round in its bore.  So a made a 0.060 inch thick piece from a 1/4" shaft to fill in the notch in the motor drive shaft.  This holds the pulley more squarely on the motor drive shaft, and helps alleviate periodic strain on the drive system.  The pulley is still kind of eccentric, and causes periodic strain on the drive system.  Its something I need to fix next.

Crawler Test Platform

I build a lot of test equipment.  This time I got a request to build a test platform which is mechanized, and moves very slowly.  I've built a few mechanized test platforms, but this is the first one that has synchronous all wheel drive.  Take a look...

The really slow motion is provided by a Worm Gear Motor, which you could think of as a clock motor.  It provides the spinning force which can be less than one revolution per minute.  The Worm Gear Motor that is on the Crawler now is 64 RPM.  I bought a couple more Worm Gear Motors also, 14 RPM, and 24 RPM.  I'm still working on the drive mechanism, and the goal is 0.1 Miles per Hour.  The overall speed has a lot of factors like the weight of the platform, and test equipment, friction through the drive train, the Voltage applied to the Worm Gear Motor, and even the friction of the tires on the surface.  With all of these factors I want the end speed to be 0.1 MPH, or 8.8 feet per minute.

1.5 inch Aluminum angle bracket, and a 5" x 10" piece of 0.625" Aluminum make the frame on the Crawler.  My starting point on this project was a robot platform with tank treads.  The treads were the first idea to get thrown out because they were hard plastic, and would do nothing but slide on a hard surface.  Then the frame of the robot platform was pretty irregular, hard to route the belts, so it got replaced with the Aluminum pieces.  This way all the mechanization is under the Crawler, and we have a large surface area for the test equipment on top.

I used Synchronous Gear Pulleys for the power transfer from the Worm Gear Motor to the wheels.  The Synchronous Gear Pulleys have teeth in them like a gear.  The Gear Belts have matching teeth which make this drive train Synchronous, meaning all four corners move at the same rate.  There is one belt that couples the Worm Gear Motor to the first drive shaft, and a second belt which couples the first drive shaft to the second drive shaft.  The length of the belt is determined by the distance between the shafts, and the outside diameter of the gear pulleys.  Two times the distance between shafts plus the outer circumference of one of the gear pulleys.  In this case the distance between shafts is 5.9 inches, and the pulley outside diameter is 1 inch, so a 15" Gear Belt was the one.

Now that I have built a Synchronous Four Wheel Drive I'll need to apply this to some other things.  For this Crawler Test Platform 0.1 MPH is the goal, but I can see the next one being a little more powerful.  The next step in this process involves Speed Control.  The one factor I have control of we can use to vary the speed of the Crawler is the Voltage applied to the Worm Gear Motor.  This motor can go from 6 to 15 Volts, and move the platform.  Previously I had blogged about using a PIC to do Speed Control using Pulse Width Modulation of a DC Motor.  That same control system will get used here to do speed control on the Crawler.  I'm also looking at using the PIC to get wireless communications to, and telemetry back from the Crawler.  The device we are testing sends data back to a piece of test software running on a computer.  So the crawling part of the Crawler is done, now I need to get to work on the brains...  Brainz!