Monday, February 11, 2008

smells like roundabout, looks like squareabout

My second robot was again based on a design by David Cook. This time from the "Intermediate Robot Building" book. This robot was called Roundabout (as it was rounded in design to help it brush past objects) and is an object avoidance design. It uses two IR detectors to determine if an object is in it's path and uses a motor control setup to steer out of the way. The design in the book is in two parts. The first only using logic chips, the second part adds a daughter board with a microcontroller to improve the avoidance abilities.

Again the book goes into a quite detailed analysis of the various components used including IR detectors, motors, motor driver chips, chassis manufacture, etc. Rather than just present one solution for most sections of the robot (power supply, motor control, sensors, control) various alternatives are presented and critiqued. This is superb as it allows you to not only build the robot but also understand the design decisions made. This is normally lack in most other robot building books I have read.

In the end I used a 7805 voltage regulator as the core of the power supply, an SN754410 motor controller chip and the presented IR signal generation and detection circuit. I had plans to do the daughter board but that ended up as a separate robot (Trackbot, details later). The motors were recycled off Sandwich who to this day still is without wheels. Power is 6x1.5V AAA batteries. Originally I used the same type of plastic lunch container as a body by then made a new body out of a casing of a CDROM drive. Hence Roundabout became Squareabout.

Squarebot in all it's glory

For the CDROM body I first gutting everything, just keeping the case itself. The case is quite good in design as there are four screws keeping the top of the case onto the body. Also the sheet metal is quite easy to machine (but less forgiving to mistakes than plastic). Disadvantages is the weight. Not an issue for the motors (as 9V they generate approx 1.5kg torque and the whole robot weights 650gm) but it does reduce the battery life. This was also a test of me being able to machine with some precision a body. However the chassis design isn't great for the task as the exposed wheels and square body means Squareabout gets caught on things. Also as the new case meant the various parts of the robot were closer together the cables are too long. I didn't bother to redo them as the chassis rebuild happened some time after the original robot was done.

The result? Squareabout goes ok, but the range on the IR emitters is quite under what is described in the book. Most of this is due to the fact I couldn't get the correct parts and the alternate parts worked, but not as well. But Squarebout still avoids objects, trundles along at a slow but steady pace and has so much torque it can climb up walls. The motor chip is quite good as it is dual voltage. 5V for the logic part of the chip and any voltage over 3.5-4V for the motors. So I ran a direct line from the 9V power supply to run the motors. All the logic chips are powered by a regulated 5V off the 7805. I found the cutting of motors from one direction to another generate a large amount of interference with the IR circuit. Originally the design used a single 9V battery but the motors drained that out two quick so I swapped to the 1.5V batteries (and started using rechargeable batteries to save money).

Since I never had time to sit down and do Squareabout in one hit I made it out of modules. Two ideas were behind this. One being that I could recycle the modules into other robots. The second being that I could compete a module in one day, this getting things done. There are three modules in the end. Power, motor control and finally detection circuit and logic brain. I used Molex connectors for all interconnections between modules. All wiring was point to point, ie by hand not using a PCB. It used predrilled blank prototype circuit boards.

The power module as stated previously is based around a 7805 voltage regulator with various capacitors to filter the output. There is a diode to provide reverse battery protection and a power switch. There are both unfiltered battery voltage outputs and 5V filtered connectors.

The power module with the 7805 in the center, connectors on the sides.

The motor control module is based around the SN754410 chip. Good dual H bridge chip capable of driving two motors in forward and reverse. Various power supply capacitors were added to the chip.

The motor chip.

The sensor/detection circuit/logic brain is straight out the book. Nothing that fancy.

Detector circuit with IR emitters and detectors out in front.

Finally a shot showing the motors and battery pack underneath the top board. This had the advantage of providing some shielding the IR circuit from the EMF off the motors which did cause some issues esp. when the motors are cutting from forward to reverse (or vice versa).

Battery pack bolted to the top plate, Sandwich motors below. Garage in the background :)

1 comment:

Calvin Tennant said...

I'm doing a project for school and was wondering if I could get some information about the sensors used in this project. I'm doing a very similar design, however I plan on using just one sensor and having the robot turn in only one direction. This simplifies the logic circuit to about three gates. Any information you can offer to me will be a great help, thank you so much.

Calvin Tennant