tinybot: unauthorized biography

• The mechanical engineering aspects of this project are ridiculously low-level: even every single gear is custom designed, machined, and manually cast in my home workshop.

• The electronics are cheap, easy, power-efficient, and extensible. The total cost of the robot is $30 if you count just the essential parts, and $90 including a pretty LCD and two fancy distance sensors. Most of the comparably featured projects on the web are several times more expensive.

• I am trying to make it look pretty, so it's purple.

1. Electronic components

• Motors: 2 * Mabuchi RF-300FA-12350 ($1.20 each),
• Traction sensor: Optek OPB609 ($0.75),
• Steering sensor: Toshiba TLP841 ($0.50),
• Distance sensors: 2 * LV MaxSonar EZ4 ($27.00 each),
• Motor drivers: 2 * Fairchild FAN8082 ($0.37 each),
• Microcontroller: AVR ATmega1284P ($7.50),
• Display: Newhaven NHD-0208AZ-FL-YBW ($9.50),
• Power supply: Zippy Flightmax 800 mAh 7.4V Li-poly ($6.20),
• Supply watchdog: Maxim MAX8212 ($3.00) + STM STP12PF06 ($0.80),
• Supply adjustment: Murata OKR-T3-W12-C ($6.20).
• ...plus several resistors, capacitors, switches, etc ($3.00).

2. Other components

The project also called for four VXB ball bearings, 16 steel dowel pins (M1, M2, M3), 30 machine screws (M1.5, M2), and one square steel rod.

The whole machininng and casting process is explained in painstaking detail in my guerrilla CNC manufacturing guide, and also illustrated on this Flickr page (an earlier project of mine).

2. Mechanical design

Rear wheels of the robot are joined together with a steel rod, mounted in two low-profile ball bearings (8 x 12 x 3.5 mm), and connected to a 130:1 gearbox that delivers about 40 RPM and 3,000 g/cm of torque, allowing the robot to effortlessly mount most obstacles. Closeup of the rear assembly:

Front wheels have integral ball bearings and rotate freely; a reflective optointerrupter, TLP841, is used to provide traction feedback for the motor, detecting a small white target on the inside of the wheel.

A second motor and a 490:1 gearbox can be found in the front; instead of the usual (and crappy) differential drive system, an oversized gear-based transmission is used to synchronize the turning of front wheels, each of them individually mounted on 3 mm dowel pins. Turn range is +/- 30°. A small slot optointerrupter, OPB609, is used to sense 0°, +/- 15°, and +/- 30° positions marked on the gears:

Gear tooth size is about 0.5 mm, pressure angle is about 18° for initial stages, and 25° for the final stage in the steering assembly. Gearwheel thickness is 1 and 1.75 mm, respectively (see the aforementioned CNC machining guide for a good primer on gear geometry).

Rendered image of the entire body, with the main PCB and the battery installed:

Positive molds machined in RenShape 460 based on CAD models:

Early stages of assembly:

Several hours of work later - all sensors installed, electronics on a breadboard, distance sensor readings displayed:

Completed, turned on, and crying "put me down":

Other images of interest: initial prototype two months ago; and the immediate predecessor to this version (note a different orientation of the motors). If you count these iterations, the project took about 25 hours of work to get to the current stage.

3. Circuitry

The battery has a nominal voltage of 7.4V (the actual range is 5.6 - 8.5V); this needs to be translated to stable 5V for the motors, sensors, and the microcontroller. To do this, I used Murata OKR-T-3-W12-C, a high efficiency 3A switched regulator. The voltage is set with a 270 Ω resistor; the value needs to be set accurately, so a 500 Ω trim pot may be more practical than shopping for a 1% resistor of this exact value.

An extra 10 µF capacitor across terminals is employed to counter motor inrush currents and the crowbarring tendencies of the H-bridge drivers used, so that the risk of resetting the MCU is minimized.

To avoid damaging Li-poly batteries, care must be taken not to discharge them below about 2.8V per cell (5.6V total). To prevent this, I rely on a MAX8212 voltage monitor, coupled with a medium power p-channel MOSFET transistor (STP12PF06 in this case, but any other rated for at least 3A will do). This serves as an input stage for the regulator; the threshold voltage is adjusted with a 250k resistor. This value also needs to be matched accurately, so a 220k resistor and a 50k trim pot may be a simpler choice.

The complete voltage control circuit looks as follows:

The remainder of the circuit consists of ATmega1284P interfaced to several peripherals:

• An HD44780-compatible LCD (NHD-0208AZ-FL-YBW) in 8-bit write-only mode (R/W pin pulled low) for displaying status messages and offensive ASCII art. Backlight brightness can be adjusted by adding an extra 500 Ω trim pot on the LED+ pin.

• Two narrow beam ultrasound distance sensors (MaxSonar-EZ4) used in one-shot mode with pulse width output. The range for these devices is about 15 cm to 6.5 m.

• Two FAN8082 H-bridge controllers for the motors, with speed control using a trim pot. FAN8082 is being phased out, but Toshiba TA7291P or ROHM BA6956AN are good substitutes.

• Basic support circuitry for the optointerrupters to limit LED current, and convert phototransistor output to CMOS levels (on the schematic, "MV" stands for traction sensing, "ST" for steering).

• A single diagnostic LED.

• An obvious and accessible self-destruct button.

Not shown on the schematic, the AVR ISP connector is also attached to pins 6-11 of the MCU to allow for easy programming.

4. Software

Click here for a video showing a test program; sensor feedback is used in this demo to execute movements very precisely.

5. Questions? Comments?

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