NanoRover

This is one of the most unconventional designs I’ve build up to date. It does not run straight, but crawls. The reason for this is that both motors are unidirectional. It uses two wristwatch lavet-type motors as it main actuators and uses two whiskers and two directional LDR light sensors for sensing it’s environment. The biggest problem with robot is the fragile connection of the wheels to the shaft of the engine.

Introduction

The Nano Rover is an autonomous vehicle that uses two wristwatch motors as its main actuators. Analog wristwatch motors are one-phase Lavet stepper motors and are driven by a biphase pulse for every step. Small actuators are hard to get, most micromotors are too big for very small scale – aka tabletop – robots and consume too much power. The wristwatch motors are a nice trade-off between cost and performance. The only drawback is that they can only turn clockwise. There are wristwatch motors that can turn in both directions, but these are very hard to find. The design of the robot is build around this limitation of clockwise turning. The movement pattern of this robot is worm-like, in which each motor runs after each other and let the robot crawl around.

The schematic

The robot is build around an AT90L2313 in an SOIC formfactor. This allows for some miniaturization of the electronics.

Driving the wrist watch motors

To drive these motors, we have to provide a bi-phase pulse train. The following two images show us the pulses that are generated by the onboard electronics of a wristwatch.

* Phase 1:

* phase 2:

The interval between each pulse is exactly 1 second, the unit of movement of the seconds handle of the watch. The next image shows a complete period of this pule train.

* Pulse train:

It is very easy to connect the wristwatch motor to a microcontroller, you connect one pin of the MCU to each end of the coil. There can be some concerns on the inductance of the coil, as the inducted potential can possibly damage the MCU. In practice , however, this has not happended to me.

Driving the watchmotor is easy as the next BASCOM AVR program shows us …

This will generate a pulse of 3ms, with a pause interval of 1 ms. the polarity of the pulse is inverted every time. The output period is 4 ms, resulting in a frequency of 1/0.004 = 250Hz. The second hand of the watch takes 60 pulses for one revolution. The speed of the second is thus 4.17 revolutions per second.

The revolutions speed of the minute hand is 60 times slower then the seconds hand. This means that the minutes hand is revolving at 4.17/60 = 0.0696 revolutions per seconds. Or 4.17 revolutions per minute.

The diameter of a wheel is 25 mm, the speed of the robot is thus 25xpix4.17 = 327.5 mm/minute.

A general formula for the revolutions per second of the seconds hand is:

Whereby:

* n := revolutions per minute of the minute hand
* t := period of the pulse in seconds

The period of the pulse (t) can be split up in two part: t = ta + tp, whereby ta the time is that the pulse is active and tp is the pause time.

The time ta should always be between 3ms and 4 ms to have a steady going hand. Off course, other brands of wristwatch motors can vary this empirically defined optimum.

The control program

The next control program allows the robot to crawl along, reacting to touch whenever one of the whiskers hits an object. This simple behavior allows the robot to crawl along obstacles. Although the light value is measured, by counting the time it takes to charge a capacitor through the light dependend resistor, nothing is done with this information. The next step is to add a few lines of code that will seek out dark – or bright – places.

Images

Some extra photos of the robot in progress:

Some photos of the finished prototype: