2002.02_Making Quick Fix Sensors for Lego Robots.pdf

FEATURE

QUICK FIXES

The sensors that come

with the MindStorm

are limited. So John

Southern decided to

make some of his own

ROBOTS: Extra sensors

HOWTO on Lego MindStorm with Linux, and

received lots of emails back from those who

asked for the PDF files of the previous article that

should have been printed in December, the biggest

request was for DIY sensors and actuators, as the

range with the MindStorm is considered small.

Sensors consist of two parts: the first connects to

the RX brick and the latter is the actual sensor. Like

most people I want to do things quick, so I’m

prepared to cut corners if the time saved outweighs

the flimsiness of the product.

When making extra sensors I

should have produced wonderful

connectors by using a lathe, turning

down machine screws and mounting

these inside a Lego brick to make

good connectors. If I had that much

spare time I would do something

more constructive such as watch TV.

A quick look through the spares box finds mini

crocodile clips, which clip onto the Lego RCX and

hold quite well. Hurrah! The easy part of the sensor,

done in a minute.

slowly to choose the correct path and we had to

drive the buggy, wait, test the temperature and

compare this with the previous result to see any

change. Another quick fix to make it a waterproof

sensor was to cover it in hotmelt glue.

The motion of the ocean

The next quick fix sensor is a mercury tilt switch. This

works as a simple contact when turned in a set

orientation. These are great fun, as by putting them

in series with resistors in parallel we are able to make

a motion detector. Changing the orientation proved

to be tricky, but hotmelt glue came to the rescue.

With one mercury switch we could sense if the buggy

was travelling on a level surface or on a slope.

With two mercury switches we have enough data

to determine whether it’s travelling uphill or downhill.

This is done by positioning one sensor so it only

contacts when the bugging is climbing and one that

makes contact when running level. If sensor one is

connected then the buggy is going up hill; if the

second sensor is connected the buggy is running

level; and if neither sensor is connected the buggy is

travelling downhill.

How to hook us

easily to the

Lego connectors

The heat is on

Now for the actual sensor. The first we build is a

temperature sensor in the hope of making the buggy

move towards heat. Adding a thermistor to the other

ends of the crocodile clips is the quickest way we can

make a temperature sensor.

The only problem that arises from

this, is that it needs a temperature

offset constant to make it accurate.

After two days of playing with this,

we found that the crocodile clips

give variable contact resistance – so

what works one day fails to be

accurate the next time we build.

Rather than be forever adjusting the program data

each time we build, we can fudge the sensor by

putting a variable potentiometer in line with the

thermistor. We can now easily adjust the resistance in

the connecting cable so the thermistor gives an

accurate reading. At this point the buggy sort of

works, except that changes in temperature are

actually very small and the thermistor requires time to

settle on a temperature. This meant it worked very

Orienteering

Next up is an angle sensor. Here we simply link a ski on

the bottom of the buggy to a variable potentiometer.

As the ski changes angle with the terrain so too does

the resistance through the potentiometer. This is much

easier that our other sensors, and means we can also

now work out the gradient.

At this point we were still plagued with cats

thinking the buggy was their personal plaything so

we will investigate imaging next time.

Te mperature probe

Hot glue – the quick fix solution.

Angle sensor using a potentiometer

LINUX MAGAZINE

Issue 17 • 2001

H aving finally read the whole of Luis Villa’s Mini-

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