Arm yourself with a robot-and have fun
ROBOTICS is an area of computer control which is becoming increasingly
popular, both with educational establishments and home enthusiasts.
Now that the cost of the controlling computer has reached affordable
levels. the major stumbling block for those interested in this
area has been the mechanical side.
Very few people with the mechanical skills to build a robot
have the electronic and computing skills to control it and vice
versa. Therefore there is a need in the market for a low cost
mechanical robot device.
This has been filled by various manufacturers by the use of
mobile boxes, turtles, buggies and so forth.
Interesting though this approach is, I have found them to be
very disappointing in practice.
The need for a large, flat uncarpeted area, and the problem
of them getting tangled up with their umbilical leads, limits,
their uses, even for educational purposes.
However. Colne Robotics has produced a BBC version of its computer-controlled
robot arm. called Armdroid. It is driven by six stepping motors
and is constructed of robust sheet steel.
The motor movement is transmitted to the various sections of
the arm by a Heath Robinson-like collection of plastic gears,
ribbed rubber belts, pulleys and twine. Despite this, it operates
quite effectively.
At the end of the arm there are three fingers that can close
and grip an object.
The arm is available either ready-built or in kit form. The
most difficult part of assembling the kit is stringing up the
pulleys.
Construction of the arm would make an excellent group project
but is quite a time-consuming task that should not be undertaken
lightly. The ready-built version is well worth the extra money.
The documentation, although a bit tatty, shows exploded mechanical
drawings of all parts of the arm. So if you do buy it ready-built
you will know how to take it apart.
The stepping motors take a fair bit of current and so a separate
power supply is provided.
The one I received for review had unfortunately upset the Post
Office and they had subjected it to a high inertial shock. The
upshot was that it rattled and the whole thing could be turned
on and off by shaking it.
The power supply is fitted with "tamper proof screws and
so I did not tamper with it. There is also a conventional on/off
switch if you are not lucky enough to have it modified by the
Post Office.
The task of controlling six stepping motors simultaneously is
not simple and so a very good demonstration program is provided.
This allows you to drive each motor by pressing keys on the keyboard.
Once you have driven the arm to a particular point you can get
the computer to remember it.
Having entered several points like this the computer can be
made to move the arm through all the points in sequence.
Alternatively, you can step forwards or backwards through each
point on command from the keyboard.
A point to note is that the arm will move directly from one
point to the next in a straight line. Therefore you must set several
points to move the arm over an object.
This is where the single stepping comes in — to allow you to
intervene if you have programmed the arm to scythe through anything.
The torque of a stepping motor decreases as its speed increases.
So if the arm is to lift anything heavy it needs to be slowed
down.
However, while the arm is getting into position it is a bind
to go so slowly. To this end the software allows you to select
a different speed at each preset point.
Also, you can set a delay at each point so that the arm pauses
before moving on to the next position.
The power required by the motors also varies depending upon
the position of the arm.
I found the motors had to be slowed down slightly when the arm
was at full stretch, even when it was carrying nothing.
The demonstration program is menu driven and fairly self explanatory.
It can even display the arm's position in "absolute coordinates"
which correspond to the number of steps the motor has received.
You can get a version of the arm with micro switches that detect
the limit of travel on all six motors. This can prevent the arm
being driven past its mechanical limits of movement.
The version I had did not have these on and although this might
sound like a disadvantage it made very little difference in practice.
This was because I was always watching the arm, and the extra
strain caused the motor to stall, thus causing no damage at all.
I found using the arm both fascinating and fun. One of the first
things I did was to program it to pick up a handkerchief, swivel
round and drop it in my lap. Then placing my son (two years old)
at the computer, the arm dutifully delivered it into his lap.
This caused great delight and he tried to give it back to the
arm. Ever since he has periodically come up to it with one offering
or another.
The use of this arm in a teaching situation would be very worth
while, not only to teach about robotics, but also project planning.
The nature of robotics dictates a carefully planned approach.
This makes the arm very suitable for group projects.
As well as fixed sequences it would be possible to program some
"intelligence" into its operation.
When the result of writing a computer program is the controlling
of objects it is very much more satisfying than words appearing
on a screen.
This robot arm should be a great source of inspiration for the
experimenter as well as being a valuable teaching aid.
Besides which it is great fun.
Mike Cook
