How to measure robot parameters?
The scaling factor will be used to convert between asserv steps and
millimeters. We will look at the left counter as this one is not corrected.
Select a flat, clean and long (2 to 10 meters) surface, then position the
robot precisely on a starting line. Draw a parallel line at the other end and
measure distance between the two lines.
Now, reset the robot counters and start monitoring left counter value. Drag
the robot as linearly as possible until it reaches the final line. While
doing this, remember any counter overflow or try to calculate how many
overflow is expected (with a wheel diameter estimation).
Position precisely the robot on the finish line and write down the left
The scaling factor is given by::
distance (mm) / augmented_counter
where ``augmented_counter`` is the counter value prefixed with the number of
Keep the right counter value as well as it can be used for the next measure.
Right wheel correction
As wheels can not be exactly the same, we will introduce a correction factor.
Do the same measure as for the scaling factor, but we need the left and right
The correction factor is given by::
left_augmented_counter / right_augmented_counter * 2^24
The ``2^24`` factor is used to have a 24 bit floating point number.
Now, it is time to check your measures. Ask the robot to drive a distance and
This is the distance between the encoders wheels, used to compute robot angle.
Place the robot precisely on a line.
Now, reset the robot counters and start monitoring counters values. Make a
large number of turns (about 10) and position the robot precisely on its
starting line. While doing this monitor counting direction, and any counters
overflow or underflow.
Write down the counters values and the number of overflow.
A counter was counting backward, so you will have to handle negatives numbers.
For example, if the current counters values are 0x1234 and 0x2345, if the
first counter was counting upward and the second one backward, and each
counter has overflowed 3 times, this gives, in the 24 bit two's complement
notation, 0x031234 and 0xfc2345 (0xfc = 0xff - 3). The second number
represents -0x03dcbb (0x1000000 - 0xfc2345).
Now make the difference between the two augmented counters values, or the sum
of their absolute values, this will give the total distance in steps travelled
by the two encoders to do the number of turns.
The encoders footing is given by::
difference / 2 / pi
The ``2`` divider is there because we summed the distance traveled by both
encoders. Then we divide by ``pi`` to get the diameter.