Overview
- The Robot Drive Motors are controlled by Victor 884 Controllers.
The 884s receive a signal from the Robot Controller(RC) board on
the Robot.
- The signal format is PWM, Pulse Width Modulation. A pulse ranging from
one to two milliseconds(ms) in length is sent from the RC board to
an 884 controller. Actual width represents a value from 0 to 255.
A value of zero is sent from the RC to an 884 as a 1 ms pulse,
and a two ms pulse represents a value of 255. The pulse width
is linear over its 0-255 range.
- The Victor draws 80ma of control signal current during a "1", and
no current during a "0."
- The RC sends out 60 updates per second, which is 1 pulse about
every 17 ms. For example, a pulse width of 1.4 ms means the RC
Board sends a logic "1" for 1.4 ms, and a logic "0" for
(17 - 1.4) or 15.6 ms. This is compatible with the 884 device,
which requires a "0" time of at least eight ms.
A bit of algebra shows that a 1.4 ms pulse width means 40% of the
way from 0 to 255, a value of 102.
Values
- A motor can run forward, backwards, or be off. Forward and reverse
are arbitrary. Here, the motor drives the wheels with a chain,
so they rotate in the same direction. Thus, a practical convention
is that the left side wheels turn forward with a signal value of
at least 138.
- The motor is off for values of 117-137. Motor on has 2 ranges, each
with 94 steps. Reverse is 116-23, with 116 the slowest and 23 the
fastest. Forward is 138-231, with 138 the slowest and 231 the
fastest. One notes slow speeds are near the center or off position
on a joystick.
All 256 values from 0-255 are valid here. Thus,
values below 23 are treated the same as 23, and values above 231
are treated the same way as a 231. One detail is that the values
here are not fully symmetrical.
The following item is from an FAQ about motors on the FIRST website.
It shows a real world example of motor operation. One notes it
doesn't quite follow the math rules above.
The method here uses an analog signal, pulse width, to simulate a
digital value.
There will be a tolerance of several steps due to variations with
oscillators. You can use the DTT to measure these values. The steps for
forward and reverse are not symmetrical. Doing a test gave these results:
VALUE STATE LED STATUS Pulse Width
0- 41 Full Reverse RED 0.856-1.06 ms
42-125 Reverse OFF 1.07 -1.48 ms
126-139 Neutral YELLOW 1.49 -1.55 ms
140-230 Forward OFF 1.56 -2.00 ms
231-254 Full Forward GREEN 2.01 -2.12 ms
From http://www.ifirobotics.com/forum/viewtopic.php?t=317
