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R/C Operation
78 AX2500/2850 Motor Controller User’s Manual Version 1.7. February 1, 2005
Losing the optical isolation increase the risks that damage in the controller will reach the
radio, although these risks remain extremely low. Without optical isolation, the radio is less
immune to electrical “noise” and therefore may operate at a reduced range.
This mode of operation is the most convenient and is the one wired in the R/C cable deliv-
ered with the controller.
Operating the Controller in R/C mode
In this operating mode, the AX2500/2850 will accept commands from a Radio Control
receiver used for R/C models remote controls. The speed or position information is com-
municated to the AX2500/2850 by the width of a pulse from the R/C receiver: a pulse
width of 1.0 millisecond indicates the minimum joystick position and 2.0 milliseconds indi-
cates the maximum joystick position. When the joystick is in the center position, the pulse
should be 1.5ms.
Note that the real pulse-length to joystick-position numbers that are generated by your R/C
radio may be different than the ideal 1.0ms to 2.0ms discussed above. To make sure that
the controller captures the full joystick movement, the AX2500/2850 defaults to the timing
values shown in Figure 50.
For best control accuracy, the AX2500/2850 can be calibrated to capture and use your
radio’s specific timing characteristics and store them into its internal Flash memory. This is
done using a simple calibration procedure described on page 82.
The AX2500/2850 has a very accurate pulse capture input and is capable of detecting
changes in joystick position (and therefore pulse width) as small as 0.4%. This resolution is
superior to the one usually found in most low cost R/C transmitters. The AX2500/2850 will
therefore be able to take advantage of the better precision and better control available from
a higher quality R/C radio, although it will work fine with lesser expensive radios as well.
Internally, the measured pulse width is compared to the reference minimum, center and
maximum pulse width values. From this is generated a number ranging from -127 (when
the joystick is in the min. position), to 0 (when the joystick is in the center position) to +127
(when the joystick is in the max position). This number is then used to set the motors’
desired speed or position that the controller will then attempt to reach.
1. 0 5 m s
min center maxjoystick position:
R/C pulse timing:
0.45ms
0.9ms
FIGURE 50. Joystick position vs. pulse duration default values