Pololu TReX Jr - Manual (Page 13)

4. RC/Analog in Detail
This section provides a detailed overview of the RC and analog modes of operation. It explains how the TReX Jr
processes its channel inputs and how those inputs influence the TReX Jr when its in RC or analog mode.
4.a. Channel Functions
The five input channels along the left side of the TReX Jr accept either radio-controlled (RC) servo pulses or analog
voltages, depending on the position of the mode-selection jumper (or on the value of the “channel input source”
parameter if the TReX Jr is in serial mode). When not in serial mode, the TReX Jr acts on the five channels as
follows:
• Channel 1: If mix jumper is in place, turn left/right at the specified speed; if mix jumper is off, set motor 1
direction and speed.
• Channel 2: If mix jumper is in place, go forward/reverse at the specified speed; if mix jumper is off, set
motor 2 direction and speed.
• Channel 3: Set auxiliary motor speed (aux. motor is unidirectional).
• Channel 4: Enable/disable flipped mode. Flipped mode causes the TReX Jr to treat motor 1 as motor 2 (and
vice versa), and to swap its notions of forward and reverse for each motor. The result is that a differential-drive
robot will behave as normal when inverted if flipped mode is enabled. This channel acts like a switch that gets
flipped when the channel value falls below a certain threshold. If your controller is not calibrated, this channel
may not function as intended.
• Channel 5: Enable/disable serial override. Serial override gives serial control of the motors for as long as
channel 5 enables it. When serial override is first enabled, each motor is set based on its most recently received
motor command, even if that command occurred while serial override was disabled. Channels 1, 2, and 3 will
not affect the motors while serial override is active (unless your serial control source is reading the values of
those channels and issuing its commands based in part on them). This channel acts like a switch that gets
flipped when the channel value falls below a certain threshold. If your controller is not calibrated, this channel
may not function as intended.
4.b. Channel Calibration
As was covered in Section 3.e, you can automatically calibrate your TReX Jr for your RC or analog controller. If
this doesn’t prove sufficient for your needs, you can manually calibrate your TReX Jr for your controller by
specifying the minimum, neutral, maximum, and deadband values for each channel. The TReX Jr can
simultaneously store both a set of RC and a set of analog calibration parameters. The calibration parameters are in
the units of the raw channel values: 0.4 us for RC, 4.89 mV for analog. In general, these parameters affect how the
TReX Jr interprets the channel input values.
Perhaps most noticeably, these parameters help determine how the channel inputs affect the motors. Motor speed is
scaled linearly (or parabolically, if the channel is set as parabolic using the “parabolic channels” configuration
parameter) from 0 at a channel input of neutral+deadband to max speed at a channel input of maximum. In the
other direction, motor speed goes from 0 at neutral-deadband to max speed at minimum. Motor speed is 0 for
channel inputs between neutral-deadband and neutral+deadband, and motor speed is at a maximum for channel
inputs greater than maximum or less than minimum. Motor direction is determined by the side of neutral the
channel input is on, and notions of forward and reverse can be switched on a channel by channel basis using the
“reversed channels” configuration parameter.
4.c. General RC Information
When the channel inputs are RC servo pulses, the update rate of the channel values is determined by the pulse-train
frequency of your RC receiver. Typically this is 50 Hz (each channel updates every 20 ms). The TReX Jr can handle
channel pulse-train frequencies of 10 Hz to 125 Hz.
Pololu TReX Jr User's Guide © 2001–2009 Pololu Corporation
4. RC/Analog in Detail Page 13 of 23