Pololu Jrk USB User Manual
Page 36
Read Variable Command Byte
Variable
Range
Two bytes Low byte High byte
Input
0 to 4095
0xA1
0x81
0x82
Target
0 to 4095
0xA3
0x83
0x84
Feedback
0 to 4095
0xA5
0x85
0x86
Scaled feedback
0 to 4095
0xA7
0x87
0x88
Error sum (integral) -32,768 to 32,767 0xA9
0x89
0x8A
Duty cycle target
-32,768 to 32,767 0xAB
0x8B
0x8C
Duty cycle
-600 to 600
0xAD
0x8D
0x8E
Current
0 to 255
0x8F
PID period count
0 to 65535
0xB1
0x91
0x92
The meaning of the variables is described below:
• Input: The input is the raw, un-scaled input value, representing a measurement taken by the jrk of the input
to the system. In serial input mode, the input is equal to the target, which can be set to any value 0–4095 using
serial commands. In analog input mode, the input is a measurement of the voltage on the RX pin, where 0 is 0 V
and 4092 is 5 V. In pulse width input mode, the input is the duration of the last pulse measured, in units of 2/3 μs.
See
for more information.
• Target: In serial input mode, the target is set directly with serial commands. In the other input modes, the
target is computed by scaling the input. The scaling can be configured in the “Scaling” box of the Input tab in
the configuration utility.
• Feedback: The feedback is the raw, un-scaled feedback value, representing a measurement taken by the jrk
of the output of the system. In analog feedback mode, the feedback is a measurement of the voltage on the FB
pin, where 0 is 0 V and 4092 is 5 V. In no feedback mode (speed control mode), the feedback is always zero.
• Scaled feedback: The scaled value of feedback. The feedback scaling can be configured in the “Scaling” box
of the Feedback tab in the configuration utility.
• Current: The value of this variable is proportional to the current running through the motor: on the jrk 21v3,
a value of 1 nominally represents 38 mA of current in the motor, while on the jrk 12v12 a value of 1 nominally
represents 149 mA of current in the motor . However, the circuitry on the motor driver chips varies between
different units, and they can vary depending on which direction the motor is driving, so these calibration values
will not always be right for every jrk. The value of this variable will always be zero unless a current limit is
enabled. See
for more information about current measurement and calibration.
• Error sum (integral): Every PID period, the error (scaled feedback minus target) is added to the error sum.
The error sum gets reset to zero whenever the jrk is not driving the motor, and can optionally be reset whenever
the proportional term of the PID calculation exceeds the maximum duty cycle. There is also a configurable
integral limit that the integral can not exceed.
• Duty cycle target: Represents the duty cycle that the jrk is trying to achieve. A value of -600 or less means
full speed reverse, while a value of 600 or more means full speed forward. A value of 0 means braking. In no
feedback mode (speed control mode), the duty cycle target is the target minus 2048. In other feedback modes,
the duty cycle target is the sum of the proportional, integral, and derivative terms of the PID algorithm.
• Duty cycle: Represents the duty cycle that the jrk is driving the motor with. A value of -600 or less means full
speed reverse, while a value of 600 or more means full speed forward. A value of 0 means braking. The absolute
value of the duty cycle will always be less than the absolute value of the duty cycle target. The duty cycle is
Pololu Jrk USB Motor Controller User's Guide
© 2001–2014 Pololu Corporation
4. Using the Serial Interface
Page 36 of 45