Jaguar Input Question

[ErikRantapaa]

I was wondering if someone could verify what kind of input signal is used to control the Jaguars.

Is the control signal the same kind of servo control signal that is described by this page: http://www.seattlerobotics.org/guide/servos.html ?

In the case of a Jaguar a pulse width of .67 ms indicates full reverse and a pulse width of 2.33 ms indicates full forward (using the default settings).

Thanks,
ER

[EricH]

According to the article you link to, servos use Pulse Coded Modulation; Jaguars use Pulse Width Modulation (PWM). I don't know what the relationship between the two is; they might be the same thing. (Anybody who does know the difference, chime in please.)

Jaguars can also use the CAN bus for control.

[RyanCahoon]

I've actually never heard servo control signals called Pulse Coded Modulation before. Pulse Coded Modulation (according to several webpages found by Google) refers to the process of digitally encoding an analog signal by sampling it at a fixed rate (computer music enthusiasts may recognize the acronym PCM). Maybe there's some relation there, but it's not obvious to me if there is.

Servo signals are usually called Pulse Position Modulated (PPM) signals, which can be considered a special class of pulse-width modulation (PWM) that follows defined timing specifications. The difference in PPM and PWM is PWM usually carries information through the duty cycle (% of the signal period the signal is "high") of the signal, whereas with PPM, the amount of time the signal stays high is the critical factor.

Jaguars accept PPM as input when using "PWM control" (the 3-wire red-black-white/yellow interface) and always use PWM for the power output to the motors. The PPM input is fairly compatible with most servo-control systems, you just have to be careful with the timing. [ErikRantapaa] gave one timing specification; another I've seen is 1ms full reverse, 3ms full forward. You can see where this might lead to problems, i.e. the motor driving one way or another when it's supposed to be stopped. In the case of the difference between the one [ErikRantapaa] specified and the one on the SRS page, it looks like the centers are the same, you just may not have access to the full range of speed/motion. If you're timing is different, you could recalibrate the Jaguars. See the documentation for more details.

--Ryan

[Ether]

excerpt from Page 6 of MDL-BDC & MDL-BDC24 FAQ:

Do I need to calibrate the Servo (PWM) interface?

It depends. The motor controller has a calibration mode, and the need to calibrate depends on the Servo (PWM) signal range generated by the robot controller.

For FRC users, the answer is probably not if you are using the cRIO system and a new motor controller. The default parameters of the motor controller are tuned for use with the cRIO. The calibration mode is provided if you want to use the motor controller with another source of Servo (PWM) signal that does not have the same range of pulse widths (e.g. an older robot controller). Or, if you have calibrated your Jaguar (MDL‐BDC) from a different source and move it back to a cRIO system, you must recalibrate.

excerpt from Pages 13 & 14 of MDL-BDC24 Getting Started Guide:

Calibrating the PWM Input

To accommodate variation in the timing of the supplied signal, Jaguar has a calibrate feature that sets new values for full-forward, full-reverse and points in between. Calibration is normally only required in applications where the PWM source has uncertainties due to analog radio links or other variables. Direct digital sources are unlikely to require calibration.

To calibrate the servo-style PWM input for a specific range, connect a PWM source, then:

1. Hold down the USER switch with a straightened paperclip for 5 seconds.
2. The LED flashes Red and Green to indicate Calibration mode.
3. Instruct the controller to send a full-forward signal for one or more seconds.
4. Instruct the controller to send a full-reverse signal for one or more seconds.
5. The LED flashes Green and Yellow quickly to indicate a successful calibration.

The MDL-BDC24 samples these signals and centers the speed range and neutral position between these limits. A calibration failure signals if an out-of-range signal is detected. This condition is indicated by flashing the LED Red and Yellow.

[ErikRantapaa]

Thanks for all of the replies.

It seems that the term PWM can mean different things depending on the context.

In the case of the Jaguar OUTPUT what matters is the duty-cycle, i.e. the percentage of time that the signal is on with respect to its period.

In the case of the Jaguar INPUT what matters is the length of time (i.e. in seconds) that the signal is on.

[EricVanWyk]

Originally Posted by ErikRantapaa

Thanks for all of the replies.

It seems that the term PWM can mean different things depending on the context.

In the case of the Jaguar OUTPUT what matters is the duty-cycle, i.e. the percentage of time that the signal is on with respect to its period.

In the case of the Jaguar INPUT what matters is the length of time (i.e. in seconds) that the signal is on.

Yep. This is why I teach it to my students as Pulse Width Modulation and Duty Cycle Modulation. PWM is an over-used phrasing, especially when you really consider what aspect of the pulses matter to you.

For the input, the positive width is critically important, and the total period just needs to be close enough.

For the output, you care about the ratio of on to off, and the frequency.