# 4.4:1 HP 12V with 48 CPR Encoder counts

Hello,

I am using the 4.4:1 HP 12V motor with 48 CPR encoder counts with the jrk G2 21v3 USB motor controller. The controller uses a single quadrature channel and the docs say that I should expect 12 counts per revolution. Is this 12 counts per revolution of the DShaft? Also, might you have a settings file for the JRK utility for this motor?

Thanks,
Mike

Hi,

My goal is to write a small application which allows the user to start and stop the motor adjust the speed and see the RPM feedback. I am using the jrk G2 21v3 USB and a 4.4 HP 12V motor with 48 CPR counts per rev. My input supply voltage is 12V and Iâ€™d like to allow users the ability to adjust the speed to the full range (0-1500 rpm). Hereâ€™s my feedback setup:

The motor runs smoothly but at max speed I see the feedback at roughly 2650.

If I try to calculate the RPM from the feedback frequency I get:

2650-2048 = 612Hz = .0016 seconds/count = 656 counts/second = 37500 counts/minute

If there are 53 counts per revolution of the motor dshaft this is 707 RPM (37500/53).

Do I have a mistake somewhere? I was expecting the motor to be able to achieve at least 1500 rpm unloaded.

Hello.

It looks like your calculation is over-simplified. You can find a description of how Feedback is calculated under the â€śPulse timing modeâ€ť heading of the â€śSetting up frequency feedbackâ€ť section of the Jrk G2 userâ€™s guide. There is a lot of information there, so for convenience here is the specific part that describes it:

So, the calculation should look like this:

\text{PulseWidth} = \frac{2^{26}}{|\text{Feedback}-2048| * \text{FrequencyDivider}}*\frac{1}{\text{PulseTimingClock}}

With your settings and feedback values put into the equation:

\text{PulseWidth} = \frac{2^{26}}{600*32}*\frac{1}{12}=291\text{ ÎĽs}

To get the pulse rate, we can multiply it by 2 to get the period (since we are assuming a 50% duty cycle) and reciprocate it:

\text{PulseRate} = \frac{1}{2*291*10^{-6}} = 1718 \text{ pulses per second}

That is the pulse rate of the backshaft, so to convert that into the RPM of the output shaft of the motor:

\text{RPM of output shaft} = \frac{\text{PulseRate}}{\text{PulsesPerRevolution}}*\frac{60}{\text{GearRatio}}
\text{RPM of output shaft} =\frac{1718}{12}*\frac{60}{4.4}=1950\text{ RPM}

Note that the PulsesPerRevolution in that equation is the number of pulses seen on a single encoder channel per rotation of the back shaft, so it would be 12 for a 48 CPR encoder.

Brandon

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Thank you for the education Brandon. Very helpful, excellent support

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