Dash output - rpm

[FoxSTI]

Can I configure one of the dash outputs to provide an analog voltage (0-5) based on rpm to feed into the analog input of another device ?

I have a logger that need an RPM signal and I don't have the budget at the moment to unlock my dash for logging.

Thanks

[David Ferguson]

Normally an RPM signal is not an analog voltage (and your C125 can't generate a continuously variable analog voltage output anyway), but rather an digital signal where the frequency of the output varies with the RPM. You can easily setup an auxiliary output from a Motec Dash to provide an RPM signal for another device, I have done this for a various Paddle-Shift Controllers.

But before you wire that up, are you sure that your logger can't accept CAN messages that could include the RPM values as well as much more?

[FoxSTI]

No CAN on the logger. It's a very inexpensive unit.
I think I'm just going to have to save for the motec logging.

I understand hat you mean about thec125 output.
So it's more of a pwm?

David do you have a same you can share for my knowledge?

[David Ferguson]

Here it how I set up a Frequency Output for Engine RPM: This should produce one pulse per revolution,

[FoxSTI]

Thank you Dave. I will play with that.
out of curiosity? how does that actually work....

[David Ferguson]

First, let's get a couple of terms correct.

A Pulse Width Modulated (PWM) signal is a digital signal generally with a fixed frequency (i.e. the same amount of time between the start of the pulses), but a varying pulse width (that is what is modulated). You can think of the PWM value as a percentage of the time that this signal is either high or low for one pulse period.

A Frequency signal can have either a fixed duty cycle (the percentage that the pulse is high or low), or typically fixed time for the pulse (either high or low depending upon the signal polarity).

A rotational speed is generally measured by a sensor that creates one more pulses per revolution. So the Frequency of the digital signal is what varies and the pulse is usually either a fixed duty cycle or a fixed time depending upon the signal source. For example, if you looked at the signal to an ignition coil, the frequency would vary with the RPM, and the duty cycle would vary with the dwell time used to charge the coil.

Using a coil signal is how a traditional electronic tachometer works -- and most data loggers and electronic instrumentation can accept a digital signal with varying frequency to represent a rotational speed -- typically measured in Revolutions Per Minute (RPM).

Now wheelspeed data is similar, but we usually like to see that result as a linear velocity (mph, ft/sec, km/h, m/s, etc), so we multiply the rotational rate by the tire circumference (there are unit conversion terms, too) to calculate linear speed.

OK - back to your question - the auxiliary output actually only grounds the pin -- the signal's high-voltage value will be determined by the device it's attached to (may be 5V or 12v-15v, sometimes you have to add a pull-up resistor). So for 3200 RPM on my given configuration, the output frequency would be 1000 x 3200/6000 = 533.33 hz. (The reason for 60000 vs. 6000 is because RPM in internally stored in .1hz increments and this is what was required to get it to work correctly).
This 533.33 hz can be converted into a the time period between pulses by dividing into 1 second ( i.e 1 / 533.33) this calculation yields a time period of 1.875 milliseconds between pulses. So the Auxiliary output will have a pulse that is low for 75% of that time (1.4ms), and high for (.468 ms or 468 microseconds).

I hope that helps.

[FoxSTI]

Never mind I think I got it.

So at 6000 rpms it will produce a pulse every 100Hz.
That is crazy fast (frequency chance) specially if you have to keep up with ever changing rpms...

[FoxSTI]

thanks very informative. I appreciate the time.

[David Ferguson]

Never mind I think I got it.

So at 6000 rpms it will produce a pulse every 100Hz.
That is crazy fast (frequency chance) specially if you have to keep up with ever changing rpms...

Just to get the nomenclature right -- at 6000 RPM one pulse/revolution, produces an output frequency of 100 hz (100 pulses per second or one pulse every 10 millisecond).

The hardware receiving this just needs to time how long between pulses to know how fast something is rotating.