micrologix 1000 high speed counting

blueyedpeter

Lifetime Supporting Member
Join Date
Sep 2011
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sydney
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Hi there guys
I once again i need your help as you might be aware i have a long term project running .
I am trying to use both inputs 0 & 1 on the micrologix 1000 to read two different input pulses for two different speed values the resolution on both inputs are relatively low both are less than 500 pulses per minuet .

Now it was suggested that i use an up counter , a timer and totalize them both together.

Sounds great but how do i do that i have looked in the help section on instructions in slc500 . nothing came of it though .

Any i dear is better than what i have found so far thanks again peter
 
Let's define the objective: As I understand it, you want to measure two discrete pulse trains as Speed signals. You want to measure them separately, and you're using a DC-input MicroLogix 1000.

Because this is a MicroLogix 1000 with only one hardware High Speed Counter, that feature is ruled out.

Is the "duty cycle" of the pulses 50% ? This means that if the input pulse rate is 500 pulses/minute = 8 pulses/sec = 125 ms/pulse, that the signal will be ON for 62.5 milliseconds then OFF for 62.5 milliseconds.

If so, you can almost certainly use ordinary instructions in ladder logic. If the "on" time of the pulses is much shorter, you might have to reconsider.

The "use a counter and a timer" method of speed measurement involves counting an unknown number of pulses in a specific amount of time.

For example, you would run a counter during a 10-second timer, and after the timer is complete you would calculate the speed. If the input was running at 8 cycles/second, you should have 80 pulses at the end of 10 seconds. You can then do math to equate that to gallons/minute or miles/hour or furlongs/fortnight.
 
The inverse method is to count the amount of time between the leading edges of two pulses. The MicroLogix 1000 has a free-running clock with 10 microsecond resolution (I'd have to check to be sure that's the case) so you can just capture the value of the S:4 register every other pulse, and subtract the New value from the Old value to get the Time between pulses.

Explain more about the physical thing you're trying to measure, and give some information about the sensor and the accuracy you need.

Simple_Speed_Example_BEP.jpg
 
Last edited:
:wish:In regards to resolution of measured components
unit number one (back wheel)
slowest aprox speed 5km/hr @ 37.9 rpm @ 1 pulse per rev about .631 Hz
@ 55km/hr @416.6 rpm @ 1 pulse per rev @ 6.94 Hz the wheel circumference is about 2.198 meters in length if need be 3 pulses per rev can be applied to the wheel.

Unit number two (the crank arms)
The cadence is measured in rotations per minuet
at a cadence of 93.6 rpm in the highest gear ratio aprox 55km/hr will be achieved
at mid range gear ratio .
and at a cadence of 8.5 rpm a speed of about 5km/hr will be achieved.

Later in the program these two ranges of wheel speed and cadence will be compared to to each other and an amount of applied ( mechanical ) load or resistance will be derived and applied to the back wheel .
and as a result it should be workable to obtain an estimated amount of work completed by the rider measured in watts.

assuming that all things are calibratable an amount of adjustment might be needed in the area of load assessment and power used but that will be later in the project .
 

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