tachometer, analogue input, and log history

crying_baby

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Join Date
Mar 2005
Location
earth
Posts
112
hello, thanks to all the past feedbacks and i had my motor up and running now.

khalil
Member
Join Date: Jan 2003
Local Time is: 04:39 PM
Location: currently in UAE
Posts: 110 Dean..

you can use a rotery encoder to count pulses..and obtain measurement of speed..but its not very accurate..and therefore I suggested a tachometer..its the right tool to measure speed ..analoq input is not complicated at all..just connect you input to the plc and read the value..use comparing bit instruction..and trigger an output when the speed reaches your preset speed...when you decide to use a plc...then post and you will get all the help you need

hi khalil, i had came across your posting in another thread and this is the scheme that i m gonna implement using an analogue card. the parameter i wanna measure is teh motor rpm and feed into teh memory of PLC.

however, i got a question, since teh inverter is already showing the frequency in Hz, which is up n running well in my lab at NO LOAD,

IF however, i put a load on the shaft, the frequency of teh inverter i assumes will maintain the same. however, the motor's rpm i doubt to maintain like it is at noload.

Dean-newbie
Member
Join Date: Jul 2005
Local Time is: 12:39 AM
Location: Aus
Posts: 3 Thanks for your thoughts guys, I guess my main goal here is to make this system as small, simple (W.R.T. wiring and construction) and cost effective as possible.

Trevor I can see your point, and I can certinly understand why you reccomend a discrete solution - not to mention that is something I could handle with my current knowledge. However I tend to have a habit of overdoing things and making the best darn project I possibly can. The reality is if I was to make this as you sugest I wouldn't be happy with it, and would end up trying to redesign it later!

Khalil, why do you suggest using a tachometer fed into an analogue input? Could a counter be configured to count pulses within a PLC? If possible Id rather have a one chip solution, or am I being unrealistic?
Also if possible Id like to have an accuracy of 1-2 km/h which relates into a resolution of approx .25 Hz. Can this be done with a PLC?

hello, i would like to know teh scheme u using for checking teh speed of the revolution. i had a similar project here but mine is to make teh feedback of teh motor whcih is controlled by a PLC. the speed controll scheme i m quite done with it and is running so far but .. i would like to add teh feedback so that, the whole system is an intelligent scheme .

ya i know, the vfd i m using got a display of the frequency presets on it but i would like to read teh motor's RPM back and fed it back into the PLC memory (by mean of a tachometer) and then, do some compare or logics to adjust such that the motor's revolution is at constant speed no matter whatever load size we put on the shaft.

howeverm, the vfd is displaying the frequency tho.. hm.. can some1 give a suggestion plz or enlighten me whether this is feasible.



--------------well actually, i had allocated budget to get a analogue module to do this(so that the tachometer can send signal to the PLC memory). however, i need to double check whether is this sensible.

Eric Nelson
Member

Join Date: Apr 2002
Local Time is: 07:39 AM
Location: Randolph, NJ
Posts: 3,022 Have a look at page 91 (PDF page 21) of the Application Notes. They explain how to create a tachometer. It should be simple to then compare the measured value with your preset max. speed, and fire an output.

I can't seem to locate any info on the input response times. This will be the limiting factor on how short a pulse a STAMP will reliably detect...

thanks eric. i found this in 1 of the thread related to my area of interest today.

--------------------------

my last question for the day is that, is PLC's memory capable to sustain a log of history of teh speeds applied w.r.t. to the time of teh day it is operating? without teh PC attached to it of course.

to my senses is that PLC do not have day time counters and how this history logs be possible to implement just like 1 of the firealarms boards that prints out a tiny sheet of history log telling, which zone's alarm was tripped at what time etc. what external tool i need to aid in this? i m planning to add this to my motor's extension on top of teh tachometer.

basically, this history is for checking when the end of teh day, i come, connect to my PLC, i load the history log into my pc and checks it.

can some1 enlighten me if this is feasible if so what do i need to use to make it possible. thank u
 
You could use the 0 to 10 output that is for actual output freq. aka speed and feed it into the plc. The voltage could then be used to display actual speed.

Remember with open loop VFD, no load and full load speed are differnt by the slip. At full load full rpm the 10 volts out from the drive is equal to the rated motor speed. For example, 4 pole motor is 1800 RPM syncronous but the name plate shows 1735 RPM rated. Too bad you VFD does not support encoder feedback.... You could control the motor rpm EXACTLY.

Last time I checked, encoder feedback is more accurate than tachometer feedback.
 
thanks leadfood, ya the suppler of mine suggested encoder feedback in this but my inverter doesnt support it tho..

however, i think the logging data requires to use a SCADA instead. the PLC itself cannot keep log history . hm..
 

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