Reading speed from Magnetic pickup

Hi Experts,

I am reading the engine speed from a magnetic pickup - magentic pickup is placed near the flywheel and when a teeth crosses it produces a pulse the output is an AC Voltage with frequency as speed increases both the voltage and the frequency will increase - and I am using a Phoenix contact transducer to read the frequency and converting that to the RPM inside siemens PLC

The problem I am facing is that when at lower speed the output from the magnetic Pickup is in the order of 200mV and the Phoenix Contact transducer is not able to read the frequency because it needs a minimum 5 Volts
even though the magnetic pickup is rated to produce 15 Vac but in this arrangement it is not able to produce that much

as engine speed up it produces more voltage and then the transducer will be able to read the frequency but i do not want to miss the speed at lower RPM

I need some ideas as how can I amplify the signal without changing the frequency to a a higher voltage minimum 5V and if there is any alternative to the the magneticpickup any other sensor which can count the teeths and I can get the speed

I have used off the shelf proximity switches, they are capable of switching at reasonably high frequencies often used these for position control on conveyors.
IFM and many others have many types from 5mm to 30mm, the 5mm are pretty good even if the teeth & troughs are small, the sensors have a cone shape field so putting it the correct distance can be effective, however, it will depend on how small your toothed wheel is & number of teeth.
You then will not require the converter as the Px will switch 24v.

Yes, a proximity switch with a high frequency rating is what you should use.

Can the voltage be measured, and characterize its value vs. speed at lower RPMs? Perhaps a diode feeding an RC circuit would convert it to DC and smooth it out some, however that could also affect the Phoenix transducer measurement.

The proximity switch seems like a better idea.

I don't think the OP really means AC voltage, it will be DC switching i.e. a sort of square wave based on the teeth , all the is doing is amplifying the pulses but at the speed he wants the pickup coil cannot react so the actual output falls into the mv range.
Try looking at these
https://www.ifm.com/gb/en/category/015/015_020/015_020_010#!/S/BD/DM/1/D/0/F/0/T/50

Or standard ones that can read 500hz probably enough for your application
https://www.ifm.com/gb/en/category/010/010_010/010_010_070#!/S/BD/DM/1/D/0/F/0/T/50
However, I have assumed you are counting pulses & not analogue conversion ?.

https://www.ti.com/product/INA333


Instrument amplifier?

Will proximity switch work to read the number of teeths that crossed per minute as shown in the picture and I am using siemens PLC do I need to get a high speed counter module also .

I don't know about the 1200 series PLC, but it probably has high speed inputs there should be an OB that works on interrupt of the I/O, on the older S5 it was possible to do it a different way (providing the pulses per second were not too high, One way we did this was to create a Program block where the peripheral image i.e. PIW 0 was transferred to the IW so updated the I/O image instantly & also did the maths, the block was called a number of times in OB1 so for example if the scan time was 20ms & the block was called 5 times then the average update of the I/O image was about 4ms fast enough for many moderate speed systems.
This was used on a machine that put mini sausages into a can the speed was so fast the only way we could see what was happening was to use a high speed camera, the process used a bowl feeder that fed the sausages into a linear transfer belt, this counted the sausages, a diverter would guide tha sausages into the can, when the count was reached the diverter moved back to the following can as it was released into the fill position, it also measured (roughly) for broken sausages & rejected them, so it was pretty fast.
We also used the same as you are using on a conveyor to measure a pallet size, again the speed was reasonably high & it was a quick retrofit so no time for sourcing a counter card etc.
Providing the pulses from the proximity are not smaller than about 10ms apart there should be no problem.
Just looked & the 1200 does have I/O interrupts so should be no problem.

qwemx said:

... do I need to get a high speed counter module also .

Click to expand...

Without knowing the number of teeth on the sprocket (25? 24? 28?) and the maximum RPM, it is hard to answer that query.

Unlikely to need high speed module or board as the integrated counters in S7-1200 are capable of upto 100Khz depending on the input chosen.


Post details of your sensor and module and how you wired them to the PLC. It sounds like analog sensor.

The magnetic pickup will produce low voltage AC signal. You need to get an amplifier/signal conditioner to turn it into square wave probably.

https://www.omega.com/en-us/communi...ioners-and-transmitters/amplifiers/p/FLSCAMPA

Agree with Calabdel, the first few I/O in the integrated PLC block are high speed inputs, some PLC's have hardware counters built in with direct access to these, the mapped input then goes through a filter usually 10ms to act as standard I/O, Interrupts are also driven off these I believe there are reserved Organisation blocks dedicated to these.

Going back to the original post of this thread though, the problem was that at low speeds (RPM; frequencies) the hardware does not generate enough voltage for the counter to detect a pulse.


So maybe @g.mccormick has the best idea i.e. a signal conditioner.

drbitboy said:

Going back to the original post of this thread though, the problem was that at low speeds (RPM; frequencies) the hardware does not generate enough voltage for the counter to detect a pulse.


So maybe @g.mccormick has the best idea i.e. a signal conditioner.

Click to expand...

Edit:Re-read the original post. Change it to a proximity switch as suggest above.

drbitboy said:

Going back to the original post of this thread though, the problem was that at low speeds (RPM; frequencies) the hardware does not generate enough voltage for the counter to detect a pulse.


So maybe @g.mccormick has the best idea i.e. a signal conditioner.

Click to expand...

Yes, these always work well.. Down to 5mv input. Adjustable

https://www.partdeal.com/dynalco-sp...Q64A2LPFhcKKrXiPKCC5axLXHGm1_J2BoC1KIQAvD_BwE