counter card (Beckhoff) not sensing pulses from open collector speed sensor-help pls

[matt_sd]

Hi guys,

I am using a Beckhoff EL1512 counter card to count pulses from a speed sensor. This sends a 24VDC pulse when close to a gear tooth.

The counter card required 15-30vdc for a logical '1' and -3 to 5vdc for a logical '0'. Typical input current 5mA.

The speed sensor is an open collector type, 3 wired (0vdc, 24vdc and output pulse) the output pulse is 21VDC with no load, as soon as connected to the couner card and a contact is made, the voltage drops to 7V, the pulse input lamp is lit but dimly on the counter card but the count is not incremented as the voltage is under 15VDC.

With my meter in series, the speed sensor is limiting to 2mA that is why there is a voltage drop.

If I use a wire and short directly between the pulse input (clock) and 24VDC, the pulse input lamp is much brighter and the count is incremented.

This is the spec on the speed sensor:-
- Open Collector outputs with
10Kohm pull-up, lmax = -20mA
The outputs are short circuit
proof and protected against
reverse polarity


My question is, is 2mA very low for a open collector speed sensor (I know a transistor is switching but seems very low)

Also the spec on the speed sensor says Imax = -20mA. why is it limiting at 2mA then?

I have tried the sensor just with a power supply (0vdc and 24vdc connected to power speed sensor) and as the output is short circut proof, by placing a multimeter on current in series with the output and the other side to 0VDC, when contact is made, the maximum current is still only limited to 2mA.

The speed sensor supplier have said they have never had any problems but 2mA output limit sounds very low.
Any help appreciated
Thanks

Matt

 

[JesperMP]

Just a thought:
Have you possibly swapped two of the wires, for example output and 0V wires ?
I have seen that when this has happened, the sensor seems to work, but behaves funny like you have observed.

Apart from that I would try to exchange the sensor.

 

[matt_sd]

hello JesperMP,

thanks for the reply. Just tried the wire swap but unfortunately it didn't make a difference

 

[JesperMP]

If it didnt even make a difference to swap these two wires, then it looks like the sensor is defective.

Also test that you multimeter's mA input isnt busted.

 

[Pego]

It seems you have a sensor with NPN output while you need a PNP.
That you get only 2 mA between the output and the 0V is because the pull-up resistor of 10KOhm is limiting the current.
Use another sensor or put an external pull-up resistor of about 1,5kOhm between +24V and the output, however you will get a signal when no gear tooth is detected....

 

[matt_sd]

Hello Pego. Thanks for your help! I see now, as the sensor is NPN, when the transistor is switched, 0VDC is set, but when the transistor is not swiched, the output is going via the resistor and ohms law(24VDC/10000r), that is why the current is being limited to 2mA

Seems strange, all the speed sensors I have seen from different manufactures are all NPN so why is becknoff's counter card PNP?

I will send an email to Becknoff asking them this and if it can be configured for NPN.

Thanks
Matt

 

[LadderLogic]

I will send an email to Becknoff asking them this and if it can be configured for NPN.

Don't bother: PNP inputs are a de-facto standard in Europe; for a European product one can expect PNP first and NPN, maybe, as an option. It is quite different from virtually all Japanese and many American products where NPN traditionally is prevalent.

While Beckhoff does have NPN discrete input modules, no counter module has NPN, as far as I am aware (we are using a lot of Beckhoff I/O in our designs).

So if you cannot find a speed sensor with PNP outputs (my guess is any European brand would have them), you might have to use some simple pull-up/pull-down resistor circuitry to convaert one to another. If I am not misaken, this topic have been discussed here many times, with schematics and all. Do some searching.

 

[JesperMP]

Well spotted pego !

Anyway, isnt this "speed sensor" just another word for "proximity sensor" ?
I'd be surprised if you cannot find a PNP proximity sensor.