Inductive proximity sensor interfacing

[xilog]

Hello All,
I have a seemingly simple question. I have a Turck AC powered inductive proximity sensor. The load has to be connected in series with the power leg of the sensor, this being the case, how can one sense the input to a PLC? I can connect a lamp in series with the sensor and watch it operate but I am lost as to how to create an input signal to a PLC. I was thinking I could use a AC relay, however I am trying to use the prox to count the links of an iron conveyor chain, so the relay would not have much life with it continuing to cycle on an off.

Thanks

[BTalbot]

Do you have any dc inputs on the plc? This would be prefered(using a dc prox sensor). The biggest gotcha wit ac sensors into ac plc inputs is "leakage current" many times their is enough to keep the input on at all times. Post the p/n of the prox and plc you are using any perhaps someone can help

[narcilla]

BTalbot,

I posted a similar query

http://www.plctalk.com/qanda/showthread.php?t=54017

what's the effect of the max off state current of 1771-IAN's being 2.3mA to Turck Bi15-CP40-FZ3X2's minimum load current having to be equal or greater than 5mA (as per its data sheet)?

[milldrone]

Originally Posted by BTalbot

The biggest gotcha wit ac sensors into ac plc inputs is "leakage current" many times their is enough to keep the input on at all times.

I believe any two wire sensor has this problem not just AC.

Originally Posted by narcilla

the max off state current of 1771-IAN's being 2.3mA to Turck Bi15-CP40-FZ3X2's minimum load current having to be equal or greater than 5mA (as per its data sheet)?

If the sensor has already been selected, then a resistor is need to help "pull down" the voltage at the PLC input. The value of the resistor needs to be calculated so that the prox and the input currents do not conflict.

Personally I try to avoid two wire devices all together (other than 4 to 20 ma analog of course).

[OkiePC]

EDIT: 5ma? What kind of input card? Use a resistor. It won't take much to bleed off 5ma...find out how much current passes in the conducting state...and apply ohms law to your circuit.

Small interfacing relays should work too, but check the specs as shown above...

Originally Posted by old school

If you size the resistors wattage at twice the estimated for 100% duty cycle and put heat shrink on the exposed leads and mount them nicely across a terminal strip and remember that they are little heaters and that they will slow cook anything adjacent to them then they are the best solution.

If you can't do the above, then the good old ice cube relay which takes more panel space, might be the low maintenance solution. It will dissipate the leakage as well as provide isolation to allow for any voltage interface and is thereby more flexible, and electrically isolating th e field device which can be a bonus (ie. short to ground fries the input card, prox, and resistor, or only the prox and the plug in relay?)

IMHO
Oakie

[narcilla]

milldrone,

Without the resistor and the proxi switch connected directly to the PLC input, how would it affect the operation of the proxi? Will it still respond to the item it should detect?

[Roy Matson]

If your PLC is 120 Volt input you need to be really careful selecting the proximity switch.
Most prox switches draw too much current in the Off state i.e. the Off state current is enough to turn on the PLCs input. I am used to the P&F brand, ony one or 2 will work with a 120 VAC input module.
You could load down the input with a resistor but even then you need to be very careful when you select the resistor. You must allow for heat dissipation assuming the input is On 100 % of the time in case that's where the conveyor stops.
Regards
Roy

[narcilla]

Roy,

My concern is the Minimum load current of the proxi which says it has to be greater than or equal to 5mA. The PLC input will not offer this load current and so, will this affect the operation of the proxi?

I'm not quite clear with the proxi's specs, the Off-state leakage current is stated as <=1.7mA and it's minimum load current as >=5mA. How does this two affect each other? Do you have to have a 5mA current through the circuit so the proxi operates reliably?

[Tom@Pton]

We use a snubber (RCS7A6V for 120v) on prox sensors to PLC inputs. Some have been operational for 10 years, give or take a couple years.

[xilog]

Thanks Folks,
From my rookie perspective, if my PLC has relay outputs, I have to determine the leak current from the prox sensor and match up a resistor value which will create a load for the AC sensor which will allow it to function. Sounds to me that using a interposing relay is as a simple solution, would the relay require a snubber circuit?

[Tom@Pton]

What is the frequency that you are counting? Hundreds of times a minute? Once or twice a day? The interposing relay is a mechanical device which may not have the response time or the the live expectancy you need at higher speeds. Look at what you are trying to accomplish first, then see if an interposing relay is a good idea.

[Tom@Pton]

My cost for that snubber is $9.50 from a supplier that is not known for competitive pricing.

[xilog]

The pulse frequency is about 100-150 ppm, i am using a sensor from some surplus equipment to retrofit a machine in our shop.What I would like to do is replace the limit switches which trigger some pneumatic solenoid valves, with a PLC controlled system to increase the precision of operation. We don't have much money to spend on R&D so I am trying to use what we have on hand

[milldrone]

Originally Posted by narcilla

milldrone,

Without the resistor and the proxi switch connected directly to the PLC input, how would it affect the operation of the proxi? Will it still respond to the item it should detect?

Originally Posted by narcilla

Roy,

My concern is the Minimum load current of the proxi which says it has to be greater than or equal to 5mA. The PLC input will not offer this load current and so, will this affect the operation of the proxi?

I'm not quite clear with the proxi's specs, the Off-state leakage current is stated as <=1.7mA and it's minimum load current as >=5mA. How does this two affect each other? Do you have to have a 5mA current through the circuit so the proxi operates reliably?

narcilla,

Chances are it will not detect the object correctly. It may work some of the time however.

[xilog]

Hi Milldrone,
Actually, I would like to use the prox sensor to count the links on a iron conveyor chain and then use that data to fire the pneumatic solenoid valve after the count reach a predetermined set point.

Thanks