Two vs three wire potmeter measurement

[somlioy]

Hello, I've got an PLC which is going to measure the position of couple of actuators by means of potentiometers.

I have the choice to choose between two different PLC-modules;
 - use two wires and measure the resistance directly or
 - use three wires and measure the voltage instead

The modules costs the same but the tree-wire version only provides 4 analog inputs whereas the two wire version provides 8 analog inputs.

What are pros/cons of doing it either way?
Are any of the methods more susceptible to noise? System is to be used in an industrial environment.

[ejeffrey]

The biggest advantage of 3-wire connection is dealing with wiper chatter.  As you rotate the pot, the wiper can chatter, causing high contact resistance or going open circuit.  In two terminal connections you normally short the third terminal to the wiper so that the maximum resistance is 100% full scale, but this can still be a problem. 

With a three terminal connection the wiper current is close to zero (just the amplifier input bias current), so any contact chatter is easily filtered out by a small capacitor.

The main downside of a three terminal connection is that it takes three wires.

[rstofer]

I would bring all 3 wires back to the controller whether I used a 2 wire module or a 3 wire module.

I always liked to experiment so, when nobody was watching, I would try both and see how it worked out.  A disadvantage of the 2 wire method is that the cable resistance is added.  This might not be a problem depending on the wire gauge.  I don't think I ever used anything smaller than #14AWG for control systems.  The voltage method has the advantage that you can measure the output at the transmitter with a DMM and possibly compare it at the PLC during troubleshooting.  Measuring resistance on an energized system, well, I'm nor sure how that would work out.

[Ian.M]

If the eight input version can also accept 0-10V analog input voltages, consider doing three wire measurement with an external precision 10V supply to the pots, or if you've got a well regulated 12V supply available, use series dropper resistors of 1/5 the pot track resistance, 1%, with a decoupling cap across the pot track.

[dom0]

I don't think I ever used anything smaller than #14AWG for control systems.

You always use at least 2 mm² wiring even for control signal circuits? That seems quite like a waste of copper (and thus money) to me.

[Renate]

In 2 wire vs 3 wire I sometimes choose 4 wire.

In 2 wire for approximate temperature a 10k PTC is not too bad.

In the old days gas gauges in cars used 90 ohm 2 wire.
They interfaced well with those clever magnetic gauges and they stayed at the same reading when you turned off the engine.
But nobody likes interfacing 90 ohm senders to a microcontroller.

Many things in cars today run off 3 wire pots running on regulated 5V.
This is a much more convenient signal to interface.

I had a propane tank that used a 90 ohm two wire.
I didn't feel like pulsing the current and measuring so I put in an I2C 3D (only 2D used) magnetic sensor, i.e. 4 wire.

[mrflibble]

I don't think I ever used anything smaller than #14AWG for control systems.

You always use at least 2 mm² wiring even for control signal circuits? That seems quite like a waste of copper (and thus money) to me.

A-ha! So that is where the global copper production is going.

As for 2 vs 3 wire, clearly 3 because 3 more better. And if this clear numerical advantage is not enough, there are also the humdrum practical considerations ejeffrey already mentioned.