Control.com - Nerds in Control

I brought a Banner (L-GAGE LG10 Series) Laser Gauging Sensor for a project that I'm working on. The sensor measures displacement of a moving target by emitting a laser which is then received by a PSD (position-sensitive device) receiver element. The output provides a variable of 4 to 20 mA. What I want to do is to connect the sensor to the DCS and produce trends and alarms. I am looking at a unipolar single ended analog input module (ABB AI810 Analog Input Module) to connect my sensor. I was told that I needed to use a signal conditioning system, which must convert the 4 to 20 mA current signal to a voltage signal using precision resistors. What is the purpose of this process??? why can't I just connect 4 to 20 mA output directly to the DCS???

Perhaps because the analogue input card on your DCS is a voltage input and the sensor has a current output?

A voltage input has a high resistance (typically anywhere between 10s of kilo-ohms to mega-ohms). A current input has a low resistance (typically 250 ohms). Voltage outputs control the voltage level, while current outputs control the current level. Voltage is usually a bit simpler, but current is less sensitive to induced electrical noise.

Using a signal conditioner to convert from one to the other is very common. Your terminal block or instrumentation vendor should be able to sell you one.

A precision resistor is put it across the (+) and (-) terminals of the analog input to create a voltage drop. The resistor size is chosen to get the required input voltage.

For instance, if the analog input is looking for 1-5V, then use a 250 ohm resistor.

Signal conditioning modules are needed for isolation if there's ground loop problem, not to convert from 4-20mA to a dc voltage, a resistor suffices.

The instrument guys know how this works. Ask 'em.

The analogue input card can measure either current or voltage on the DCS (4 - 20mA & 2 - 10V). The problem is the sensors response frequency is to fast for the DCS and since I am measuring vibration the trends on the DCS will be a series of oscillations. I still don't understand how a signal conditioner can help me solve the oscillation for a more constant output trend, making setting alarms and trips easier?

Well, you didn't mention that there was a frequency response problem, so no one discussed that point.

It appears that what you need is a frequency to current (or voltage) converter. There are lots of ways to do this. I can't recommend any particular model, but there are lots of these available from instrument and terminal block suppliers.

I looked up the L-Gage LG10 sensor and it appears to offer a 4-20mA sourcing output option and a voltage output option, depending on what you purchase.

The ABB AI810 card appears to be an 8ch Single-Ended Analog Current Input Card.

If you have the 4-20mA output option on the LG10, I would expect wiring the two together would work for you, unless I'm missing something. You may want to double-check things with Banner and ABB.

In general, a signal conditioner would be useful to convert the signals, amplify if needed, isolate the circuits if grounds were a problem, change between a loop-powered current and a sourced voltage/current signal, or offer a level of surge protection on the loop.

If you find you may need signal conditioning, send me an email. Acromag can assist.

Kind Regards,

Donald Lupo
dlupo [at] acromag.com