Sensor Voltage Problems

Hi Guys and Gals,

I have an AST4600 Pressure sensor connected to an Allen-Bradley Micrologix 1200 PLC via the 1762-IF4 module.

The problem is that the sensor reads the correct output voltage when disconnected from the PLC, but gets "pinned down" in voltage when connected.

Lets say I have 5800psi on the sensor, it will read 4.127V with the sensor disconnected which is just fine. When I connect it I read 3.083V at that same pressure, which is wrong. When I reduce the pressure on the sensor, it remains at 3.083V until I get to 3800psi in which it will read correctly and give me the correct voltage that I can translate to the correct pressure. So its basically Translating any pressure above 3800psi to 3.083V when the sensor is connected to the input module.

What is going on here? I had the same problem when trying to use small voltage dividers (potentiometer and resistor to adjust voltage) to simulate a pressure sensor on the 1762-IF4 input, however it worked with a benchtop power supply using the voltage. I have called AST and they can't seem to see any reason for it.

The specs of the sensor are:

10-28VDC Excitation
1-5V output (0-7500psi)
output impedance <100ohms, nominal
current consumption <10mA
Bandwidth (-3dB): DC to 1kHz
Output Noise <2mV RMS
Output load 5k Ohms Min.

Thanks in advance....

It may be an incorrectly configuration/connection at analog input module. If the module configured for 4-20 mA input it may bring your voltage down when you connect your transmitter to it. Check you analog module configuration and connection. Sometime it is require some jumpers to be moved also.

As val_99 says. Heres a picture. ( note factory default is current)

100 ohm output impedance of the transmitter trying to drive 250 ohms at the input, an equivalent circuit of 71.4 ohms. No wonder the voltage signal caved in . . .

Dan