improving my signal

[waynb48]

Hi I am using a plc to readin an analog signal. I tested the plc with a potentiometer and it recorded the correct voltage levels.when I connect my sensor to the plc terminals i can hear the contacts opening and closing real fast therefore the correct voltage levels does not register on the PLC. I tried using a filter i.e. capacitors improved slightly but not enough any suggestions please

 

[Ron Beaufort]

Greetings waynb48 ...

we're going to have to know a lot more about your system in order to help you ...

what type of PLC ... what type of input module ... where are you connecting this potentiometer to the module (screw numbers, etc.) ...

big ticket item: there aren't any "relay contacts" associated with an analog input ... best guess: your program is looking at the input signal, making an EQUAL comparison, and using that logic to turn a digital output (relay) on and off ...

if my guess is correct, then you need to understand that the analog input will not be "stable" ... specifically, its numeric value will constantly be shifting by several "points" ... that's just the nature of the beast ...

one step further: since the value is constantly shifting, an EQUAL comparison will constantly be turning on and off ... again, if I'm guessing correctly, an LIM (Limit) instruction might be more in line with your requirements ...

tell us more about your system and we'll try to help ...

 

[waynb48]

Reading in signals to my logo

My system consists of a sensor an ADXL103 the output of the sensor is an analog voltage which varies with the angle of tilt .The datasheet says the voltage varies between 0.3 and aroun 4.5 volts but I never got this.The aim was to connect the sensor to the plc and have the PLC read in the voltage and convert the voltage to an angle in degrees. Anyway I was helped out with a program as I had problems coming up with a scaling question. Initially I was told this wasnt possible using the logo but it is an amazing little device. Very versatile.I connected a potentiometer to the I7 (AI1 )input one end to the L (input power 12V) input and the other to I1 and managed to simulate a varying voltage at the input. It worked well but the signal from the sensor gave me a lot of problems Im sorry I dont have a circuit diagram. But I can upload the program

 

[BobB]

I would suggest that the first port of call is to check the maximum impedance that the device can power and the impedance of the input card. For example, if the device will only supply an impedance of 300 ohms and the PLC card impedance is 500 ohms it will never work properly. The device must be able to supply to a greater impedance than the impedance of the receiver.

 

[keithkyll]

The ADXL103 is a single IC, not a sensor in the sense we would think of on this forum. It's a G force sensor. +/- 1.7G. Typical application for these type of sensors is the device in your car that trips the airbag. This particular unit is sensitive to gravity, so it can be used as a tilt sensor.
The device needs further support electronics before it can be connected to the outside world.

Design info if Wayne wants to go further:
The datasheet shows an example using the 2 axis ADXL203 as a tilt sensor (fig 23, pg. 9). Not sure what one axis (ADXL103) will do. Output needs to be filtered. Minimum is 2000 pF. I recommend max of 4.7 uF for 1 HZ bandwidth. Output is through a 32K resistor. Buffer output with a opamp. I assume output swing with tilt is 1.5 to 3.5 volts, based on what I see for the ADXL203. Do offset and gain to shift it to the intended 0-5 volts. Power supply needs isolation to prevent noise. That means the zero reference of the Analog input on the PLC establishes the ground reference.

 

[John Soltesz]

The LOGO may have too low of an input impedance. If I remember correctly the input is a modified digital in that they can do a voltage compare. If so you could try an isolator, powered type, between the two devices. Moore Industries, Action Instruments, Phoenix Contacts all make these and you can adjust for your sensor to give you a typical output like 0-5 vdc.

 

[waynb48]

Hi I made a really simple mistake .I rushed to connect the sensor and didnt make sure the connections were soldered proper;y.The wires were moving about with proper soldered connections I got not perfect rreadings but much better. I configured the message text to show the sensor voltage output and the corresponding angle the last digit of each figure still changes rapidly I think if my resolution was say changed so I would have a change every 5 degrees then that might improve the readings. I think with this sensor noise has to be reduced from outside sources additional signal condition can be kept to a minimum