ladder logic for level transmitter

What Ladder Logic programmer are you using?

Does the PLC itself have 4-20mA analog inputs?

Does the PLC have Relay Outputs?

If you are using either CScape (Horner) or Cimplicity/Proficy (GE Fanuc) I could probably send you an image of a simple ladder diagram to do this.

Mike

1. RSlogix 5000 and/ 500

2. The PLC has a 4-20mA analog inputs and relay outputs

However, I will still welcome any ladder logic diagrams from any programmer and use that idea for the rests.

Thanks Mikerho

Dear Guest, Am I correct in assumming that you have a PLC with the level transmitter and motor already connected and you want to make this modification ? If so then if you could forward the make/ model then you will find assistance here on CR4.

Kind Regards

The make of the PLC is allen bradley and the level transmitter is a radar from www.vega.com. Besides the motor is an ABB (22kW) model.

Regards

• Range the input signal 4-20mA into 0-100% engineering units
• Raise an out-of-range error if the signal is <0% or >100%
• Compare the input in engineering units with 15% and use that to drive the motor

3 rungs. Maybe less.

Some PLC's contain internal function blocks for what you require, you just put in the required scale, raw value (analog channel), a couple of more parameters and it will return the scaled value. It will also indicate error conditions (<0% and >100%). This information will be contained in the PLC documentation.

Set up a rung with an analog (4-20mA) input for the ore level (condtioned upon your equipment start permissive logic directly to the enable input) to a geater-than (relational) block PV with the output of the GT (greater-than) connected to a "Set" bit/coil. This would be to Start the motor when GT 15%.

Do the same type of logic on the next rung and use the same ore level analog (4-20mA) to feed a Less-than-or-equal-to" LE block with the output of the LE going to the "Reset" of the same bit/coil used to start the motor.

You will need to program a "deadband" of operation into your logic to prevent "short-cycling" of the motor which will severely shorten the life of the motor. This "deadband" must be based upon the motor nameplate DATA and "duty-cycle" rating.

The next rung would be a series logic bit string that uses the "Set-Reset" bit/coil connected to a discrete output that switches the motor control circuit.

The analog PV value from the 4-20mA ore level must be either linked to an HMI graphic display tag or in series with an analog display meter scaled to properly reflect/display the level in 0 -100% or 0-xxFt.

If you are not using a HMI graphics display, you may contact any display meter manufacturer and they will be more than happy to create whatever scale on the display meter you need.

Please keep in mind that you may need to "condition/filter" the analog signal if the dynamics and/or noise is excessive in your circuit wiring.

You may also want to "truncate" the analog PV value before displaying to provide a whole number if decimal points are not needed.

Also if you are using "seal-in" / maintained motor stop logic, you will need to add a separate rung dedicated for the stop logic.

Good luck!