Hi,

I have a control system with a PLC and a PC based HMI (human machine interface).

I have an application where I have two bare resistance strips through which I want to pass a current. (I will be using these to seal the edges of two pieces of mylar). The current through these strips will typically be about 8 amps at 115 VAC.

I would like to be able to control and monitor the current through these resistance strips using my PLC/HMI. Basically, if an operator enters a setpoint of 8 amps on the HMI, I need a way for this setpoint to be communicated to the current controlling device. I also would like to be able to read the actual current into my PLC/HMI so that I can check to make sure the actual current equals the setpoint current.

Does anyone know of an instrument that would allow me to do this?

Thanks in advance,
Paul

 

How to communicate to the current controlling device depends of the input the controlling device want to have. If it need a voltage input signal or a current loop signal the PLC/HMI must be able to send out that signal. Another option could be too use some kind of serial communication.

One sudgestion to measure the current is to use a clamp meter able to send out a voltage. This is only possible if the distance between the point of measurement and the analog input of the PLC is no more than a couple of meter. Othervise use an instrument with a 4 - 20mA current loop. More information about the PLC is also needed to give a good answer.

 

If you are going to use AC voltage and thus AC current you can use TRIACS to control the average current through the resistors and use a current transformer in combination with a resistor and a low pass filter to monitor the current.
Otherwise, you can use DC voltage and use PWM to control the current and use a hall effect sensor in combination with a low pass filter to monitor the average current.

 

mik3,

I was thinking along the lines of what you suggest, except using hall-effect sensors for current monitoring. Unless the inputs to the PLC are isolated, then you can use a sense resistor and an amplifier which incorporates a low-pass filter.

Anyways, he wants a ready-made solution by the sounds of it. Maybe some sort of motor drive? Then rig up the sense resistor/amplifier yourself?

Steve

 

Steve,

I thought of the sensing resistor solution but in my mind was the fact that it will dissipate energy as heat so I told him to use hall effect sensors. Of course a resistor solution is easier and cheaper, its up to him and his design specifications.

 

Sorry, I missed that.. I was referring to the AC part of things, which you suggested a current transformer. You are able to use a hall-sensor for AC as well, you just need to make sure to get the bidirectional type.

Steve

 

In my opinion, an improvement to this machine is to monitor the current through and the voltage across it (thus the power) as to be able to calculate the temperature of the resistor and keep it constant rather than care about the current to be constant. This is because if you keep the current constant the temperature will rise and at some point the resistors will burn.

 

If it's a plastic sealer, then the time current flows will be brief - on the order of a few 10's of milliseconds.

 

If the time the current flows is soo brief, then why not modulate in the time domain, rather than in amplitude (current)? It would be far easier to time 30mS or whatever, then trying to control current. This leads to the problem of having such a low-frequency waveform, since you would need to ensure that you switch on at certain instances.

Steve