Firstly, I must state that I have never programmed a microcontroller before, although I'm used to designing CMOS logic circuits and programming other forms of instrumentation. Also, I feel that function block software would be more appealing to me than ladder logic.
I want to purchase a "smart relay" or microcontroller that is 24VDC powered and that can accept at least one analogue process input (4-20maDC preferably, but 0-10VDC will do).
The function of the controller should be as follows:
The input signal is compared against a high setpoint and a low setpoint. If the signal is below the low setpoint, a relay output is energised. If the signal is between the high and low setpoints, the relay is de-energised. If the signal is above the high setpoint, the relay output is energised.
I would like also like to delay the relay energising by an adjustable amount (0-5 seconds). I don't want the relay to respond to short term input changes.
Once programmed, I would like to be able to adjust the high and low setpoint values and the time delay by means of pushbuttons and LCD display on the front of the controller.
I've looked at a few makes - Siemens "Logo", IDEC "SmartRelay", Mitsubishi "Alpha", Telemecanique "Zelio" (I'm based in Europe).
Could anyone kindly comment, please, on their experience, good or bad, with the ADC stability on the analogue input to the above controllers. Does one particular make of controller lend itself to my application?
I've got to go through the learning curve of the programming software and would appreciate some advice on which is the best controller to plump for.
Many thanks.