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Two tanks with three continuos, redundant level sensors each need to be supervised so that when a certain total volume is reached, a specific actor is triggered. This could be done via the installed PLC system, however it is likely that a regulatory body will demand a hardware solution. However SIL compliance is not demanded. The important thing is that it's hard to alter the function or change parameters.

So I need a device that can do the following:

  • Take average from three analog signals for tank 1, add offset
  • same for tank 2
  • add both values and give a digital signal to trigger if total exceeds a certain value
  • don't add value for tank 1 or 2 to the total, if a maintenance switch assigned to this tank is on

Can this be done in a logic module? If so what specifications (not the specific model) do I need? If not, what's the device called and how to specify?

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  • $\begingroup$ Can you please tell us more about the redundant level sensors? Is it possible to share a picture or sketch of the tank? What are electrical characteristics of the three analog signals? $\endgroup$ Sep 3 '21 at 12:19
  • $\begingroup$ no picture, just three sensors measuring essentially the same level $\endgroup$
    – mart
    Sep 3 '21 at 12:24
  • $\begingroup$ How about electrical characteristics of the three analog signals from the sensors? $\endgroup$ Sep 3 '21 at 12:27
  • $\begingroup$ You can use op amps to add the signals, or you could use a parallel output ADC (if those still exist) followed by a whole bunch of logic chips. Neither is a very elegant solution. $\endgroup$
    – Drew
    Sep 4 '21 at 1:41
  • $\begingroup$ 4-20 mA each. I need a prepackaged solution. $\endgroup$
    – mart
    Sep 4 '21 at 19:58
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All the major industrial PLC manufacturers make micro PLCs.

enter image description here

Figure 1. Siemens LOGO! base module with built-in HMI and keypad. A larger panel-mounted remote display is available.

The base unit has up to four analog inputs which are 0 - 10 V. You may require 3 × 2-channel 4 - 20 mA analog input modules.

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Figure 2. Most of the program requirements done for Siemens LOGO! using Logosoft Comfort and running in the simulator. Download over Ethernet. Double-click for full size view.

  • The analog inputs go through an analog amplifier for scaling. The AM (analog markers) are required to terminate the analog signal as far as I can remember.
  • AVERAGE 1 takes sums the three analog signals and divides by 3.
  • The SUM block sums TANK 1 and TANK 2.
  • The COMPARE block compares against the setpoint. I've added some hysteresis.

The only thing left for you is to add in some MUX (multiplexor) blocks to output a zero if the maintenance switch is on.


I strongly recommend that you avoid hobby boards for industrial controls. PLCs are hardened for industrial environments, have proper isolation between inputs / outputs and control and are very reliable and have a large userbase.

If there is any danger to personnel or property then you need a fail-safe system or fail-safe backup system in conjunction with the PLC. Once you install this you will be liable.

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  • $\begingroup$ How would you tamper proof a micro plc? $\endgroup$
    – mart
    Sep 7 '21 at 8:54
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    $\begingroup$ Use password protection. The Siemens LOGO! series have it. $\endgroup$
    – Transistor
    Sep 7 '21 at 9:27
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Take a look at Analog input modules from National Instruments similar to the one below.

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This is prepackaged solution Labview software controlled solution.

Another good place to look at is hobby electronic market.

Some suggestion are

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Based on the information provided either of the above will be able assist in achieving your end goal. All of the above have the ability to accept analog inputs.

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