This contains the Main card with a Power Supply and Relay Control. On this card is connected the Display ICL7107 – Temperature controller.
The Thermocouple and Control Modules can be plugged into this card, these change the type of control and type of inputs. This way this can be made into any parameter controller with any type of input and output. But it is all set in production, not configurable at site.
So even if you make a 4-20mA output Flow Controller with this, the Main card and Display card remains the same. Only the Modules change. No Connectors are used, to make it vibration resistant.
The PCB Layout is here
This is the Display Circuit and PCB part of of section Temperature Control.
The above circuit is powered by +5 and -5 from a LM7805 and LM7905 pair. If +/- 12V or +/- 7.5V is used in opamp or digital parts, then use below circuit for the DPM section.
The PCB for above
The PDF Circuit for above Display Card STC1000
A Two Point Process Controller or Temperature Controller would look like this. When a High-Low Alarm Protection for uC based Controller was needed the same was modified and used.
Sometimes uC based instruments just forget and that can be costly, the watchdog timer has solved this problem. In Industrial Process Control we cannot take any chances, so a two tier or even three tier protection is required, especially so if the Job or Raw material being processed is expensive. So Real Time Analog Trip + uC Controller is safe. Then you can still have Electro Mechanical Devices for Extra protection against Fire or Meltdown.
Temperature is the most common process parameter, Pressure seems to be next. Flow, Level, RPM, Distance and many more follow.
When a physical parameter let us say distance is measured, we first need a sensor that converts it into electrical values, digital or analog. The sensor used for length is a Linear encoder and it creates the electrical information for the DRO to Display.
Now many sensors like a strain gauge produce very low and weak signals and some are non-linear. So the signals undergo Amplifying, Temperature compensation and Linearizing together called Signal Conditioning. This stage outputs some standard values like 4-20mA, 0-10 V, 1-5V, or even directly digital or wireless in the new technologies. Even when a digital interfacing standard is used, amplification and some signal conditioning is required at sensor end. Compensation and Linearizing can be better done with a uC using Math or Look up Tables.
When you have many different parameters then it is better to use process controllers with a 4-20mA input standard. Then the inventory of controllers can be reused for any process.
It is always better to use sensor-end transmitters to get 4-20mA to your Controller, Recorder or SCADA System.Weak signals are corrupted by noise and dont travel long wire distance. Junction EMF at joints and terminal blocks add to errors. These are overcome by the 4-20mA signal that has the juice and punch to transmit the data over the shop floor to the control panel rooms.
Read more here Temperature Measurement and Control