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
This was a Student Project of mine, it is a very basic instruments that could do most of the tasks on a test bench.
The cabinet used, was available as a part of a Hi-Fi DIY System, The Range and Mode switches were Modular Stackable Interlocked MultiPole Switches. The rest of the Interface were wirewound pots, jacks, sockets and plugs. The front panel was Engraved Acrylic, the panel was the toughest issue we tackled.
I wanted it to look well engineered, at least on the outside. It was a total analog instrument except for the digital indication of ICL7107. Thanks to ‘Intersil Hot Ideas’ Applications Book.
The above belong to an instrument Measureall shown below.
This was a Heater which was custom built for a CRT manufacturer, It comprised of many Tubular finned heaters. And probably a big fan that worked at high temperatures. This was used in a Heating Chamber which aged or tested CRT monitors. It had a temperature controller, a remote indicator. Learn about CRT here Cathode-Ray Tube .
This is a Multi Zone Temperature Control panel used in Plastic Extrusion Moulding. We made many of these and it helped us learn cable management techniques. The panel is a standard way it was built and some of the merits were the selection of panel components and wiring expertise.
This is a Regulated 0-30 V DC Power Supply manufactured by me in small numbers, 0-30V 5A and 5V -5A. It had a Switching Preregulator. This keeps the Vce across Transistor Bank at the Back of Instrument, at the lowest possible level, in order to keep them Cool. When Vce and Ic both are more in a Power transistor it will get heated more, then the efficiency of power supply is low.
The Output had both Constant Current Control and Constant Voltage Control. The CC control is also a Short circuit protection. The Output was protected from outside voltages damaging output circuits even when equipment is switched off. The back panel is a finned anodized black aluminum heat sink with many transistors.
I looked for the circuits to scan them, i could not find it. I made-n-sold quite a few of them years ago. All of them work well even today as i over-rated most components. The cabinet was Steel for EMI-RFI Immunity, it was painted with Stipple Matt Powder Coating, the Cabinet maker had just learnt to give such a finish to his boxes.
Some pages related to power supplies below. Power Design Section
Series and Parallel Transformers
Series and Parallel Connections in transformers. Observe the Dot-Polarity. When windings are in parallel the current capacity doubles. When windings are in series the voltage doubles. Wrong dot polarity in series connection just cancels the voltage and a small residual imbalance voltage remains. If wrong polarity connection is done in parallel, then a short circuit or overload may occur.
Basic Electrical Circuits
Center Tap Transformer
This is when you need to build a symmetrical dual supply like +12V and -12V supply for an opamp design.
This is also good for a two diode full wave rectifier. But when you make a dual supply use a bridge and the center tap is ground. If you use just two diodes then one winding may be loaded more than other. That may not be good for long term transformer life. The core may get magnetized.
Below is a Electrical Control Panel, It has a Strip Chart Recorder which is electro-mechanical, It has a PLC Inside. A handheld PLC Programming unit. The computer is showing the Ladder Logic Program. PLC is based on a embedded uC. The DIN Controllers are Analog + Digital circuits. So you see it is a Electrical Control Panel with Electronics and the PLC Ladder Logic means, Software too.
This is a PID Controller based Control Panel using a PLC from GE Fanuc Automation. The Computer is a 386 40MHz with 40MB HDD.