This is the RTD Signal Amplifier part of Temperature Controller or Indicator. The card also contains a relay on-off control circuit.
The Circuits are –
The PCB Boards –
This is the pcb board details of a Two Set Point Controllers for any process, shown here for temperature. For new types of transducers or input types, module card has to be designed or modified. The other cards remain the same.
The cabinet of these process controllers were made of steel for shielding, but the display card would still pickup EMI in some cases. These were more in instances where the Instrument supply was derived from the motor 3-phase supply. Instrumentation Supplies 230V AC must come from a Lighting Circuit of another supply arm, this has to come after conditioning with EMI-RFI filters and Servo Stabilizers or UPS if possible. This way the load spikes-glitches due to turn-on and turn-off of Motors and Heaters. dont act as a feedback to instruments. If line-load regulation is bad and mains voltage unstable, more problems can be expected.
This front panel shielding was done with a semi farady cage, by having a ground plane on the front of PCB, facing operator. This is just the negative of solder mask, but is the copper layer in front, no pth processing, even though it is two layer pcb. The solution worked well.
When you need a proportional control output, either 4-20mA or Time Proportional On-Off, This module is used. It does a slow PWM control, the cycle time for SSR or Thyristor Banks can be closer to Mains Frequency. The 4-20mA can be used to drive motors for turning valves for fuel or fluid heat control.
Schematics of Module
Board of Module
This input module converts J, K Thermocouple and 4-20 mA Inputs to 0-2V Full Scale. These can be used for any voltage/current inputs too. The RTD module can be modified more easily for Voltage inputs. The control output can be On-Off or 4-20 mA/Proportional with another card. The 4-20mA I/O STC1000I is not complete in documentation.
This is a Input Signal Conditioning Card for the Temperature controller. The voltage levels from sensors are either too low or need to be translated in level and span. Then for greater accuracy some linearization methods have to be used for a more precise reading. This also increases the cost. The circuits here do no cover the linearization see others in this and my related pages.
The step or segment linearization can be done by transistor, diode or CMOS switches to accomplish varying attenuation/gain for stages of the curve or voltage levels. In Microcontroller systems it can be done by lookup tables or math.
In some older digital systems without a MCU, the A to D drives the address of an Eprom Array to get a Digital Data for Display, as a linearized Reading. This Corrected Data was in turn made into analog using a D/A and then on to a Chart Recorder. This was a Logic only System of the early days. Microprocessor systems was expensive, power consuming and use to frighten people by getting lost in loops or a short nap.(they have fixed that, make sure you code properly).
Input Module – J and K Thermocouple with 4-20 mA
PCB Boards of the Module
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