Temperature
Control
in
Automation
:
Temperature Controller 48-96
A 1/8 DIN Indicating Temperature Controller. This
is a Analog Controller built around ICL7107.
This is a 48-96 DIN Temperature Controller, it is
mains operated, it is in a Industrial Plastic Housing. It is not meant
for very harsh environments. Suitable for Control Panels in Control
Rooms. It has a accuracy and linearity of 0.5%. It had options of
on-off or time proportional control.
Complete designs and PCB of a Generic Analog
Controller
Main Cards having Power Supply and Digital Display
Signal Conditioning and Control Modules on
BergStick/soldering.
Two Setpoint Temperature Controller
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 with 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, Humidity, Strain, Torque, 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, pulses(events/limit) 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, EMI/RFI or
Noise Cancellation 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 of today. Even when a digital
interfacing standard is used, amplification and some signal
conditioning is required at sensor end. Compensation, Linearizing and
Noise Rejection
can be better done with a uC using Math, Look up Tables, DSP, Software
Filters etc..
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 limited, as it can be reused for
any other process.
It is always better to use sensor-end transmitters
to get 4-20mA to your Controller, Recorder, Data Logger or SCADA
System.Weak signals
are corrupted by noise and dont travel a long wire distance. Junction
EMF
at joints and terminals 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. Some times Optical or
Magnetic isolation and modulation of analog signals is required. When
the signal wires have to mesh with high power lines on their way back
(not a good practice) or the sensors could get accidental exposed to
live energy points; the control room or panel instruments can be
damaged.
Lastly where corrosive materials are being
processed, assume it is always so; Protection is needed for sensors,
junctions and wires. When volatile fluids are being processed, the
sensors, wires, control units all must perform at low energy levels;
sealed hermetically, etc. A tiny battery spark may undo all the
measurement and control efforts !.
Anantha Narayan
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