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DT231/2 Control and Instrumentation MiniProject
Student : Alan Sherry
Date : 17/December/2003
Lecturers : Ciaran Young & Gavin Duffy
INTRODUCTION:
For this project an electronic temperature sensing instrument was successfully
designed and built using VERO board.
An RTD was used as a transducer to transform ambient temperature into an electrical
signal. The RTD used in this experiment consisted of a platinum winding encased
within a stainless steel sleeve. This signal was then conditioned using a wheatstone
bridge to give an output range of 0 to 100 mV.
This works because an RTD transforms temperature applied to it into a proportional
change in resistance and the signal conditioning circuit can be used to scale this
output into a more easily readable output signal.
This is described in block diagram form below.
The required linear output was successfully obtained in the ranges as follows :
Output Range :
Input Temperature Range:
0 to 100 mV
0 °C to 100 °C.
An RTD (Platinum resistance Wire) Temperature Sensor RTD100 was used as the
temperature sensing element . RTD sensors generate output signals in one of two
ways: through a change in output voltage or through a change in resistance of the
sensor's electrical circuit. The Platinum RTD, Resistance Temperature Detector is one
of the most linear, stable, and reproducible temperature sensors .An RTD’ resistance
vs temperature characteristics are stable, reproducible, and have a near linear positive
temperature coefficient from 200 to 800 °C.
These attributes establish RTDs as a de-facto industry standard and it was for this
reason that I wanted to have an RTD in my project even though the time constant is
longer than it would be for a thermocouple.
It was decided to input a supply voltage of 15 Volts into the WHEATSTONE
BRIDGE signal conditioning circuit .In this case ,it was not required to build a linear
amplifier to obtain the desired output range .If one had been required I could have
chosen to condition the output of the wheatstone bridge with an instrumentation
amplifier using UA741 OP AMPS or even a differencing amplifier .
In such an event the output of the amplifier would have to be within the saturation
range which for a UA741 is approximately +-15 Volts
The resistance of the RTD increases with temperature and in such circumstances a
corresponding proportional increase in voltage is detected at the wheatstone bridge
output .
The RTD Temperature Circuit was tested in the environment of the lab using
beakers of boiling and warm water and also using a beaker of ice and icy water .
The RTD was wired into the signal conditioning circuit ( a wheatstone bridge used as
a deflection bridge) and was brought into contact with temperatures over the required
range .
Design:
When building my temperature sensor, I had different choices available to me
(Thermocouple or thermistor with their different advantages and disadvantages). I
had to consider the different constraints imposed by different design systems.
Below is an outline of the design of an RTD. As I choose to use an RTD for the
project , I will only briefly mention other types of sensors.
RTDs:
These are precision temperature-sensing devices. They're the ones to use when
applications require accuracy, long-term electrical (resistance) stability, element
linearity, and repeatability. The devices can work in a wide temperature range—some
platinum sensors handle temperatures from 328ºF to 1202ºF.
The sensing element in RTDs is typically a fine platinum wire winding or thin
metallic layer applied to a ceramic substrate. The platinum resistance thermometer is
the primary interpolation instrument used by the National Bureau of Standards in
applications with operating temperature ranges from 436ºF to 1135ºF.
RTDs as a class are divided into two types:


Resistance wire RTD
Thermistor (thermally sensitive transistor)
RTDs work by producing a predictable resistance at a given temperature. Resistance
wire RTDs (generally platinum) have a positive coefficient by increasing resistance
with temperature increase. The resistance of most metals increases reasonably linearly
with temperature in the range –100°C to 800 °C. This is the physical principle the
instrument is based on. Nickel, copper and platinum are the main materials used .
Platinum has the most linear characteristic over the widest range and thus is the most
desired and the most expensive. Thermistors are generally negative coefficient by
decreasing resistance with temperature increase.
Some of the advantages for choosing the RTD for the lab experiment are set out
below:
RTD Advantages:
They are possessed of a wide temperature range for example , Watlow Gordon
platinum sensors cover temperatures from -328 to 1202°F (-200 to 650°C),with
repeatability and stability: The platinum resistance RTD is the primary interpolation
instrument used by the National Bureau of Standards from -436 to 1135°F (-260 to
630°C). Precision RTDs can be manufactured with stability of 0.0025°C per year.
Industrial models typically drift less than 0.1°C per year.
They have a high output ( The current drop across an RTD provides a much larger
signal than thermocouple voltage output) and linearity of output (extremely important
for this project, platinum and copper element RTDs follow a more linear curve than
thermocouples or most thermistors).
They have a low system wiring cost. Unlike a thermocouple, an RTD uses ordinary
copper leads for extension wires and requires no cold junction compensation.
The coils inside the stainless steel sleeve of the RTD take advantage of area averaging
for temperature response. Point measurements, while often desirable, may cause
errors. An RTD element can be spread over a large area, improving control with area
averaging, a technique impractical with thermocouples.
Precision thermometers can be manufactured with stability of 0.0025ºC per year.
However, industrial models typically drift < 0.1ºC per year. RTDs with platinum and
copper elements follow a more linear curve than thermocouples or most thermistors.
Unlike a thermocouple, an RTD uses copper wire products for instrument connection
and requires no cold junction compensation. As a result, system cost is often lower.
Although point measurements are often desirable, they can cause errors. An RTD
element can be spread over a large area, improving control with area averaging, an
impractical technique with thermocouples. The voltage drop across an RTD provides
a much larger signal than thermocouple voltage output.
Effects of Leadwire Resistance :
Because the RTD is a resistive device, any resistance elsewhere in the circuit will
cause errors in the readings for the sensor. The most common source of additional
resistance is in the leadwires attached to the sensor, especially with assemblies that
have long extension leads as part of the assembly. The amount of error introduced
into the system will depend upon the length and AWG of the wire as well as the base
resistance value of the RTD. Leadwire error can be significant, especially with long
runs of small diameter leads or low resistance elements. The use of a 3-wire or 4-wire
system will reduce errors to negligible levels in most applications. The need for a 3wire or 4-wire system will be dependent upon the resistance value of the sensing
element, the length and AWG of the leadwires as well as the accuracy required.
The drawbacks to this sensing technology are slower response time (40 seconds
recorded for this project, due to large element size), sensitivity to shock and vibration,
small resistance change (low sensitivity) for temperature variations, and low base
resistance. Low base resistance and small resistance change for corresponding
temperature change become a concern when long lead lengths are required because
the leads create additional resistance. When added to the resistance of the RTD
element, the lead resistance can result in measurement errors. To overcome leadlength problems, you should use 3- or 4-wire lead circuitry; this allows the effect of a
bridge circuit to measure the resistance change based on temperature. Wire-length
errors are minimized because the resistance change occurs at the RTD sensing point.
Accuracy of the measurement is primarily dependent on the accuracy of the signal
conditioning circuit in the controller or measuring device.
An RTD can be classified as a contact sensor. It is designed to be a platinum coil
contained within a stainless steel sleeve.
Contact Temperature Sensor Types :RTD
There are two methods of temperature sensing: contact and noncontact. Contact
sensing brings the sensor in physical contact with the substance or object being
measured; you can use this approach with solids, liquids, or gases.
This was the approach taken in the laboratory when the RTD was brought into contact
with ice and boiling water .
RTDs are best for most industrial measurements over a wide temperature range,
especially when sensor stability is essential for proper control.
Now to explain the operation of the signal conditioning circuit i.e the deflection
bridge:
Wheatstone Bridge Background
A wheatstone bridge is an electrical circuit for the precise comparison of resistances.
It is a DC device and can also be used as a deflection bridge as in this project where
it was used to modify a change in resistance to a change in voltage which could be
more easily be used for display purposes.
It consists of a common source of electrical current (such as a battery) and a
galvanometer that connects 2 parallel branches, containing 4 resistors, three of which
are known .
A current flows in an electrical circuit driven by the potential difference at the battery.
Resistance, current and voltage are connected by Ohm's law
U=RI
where U is the voltage, R the resistance and I the current.
The current and potential difference (= measured voltage) in each part of the circuit
can be calculated with the help of Kirchhoff's rules
1. The sum of the potential drops around any circuit loop must equal the sum of
the potential increases
2. At a junction point in a circuit where the current can divide, the sum of the
currents into the junction must equal the sum of the currents out of the
junction.
Look at the following setup: This is an example of a wheatstone or deflection bridge:
The potential drop across the multimeter is zero if
I1*R1=I3*Rx and
I2*R2=I4*R4
and as the potential drop over the galvanometer is zero, so is the current and we have
I1=I2 and
I3=I4
With this equation we see that the unknown resistance is given by
Rx=(R1/R2)*R4
For this experiment a wheatstone bridge was used as a deflection bridge in which
case 3 resistance values were fixed and not adjusted and the output voltage varies
proportionally as the fourth resistor ,the RTD , changes distance in accordance with
its temperature response .