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Introduction:
A basic instrument system consists of a sensor (Already posted) , a processor and a receiver. This tutorial is about the
processor and the receiver. The technology used i n signal processing is also important for automatic control
systems.
SIGNAL PROCESSING AND CONDITIONING :
You should now be familiar with transducers and sensors. These were PRIMARY TRANSDUCERS. We must now examine how to process the output of the transducers into the form required by the rest of the instrument system. These may also be transducers but in this case, SECONDARY TRANSDUCERS. Most modern equipment works on the following standard signal ranges.
- Electric 4 to 20 mA
- Pneumatic 0.2 to 1.0 bar
- Digital standards
- Older electrical equipment use 0 to 10 V.
The vast array of instrumentation and control equipment available uses many forms of signals. Here is a summary.
ELECTRICAL - Voltage, current, digital.
MECHANICAL - Force and movement.
PNEUMATIC AND HYDRAULIC – Pressure and flow.
OPTICAL – High speed digital signal transmission.
RADIO – Analogue and digital transmission.
ULTRA VIOLET – Similar application to radio over short ranges.
Processing may do the following things.
- Change the level or value of the signal (e.g. voltage level)
- Change the signal from one form to another. (e.g. current to pneumatic)
- Change the operating characteristic with respect to time.
- Convert analogue and digital signals from one to the other.
AMPLIFIERS
Amplifier gain may be expressed as a ratio or in decibels. The letter W indicates it refers to power gain. The gain in dbW is given by $Gain(dbW)=10{{\log }_{10}}\frac{Poweroutput}{Powerinput}$
Since electric power into a resistive load is given as
The letter V indicates it is a voltage gain.
DIFFERENTIAL AMPLIFIERS
These have two inputs and the difference between them is amplified. The electronic symbol is shown.
ATTENUATORS Sometimes a signal is too big and must be reduced by attenuating it. Electrical signals are attenuated with resistors which dissipate the electric power as heat. Step down transformers and gear boxes for example, are not strictly attenuators because they reduce the level, not the power. The gain of an attenuator in db is negative as the next example shows.
The term amplification is often used when the level of a signal is increased but not the power. Strictly speaking, such devices should be called TRANSFORMERS. For example an A.C. electric transformer may increase the voltage but not the power. We have voltage amplifiers and current amplifiers which do not necessarily change the power level.
TRANSFORMERS
ELECTRICAL: Many devices only change the level of a signal without changing the power. A voltage amplifier is one example. An electrical transformer for alternating voltages basically consists of two windings, a primary and a secondary. The coils are wound on a magnetic core.
In an ideal transformer there is no energy loss and so the power in and power out are equal. $V_1*i_1=V_2*i_2$. It follows that if the voltage is stepped down, the current is stepped up and vice versa.
MECHANICAL
Mechanical transformers are levers and gear boxes which change movement, force, speed and torque but not the power. The are used in many instruments (e.g. a mechanical pressure gauge and the nozzle flapper system described later).
The gear ratio is in direct proportion to the pitch circle diameters (mean diameters) or number of teeth on each wheel.
The lever movements at the ends are in direct proportion to the length each side of the fulcrum.
HYDRAULIC:
The hydraulic pressure amplifier shown increases the pressure in direct proportion to the areas of the pistons. It is also called an intensifier.
Now let's examine processors which change the form of the signal.
SIGNAL CONVERTERS : Signal converters change the signal from one form to another. Where ever possible, these are the standard inputs and output ranges.
All these examples have opposite versions i.e. I/P, P/M and so on. Most signal converters have two adjustments – zero and range and this is explained in the next tutorial.
NOZZLE – FLAPPER and DIFFERENTIAL PRESSURE CELLS : The nozzle – flapper system is widely used in D.P. cells. The form shown below converts differential pressure (e.g. from a differential pressure flow meter) into a standard pneumatic signal. This is widely used in the control of air operated pipeline valves.
The bellows respond to the differential pressure and moves the lever. This moves the flapper towards or away from the nozzle. The air supply passes through a restrictor and leaks out of the nozzle. The output pressure hence depends on how close the flapper is to the end of the nozzle. The range of the instrument is adjusted by moving the pivot and the zero position is adjusted by moving the relative position of the flapper and nozzle.
This system is used in a variety of forms. Instead of bellows, a bourdon tube might be used and this is operated by an expansion type temperature sensor to produce a temperature - pneumatic signal converter.
ELECTRICAL D.P. CELLS : These provide the same functions as the pneumatic versions but given an output of 4 – 20 mA using electrical pressure transducers. They are typically used with D.P. flow meters.
CURRENT/PRESSURE CONVERSION : The pictures below show typical units for converting 4 – 20 mA into 0.2 – 1 bar and vice versa. They contain adjustments for range and zero. They are widely used for converting the standard pneumatic and electric signals back and forth. They can also be adjusted to work with non standard inputs to convert them into a standard form.
ANALOGUE – DIGITAL CONVERSION : So m any modern systems now use digital signals that it is important to appreciate how analogue signals are processed into digital form and vice versa. Analogue to digital conversion is a process of turning an analogue voltage or current into a digital pattern which can be read by a computer and processed.
Digital to analogue conversion is a process of turning a digital pattern from a computer into an analogue voltage or current.
REVISION OF BINARY NUMBERS : A number may be represented in digital form by simply setting a pattern of voltages on a line high or low. It is normal to use 4,8, 16 or 32 lines. An 8 bit binary pattern is shown below.
The total pattern is called a word and the one shown is a n 8 bit word. The pattern may be stored in an 8 bit register. A register is a temporary store where the word may be manipulated.
Bit zero i s called the least significant bit (LSB) and the bit with highest value is called the most significant bit (MSB).
Each bit has a value of zero when off (low) or the value shown when on (high). The maximum value for an 8 bit word is hence 255.
PRINCIPLES OF A/D CONVERSION DIGITAL TO ANALOGUE CONVERTERS : These are devices for converting a binary number into an analogue voltage. The change in the binary value from zero to a maximum corresponds with a change in the analogue value from 0 to a maximum.
RESOLUTION :When a digital number is converted into a voltage, each increment of the binary value corresponds with an increment in the voltage output. The value of this increment is the resolution.
ADDER TYPE CONVERTER : One way to convert a digital pattern into an analogue voltage is with the adder type shown below. The diagram shows a 5 bit pattern.
The output voltage is controlled by the state of the I/O switches (these might be a register).The output of the summing amplifier is
The maximum number represented by the digital number is 31 for the 5 bit system shown. An increase of one produces a voltage increment of $\frac{1}{32}$V. The maximum voltage output is 31V/32.
One problem with this system is that the theoretical resistance values can become ridiculously large
LADDER TYPE : Another way to convert a digital signal into an analogue signal is with the ladder type illustrated below.
The output voltage is given by
ANALOGUE TO DIGITAL CONVERSION
: One way to convert analogue into digital is to utilise a counter as shown below.
COUNTER TYPE
The output of the counter is a digital pattern and this is converted back into an analogue signal by the D/A converter and compared with the input. The input is compared with the feed back signal and so long as it is smaller, the counter will go on running. When the two become equal, the counter stops and the digital value of the counter is the digital value of the analogue signal.
Another type uses an integrator as shown
INTEGRATING TYPE :
This time no feedback is used. The reference voltage is integrated so the output of the integrator is a ramp which grows with time. The time taken for the ramp to reach the same value as the input is converted into a digital form by a counter.
SIGNAL CONDITIONERS:
Now let's look at processors which change the characteristic of the signal. These are often referred to as SIGNAL CONDITIONERS. One example is the square root extractor. For example DP flow meters produce an output which is directly proportional to the square of the flow. A processor might be used along with the DP cell to extract the square root so that the resulting signal is directly proportional to flow.
Other conditioners bring in the time element. For example the flow meter produces a signal representing the flow rate. If the signal is integrated with time, the output is quantity. Such a unit is called an INTEGRATOR and there are mechanical, pneumatic and electric methods.
Some meters produce a quantity. For example a movement transducer produces distance. If the signal is differentiated with respect to time, we get the rate or velocity. Such units are called DIFFERENTIATORS.
FILTERS : Another form of signal conditioning is the SIGNAL FILTER. This is used with electric signals to remove unwanted parts of the signal. For example a signal may contain AC and DC signals. A simple capacitor would filter out the DC signal. The signal may contain AC signals of different frequencies. A more sophisticated filter would remove unwanted frequencies.
MULTIPLEX SYSTEMS: A modern industrial information transmission system requires that many channels be sent from one place to another using either a single frequency of transmission or a single pair of wires such as a telephone line. A system that does this is called multiplexing.
Modems are units that receive and transmit the information to each other. They require a marker signal to let each other know when to receive or transmit. The modem acknowledges a request to receive and this is called handshaking. When acknowledgement is received by the transmitting modem, it sends the data to the receiving modem.
The channels are sampled very rapidly one after another for a fixed period of time. This is called Time Division Multiplexing (TDM). Each sample is tacked in a queue (a buffer) and sent one after another. The process is continually repeated. The receiving end reverses the operation. The incoming data is placed in a buffer, extracted in sequence and diverted to the correct output.
If the information is digital, it sends the information for each channel as a package of digits one channel at a time.
BUFFERS and STORES : Very often a signal value needs to be stored. This is usually done by converting them into a digital number and storing them in a memory chip. If a system is receiving or sending serial data but the incoming data is arriving quicker than it can be processed, then the incoming signals are put into a temporary store called a buffer where they wait to be processed.
SIGNAL RECEIVERS : The input of an instrument system is sensed with a primary transducer and then processed or conditioned before being sent on to the last item in the chain which is the signal receiver. The signal receiver may be
- an indicator
- a chart recorder
- a plotter
- a printer
- an alarm
- a computer
INDICATORS : Indicators may be analogue or digital. Analogue indicators have a scale and a pointer moves along the scale to indicate the value. Digital indicators may be a display of numerals which change in fixed steps. Sometimes a digita indicator is made to simulate an analogue scale (e.g. a digital watch with hands). The pointer however moves in fixed steps. Digital indicators have a minimum resolution and usually this is smaller than the resolution a human eye can make on an analogue scale so digital indicators are often more accurate to read.
CHART RECORDERS : Chart recorders keep a record of the measurements over a period of time. There are electrical and pneumatic versions. The recorder may use paper strips or polar charts. The paper moves one way and the marker pen moves at right angles to this. Often they use more than one pen to record several channels. The recorder is often part of a controlled system and contains other equipment to do with the control.
PLOTTERS AND PRINTERS: Plotters draw graphs or charts. The pen recorders described above are types of plotters specifically for recording a signal over a long time period. The term plotter usually refers to a system where the paper is stationary and the pen is able to move in the x and y directions. Often the x direction is time and so the plotter become a recorder. Plotters are more often used to plot one signal against another. They are very commonly used for producing engineering drawings created on a computer.
Printers refer to a system where the paper is marked by a printing head. There are many types such as dot matrix, ink jets, laser and other electrostatic devices. Printers will print text or they may be used to print pictures and drawings reproducing on paper what can be seen on a VDU.
ALARMS : The purpose of an instrument may be to set off an alarm such as a bell, siren, flashing light and so on. They are switched on by the signal processor. An example is a smoke detector.
COMPUTERS : Computer technology is taking over many rolls performed by traditional equipment. The signal is processed into digital form, in other words into numbers. The computer processes the numbers and stores the data, sends it on to a VDU or to other control equipment.
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