4-20 mA loops are a 2 wire system for reading a value from a sensor. Its advantages are: a) Only 2 wires are required b) In a fault mode the current is less than 4 mA thus indicating a fault c) Robust to electrical noise d) Transmitter does not need to be calibrated with its cabling e)Supports the use of long cable lengths
It is the closeness with which an instrument reading approaches the true value of the quantity being measured.The accuracy of a measurement means conformity or truth.
Ability of scales to adjust to the range of plotted values. On graph scales, auto scaling determines maximum and minimum scale values.
the basic elements of ADAS are:
transducers:transducers convert physical parameters into electrical signals.
signal conditioners:this is used to process the electrical signals.the siganls are suitably modified. the signals may be amplified or otherwisw processed.
display devices:VDUs are needeed for obtaining continous monitoring of the signals.
graphic recording instruments:these instruments are required for obtaining permanent records of the input data.examples are optical recording systems, stylus and ionk recorders.
magnetic tape instrumentationthis incorporates:acquisition of input data ,preservation of the original electrical form of the the input data and reproduction of this data when required for further analysis.
Pictorial description or representation of a program or algorithm. The block diagram consists of executable icons called nodes and wires/paramaters and variables that carry data between the nodes. the block diagram is the source code for DCS/PLC. These come along with the software installation.
series of circular parts which resembles the fold of an accordian, used as pressure transducer.
Physical: a terminal or pin at which you can measure or generate an analog or digital signal. A single physical channel can include more than one terminal, as in case of differential analog input channel or a digital port of eight lines. Virtual: a collection of property settings that can include a physcial channel, its tag name, input terminal connections and other informations like analog/digital, range , and the data quality.
When measuring a thermocouple, a volt meter is usually connected. This connection results in another junction with two new thermocouples, each adding/subtracting mVs to the measured value. This is called the cold junction.
Originally the junction was contained in a bath of ice, hence the name cold junction. This kept the temperature of the junction at 0˚C to eliminate ambient temperature influences and allow the temperature at the “hot” end to be determined. The modern method is to measure the temperature where the thermocouple is connected to the copper connectors in the instrument using an accurate RTD
A contactor is a 3-pole electromechanical switch whose contacts are closed by applying voltage to a coil. When the coil is energized, the contacts are closed, and remain closed, until the coil is de-energized. The contactor is specifically designed for motor control, but can be used for other purposes such as resistive and lighting loads. Since a motor has inductance, breaking the current is more difficult, so the contactor has both a horsepower and current rating that needs to be adhered to.
Ethernet Device when communicating or sending messages listen on the channel to ensure that they are the only one sending packets of info in the channel. If more than 2 devices send messages then a collision occurs in the netwrok leading to messed up signals.
CSMA/CD is Carier Sense Multiple Access & Collision Detection is a method to detect collision in network and retry sending messages until successful or time out.
Acquiring and measuring analog or digital electrical signals from sensors, transducers and probes or fixtures. Also generating analog or digital electrical signals forms a part of DAQ.
A device that acquires or generates data and can contain multiple channels and conversion devices, DAQ devices include plug0in device, cards, I/O cards, Linking Devices etc.
The main function of a data acquisition system is to measure and record electrical signals. in practice, electrical signals are obtained in two ways:
signals from transducers such as strain gauge etc.
signals generated from direct measurement of electrical quantities like voltage, current,etc.
instrumentation systems can be broadly classified as analog and digital
An analog system deals with measurement of information in analog form whereas a digital DAS handles information in digital form
Instruments fuse, combine or reduce data from sensors in many different ways.Processing can be simple calculations like filtering and self calibration which may be piece wiseor polynomial fitting of data to describe the transfer function of sensor. More complex processing can be correlation or multiple channels of filtering like scrne matching data from video images.
Instrumentation systems must collect, record and process data in a correct and timely fashion. a DAS that monitors an experiment and collects measured values needs to do so in an acceptable manner to ensure correct understanding of sensed phenomena. If the system misses the sampled points or alters them, the resulting data represent a skewed or worse an incorrect view of phenomena. A number of parameters play into real time performance architecture,processor capability, resource management, communication channels and appropriate scheduling.
It is the time before the instrument begins to respond after the measure quantity has changed.
Largest change in the input quantity for which there is no change or no output at the instrument.
Evaluation of the system's ability to faithfully tranmit and present all the pertinent information included in the input signal and to exclude all else.
Differential pressure sensors measure the pressure difference between two points, one connected to each side of the sensor.
4-20 mA analog signal
ADAS(analog data acquisition system)
Cold Junction Compensation
Collisions & CSMA/CD