CAS Newsletter March 2012
IN THIS ISSUE
The older Modbus systems limited the number of objects of each type to 9999.
I.e. Max Input Register = 39999 and Max Holding = 49999
Whereas the protocol has space to read up to 65535 objects of each type, older Modbus systems imposed a stupid validation and choked the address space to the 9999 limit. Newer systems like the CAS Modbus Scanner do not choke the address space and you can read any registers.
To read address Holding Register = 512345 (also known as Holding Reg with offset 12345 in PDU terminology)
Display of the request – we show both the 5 and 6 digit address.
1. Install the FieldServer Utilities.
2. Start a CMD session.
3. Power down the gateway.
4. Delete the existing configuration.
“C:\FieldServer Technologies\FieldServer Utilities\Bin\ruinet” –i192.168.1.168 -zconfig.csv
192.168.1.168 == IP address of FieldServer. This one is provided for an example only.
The quotation marks are required.
When you have completed the line push the Enter key.
If the command complete OK you will see a screen similar to this one. The folder name and IP address may be different. Note the DELETED message. That is what we want to see.
If it completed OK then:
If not then:
Since force is a vector quantity, it has got both direction and magnitude. In case, a body is in motion, the energy of its motion can be quantified as the momentum of the object i.e. the product of its mass and its velocity. When the body is free to move, its velocity will be changed by the action of a force.
Units of Measurement
Pounds per square inch (psi)
Whenever an object is subjected to an external stress i.e. pressure with the aim to cause a reduction in its volume, this process is called compression. The majority of liquids and solids are practically incompressible, whereas gases are not.
Third Gas Law
Various means like mechanical, optical, acoustical, pneumatic or electrical can be used to measure deformation (strain) of an object. Earlier mechanical devices such as extension meter (extensiometer) were used to measure strain by measuring the change in length and comparing it to the original length of the object. However, mechanical strain gauges offer certain limitations like low resolutions. Besides they are bulky and difficult to use.
Further, capacitance and inductance-based strain gages were introduced but these devices’s sensitivity to vibration, their mounting requirements, and circuit complexity restricted their usage.
Next are the photoelectric gauges. These gauges use a light beam, two fine gratings, and a photocell detector to generate an electrical current proportional to strain. A photoelectric gauge can be as short as 1/16 inch but its usage proves to be extremely costly and delicate.
In 1938, the first bonded, metallic wire-type strain gage was introduced. The metallic foil-type strain gage is constructed of a grid of wire filament of approximately 0.001 in thickness, bonded directly to the strained surface by a thin layer of epoxy resin. When a load is applied to the surface, it gets strained and experiences a change in length. This resulting change in length is conveyed to the resistor and the corresponding strain is measured in terms of the electrical resistance of the foil wire, which varies linearly with strain. Other types of Strain Gauges are described below.
Semiconductor Strain Gauges
In the year 1970, the first semiconductor strain gages were developed for the use in automotive industry. Semiconductor strain gauges exhibit following key features:
Thin-film Strain Gauges
Thin-film strain gage is more advanced form of strain gauge as it doesn’t necessitate adhesive bonding. A thin film strain gauge is constructed by first depositing an electrical insulation, usually a ceramic onto the stressed metal surface, and then depositing the strain gage onto this insulation layer. Techniques used to bond the materials molecularly are:
Diffused Semiconductor Strain Gauges
A further improvement in strain gage technology was introduced with the advent of diffused semiconductor strain gages since they purge the need for bonding agents. Main features are listed below:
Bonded Resistance Gauges
Following are the chief characteristics of bonded resistance strain gauges:
Construction of a Bonded resistance strain gauge is shown in the figure below:
Selection of a Proper Gauge
In addition, choosing the right carrier material, grid alloy, adhesive, and protective coating plays an important role in the success of the application.
Load Cell is a specific type of transducer or sensor capable of transforming force (load) into a measurable electrical output. Load cells measure tension, compression, or shear. A typical load cell device comprises of four strain gauges in a Wheatstone bridge configuration.
Classification of Load Cells
Type of output signal generated
Way they detect weight
Hydraulic Load Cells
Pneumatic Load Cells
Strain-gauge Load Cells
Advantages: As compared to other load cells strain gage load cells offers more accuracy and lower unit costs. Although there are several varieties of load cells available, strain gauge based load cells are the most widely used type.
Piezo-electric Load Cells
Selection of a Load Cell
The demand for pressure measuring instruments arises with the advent of steam age. Mechanical methods of measuring pressure such as Bourdon tubes or bellows, where mechanical displacements were transferred to an indicating pointer were the first pressure instruments. Initially, these tubes were constructed of glass, and scales were added to them as per requirements. However, these mechanical motion balance pressure measuring arrangements were large, cumbersome, and not well suited for integration into automatic control loops. Consequently, as control systems evolve to become more centralized and computerized, these devices were replaced by analog electronic and, more lately, digital electronic pressure transmitters. Pressure transmitters or transducers are ready to use instruments employed for measurement of pressure. These are OEM transducers with
The terms pressure gauge, sensor, transducer, and transmitter can be used interchangeably. Majority of modern pressure sensors operates on piezoresistance principle. Due to pressure, a material generates electricity at a certain rate, which leads to a specific level of charge flow related with a specific level of pressure. This charge is supplied to a wire which leads to a control panel and display for human analysis.
When transducers are directly interfaced with digital data acquisition systems and are positioned at some distance from the data acquisition hardware, high output voltage signals are preferred and these signals must be guarded against both electromagnetic and radio frequency interference (EMI/RFI) when traveling longer distances.
Types of Pressure Sensors
In industrial applications, good repeatability is considered more significant as compared to absolute accuracy. For pressure variations over a wide range, transducers with good linearity and low hysteresis are the ideal choice.
Maintenance and Calibration
Pressure transducers can be recalibrated on-line or in a calibration laboratory. Laboratory recalibration is usually chosen over the other one. In the laboratory, there are generally two types of calibration devices:
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