My Mind, My Heart, My File

The programmable logic controller (PLC) is a microprocessor-based system that accepts input data from switches and sensors, processes that data by making decisions in accordance with a stored program, and then generates output signals to devices that perform a particular function based on the application. The traditional motor control circuit is normally a hardwired system; therefore, any required circuit design change is a rather involved process in terms of material and labor.

With roots established in the manufacturing and automotive industries, the PLC was born out of a desire to automate the motor control process in a way that offered flexibility to make circuit design changes easier. The interesting challenge early on in its development was to design a programming language that would allow the industrial electrician a familiar way to communicate with the electronics of the PLC. This programming language would use symbols encountered in conventional ladder diagrams of the hard-wired variety.

The original purpose of the PLC was to allow electro-mechanical and electronic input devices to communicate with a computer that would perform logical operations on the input data and output a corresponding signal to some form of output device. (See PLC Logic Functions below.) To truly understand how a PLC works, you have to speak the same language. Let’s take a look at how to make the translation.

Understanding inputs and outputs

The magnetic motor starter is the controller that operates the connected motor load. The 2-wire and 3-wire control circuits use various types of input devices to energize the starter’s coil. These input devices use a momentary or maintained contact configuration.

Typical input devices are push buttons, proximity sensors, liquid level sensors, photoelectric sensors, selector switches, and pressure transducers. Typical output devices are contactors, magnetic motor starters, solenoids, pilot lights, and intelligent display panels. These output devices behave according to the connection of the input devices.

Fig. 1. A 3-wire control listing input and output functions.

PLC operation is a function of these inputs and outputs. In the standard 3-wire control circuit, as shown in Fig. 1 on page 18, you’ll notice a normally closed (N.C.) momentary stop push button, and a normally open (N.O.) momentary start push button. These contact devices represent the input function. The coil of the magnetic motor starter represents the output function.

The N.O. start push button energizes the coil of the magnetic motor starter, and the N.C. stop push button de-energizes it. Here, the PLC recognizes two input functions, which are the stop and start push buttons, and one output function, which is the coil of the magnetic motor starter.

In very simple terms, a PLC is designed to perform three tasks: check the input status, execute the program, and update the output status. The PLC checks the input status by scanning each input to determine if the connected device is on or off and then records that information in memory. Next, the PLC has to execute the user program (one line at a time) to make decisions. For example, suppose the user program tells the PLC to turn on an output device if Input #1 is on, and then turn off another output device if Input #2 is off. The PLC will analyze these conditions, execute the appropriate action, and then store that information in memory.

Lastly, the PLC has to update the output status. This means it will send data to an output device, such as the coil of a magnetic motor starter, to enable some type of manufacturing process to begin. The time it takes the PLC to go through this cycle is called the scan time.

Fig. 2. Ladder diagram as entered into PLC programming.

The PLC uses a programming language based upon readily identifiable symbols common to motor control. Hand-held programmers or PCs are the most common methods for programming the PLC. Figure 2 on page 18 is an example of a programming code setup to perform an “AND” operation. Switch #1 and Switch #2 are connected in series to the coil of a relay. The first rung of the ladder diagram shows two inputs, namely Switch #1 and Switch #2, and an output, namely the relay. Each rung of the ladder diagram should contain input(s) and output(s). The input(s) should be the first listed instruction and the output(s) the last listed instruction. Usually, programming code requires the END command to be listed as the last instruction on the last rung of the ladder diagram.

The following specifications will give you a sense of the type of information that is important in the selection and application of the PLC:

• 80188 CPU 8MHZ clock speed
• Input points — 16
• Output points — 12
• 8226; High-speed counter — 10KHZ
• Maximum user program — 1K
• Registers — 256 words
• Internal coils — 2560
• Memory backup with lithium battery — five years
• LED status indicators for I/O and CPU status
• Scan rate — 18mS/1K of logic.

Source :  www.ecmweb.com

Vidal is president of Joseph J. Vidal & Sons, Inc., Throop, Pa.

THE 4-20mA CURRENT LOOPThe 4-2OmA current loop has been with us for so long that it’s become rather taken for granted in the industrial and process sectors alike. Its popularity comes from its ease of use and its performance. However, just because something is that ubiquitous doesn’t mean we’re all necessarily getting the best out of our current loops.

A big benefit of the current loop is its simple wiring just the two wires. The supply voltage and measuring current are supplied over the same two wires. Zero offset of the base current (ie. 4mA) makes cable break detection simple: if the current suddenly drops to zero, you have a cable break.
In addition, the current signal is immune to any stray electrical interference, and a current signal can be transmitted over long distances.

Typical wiring for current output transducer.

You can think of the current loop itself as being analogous to a water system. You have a hose pipe (the wires) and a source tap (the power supply). You have a spray gun that regulates the flow (the transducer). You can have other equipment on the line, but it all has to be connected together in a ring Ioop. The more holes (devices) you have on the hose pipe, the higher the pressure will be required from the tap. Relating all that back to the current loop, you see a power supply, a transducer and one or more pieces of instrumentation all connected together in a ring.

You’ll often hear things referred to as being either active or passive. Some instruments have an active output which includes both the control of the current in the loop as well as provide the supply voltage. This is typically specified as being a 4-20mA output into 10-750 Ohms, or something similar. A passive input would be a simple resistor input that has a voltage drop to be factored into the equation once the supply voltage is chosen. This is typically specified as a 4-20mA input into 10 Ohm.

Working out the power supply requirement is a simple matter of adding up all the units in the loop at maximum current of 20mA. As an example, suppose you have a sensor ‘regulator’ which requires minimum 12V DC and instrumentation of 10 Ohm input:

10 Ohm x 20mA = 0.2V

So, for this circuit, a 12.2V minimum supply is required, the sensor’s maximum voltage might be specified at 30V, so a 24V supply would be all the circuit requirements with spare capacity to boot.

In order to measure the current loop it is necessary to break the loop and insert a current meter into it. You can also measure the voltage across the various components by in the loop, such as the voltage out of the power supply, the voltage over a sensor, and the voltages over the various pieces of instrumentation. This information will give you a good picture of what is happening within the loop.

Multi-instrument 4-20mA current loop with panel meter,
chart recorder, computers, etc.

A question which is sometimes asked is whether it is possible to use single power supply over several loops. This is possible, but you have to ensure that the power supply can give enough current to meet the needs of multiple loops. It is also the case that the current loops will have the same zero negative reference, which can cause a ground loop. In addition, interference from one loop can affect all the other loops driven from the one supply.

Source : http://www.sensorland.com/

 An Overview of Fiber Optic Technology

The use of fiber optics in telecommunications and wide area networking has been common for many years, but more recently fiber optics have become increasingly prevalent in industrial data communications systems as well. High data rate capabilities, noise rejection and electrical isolation are just a few of the important characteristics that make fiber optic technology ideal for use in industrial and commercial systems.

Most often used for point-to-point connections, fiber optic links are being used to extend the distance limitations of RS-232, RS-422/485 and Ethernet systems while ensuring high data rates and minimizing electrical interference. Conventional electrical data signals are converted into a modulated light beam, introduced into the fiber and transported via a very small diameter glass or plastic fiber to a receiver that converts the light back into electrical signals. Fiber’s ability to carry the light signal, with very low losses, is based on some fundamental physics associated with the refraction and reflection of light.

Fiber Optic Principles

Whenever a ray of light passes from one transparent medium to another, the light is affected by the interface between the two materials. This occurs because of the difference in speeds that the light can travel through different materials. Each material can be described in terms of its refractive index, which is the ratio of the speed of light in the material to its speed in free space. The relationship between these two refractive indices determines the critical angle of the interface between the two materials.

There are three actions that can happen when a ray of light hits an interface. Each action depends on the angle of incidence of the ray of light with the interface. If the angle of incidence is less than the critical angle, the light ray will refract, bending toward the material with the higher refractive index. If the angle of incidence is exactly equal to the critical angle the ray of light will travel along the surface of the interface. If the angle of incidence is greater than the critical angle, the ray of light will reflect.

The refractive index of vacuum is considered to be 1. Often, we consider the refractive index of air also to be 1 (although it is actually slightly higher). The refractive index of water is typically about 1.33. Glass has a refractive index in the range of 1.5, a value that can be manipulated by controlling the composition of the glass itself.

Fiber Optic Characteristics

Optical fibers allow data signals to propagate through them by ensuring that the light signal enters the fiber at an angle greater than the critical angle of the interface between two types of glass. As shown in Figure 1, optical fiber is actually made up of three parts. The center core is composed of very pure glass, with a refractive index of 1.5. Core dimensions are usually in the range of 50 to 125 um. The surrounding glass, called cladding, is a slightly less pure glass with a refractive index of 1.45. The diameter of the core and cladding together is in the range of 125 to 440 um. Surrounding the cladding is a protective layer of flexible silicone called the sheath.

 See detail :  An Overview of Fiber Optic Technology

 RS422/485 Application Note

Introduction
The purpose of this application note is to describe the main elements of an RS-422 and RS-485 system. This application note attempts to cover enough technical details so that the system designer will have considered all the important aspects in his data system design. Since both RS-422 and RS-485 are data transmission systems that use balanced differential signals, it is appropriate to discuss both systems in the same application note. Throughout this application note the generic terms of RS-422 and RS-485 will be used to represent the EIA/TIA-422 and EIA/TIA-485 Standards.

Data Transmission Signals Unbalanced Line Drivers
Each signal that transmits in an RS-232 unbalanced data transmission system appears on the interface connector as a voltage with reference to a signal ground. For example, the transmitted data (TD) from a DTE device appears on pin 2 with respect to pin 7 (signal ground) on a DB-25 connector. This voltage will be negative if the line is idle and alternate between that negative level and a positive level when data is sent with a magnitude of ±5 to ±15 volts. The RS-232 receiver typically operates within the voltage range of +3 to +12 and -3 to -12 volts as shown in Figure 1.1.

Figure 1.1 RS-232 Interface Circuit

See detail :  RS422/485 Application Note

AC meters are used to measure the RMS value of sine waves, such as AC power-line voltage.

True-RMS meters are required to measure the RMS value of complex waveforms, such as sine waves chopped by a triac or square waves.

True-RMS meters can be connected to read AC plus DC components or AC components (AC ripple) only.

An Update on Cable and Switchgear Incipient Fault Monitoring
Cliff Walton
Cre8 Innovation Solutions Ltd, UK
Abstract
There has been a rapid escalation in interest in on-line techniques for the detection and location of incipient faults in both cables and switchgear since the last review [1] presented at Euro TechCon in November 2003. This update summarizes some of the reasons for these developments, identifies some key developments, promising techniques and trends that have been presented at recent conferences and then points a possible direction for future research and development.
Introduction
The United Kingdom’s electricity supply industry, like those in many other developed countries, will face a new set of challenges over the next decade as it strives to improve and maintain supply standards while minimizing operations and maintenance (O&M) costs and embarking on major asset renewal programmes.

See detail :  An Update on Cable and Switchgear Incipient Fault Monitoring

As the population increases so do the demands placed on our electricity generating and distributing networks. At peak periods transformers can experience rapid rises in temperature producing hot-spots within the transformer. Overheating can lead to the premature breakdown of the cellulose insulation between the transformer windings, which in turn can seriously reduce the life expectancy of a large transformer and if left unchecked can lead to failure.
Reliable detection and monitoring of hotspots permits remedial action to be taken in the event of overheating and the life of the transformer to be extended with confidence. Conventional electrical temperature sensors may be placed at the top and bottom of a transformer to indicate global temperature changes but they cannot be used on the windings to detect hotspots directly. Only fibre-optic sensors, being all-dielectric, are permitted.

See detail :  Power Transformer Hotspot Monitoring