Relays

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.
Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

SCADA System

SCADA is an acronym that stands for Supervisory Control and Data Acquisition. SCADA refers to a system that collects data from various sensors at a factory, plant or in other remote locations and then sends this data to a central computer which then manages and controls the data.

Advanced Process Control: Bridge the Gap

A thirty-year focus on risk aversion has left a deep wound in the pharmaceutical industry. Manufacturers are still far more focused on avoiding mistakes than they are on continuously improving their processes. For many, quality is still measured more by the degree of error-free documentation than it is by fundamental process knowledge.

Distributed Control Systems (DCS)

Distributed control systems (DCS) use decentralized elements or subsystems to control distributed processes or complete manufacturing systems. They do not require user intervention for routine operation, but may permit operator interaction via a supervisory control and data acquisition (SCADA) interface. Distributed control systems (DCS) consist of a remote control panel, communications medium, and central control panel. They use process-control software and an input/output (I/O) database. 

Cycloconverter

A cycloconverter or a cycloinverter converts an AC waveform, such as the mains supply, to another AC waveform of a lower frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link (Dorf 1993, pp. 2241–2243 and Lander 1993, p. 181). They are most commonly used in three phase applications. In most power systems, the amplitude and the frequency of input voltage to a cycloconverter tend to be fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter tend to be variable. The output frequency of a three-phase cycloconverter must be less than about one-third to one-half the input frequency (Lander 1993, p. 188). The quality of the output waveform improves if more switching devices are used (a higher pulse number). Cycloverters are used in very large variable frequency drives, with ratings of several megawatts.

Variable Speed for Motor Control

An introduction to variable-frequency drives

Find more articles on: Variable Frequency Drives

Speed, torque, and horsepower are three inter-related parameters in motor control. The speed of a motor, measured in revolutions per minute (rpm), defines a motor's ability to spin at a rate per unit time. The torque of a motor, measured in foot-pounds (ft-lb), is a rotational characteristic of the motor that is the algebraic product of force multiplied by distance. Electrically, one horsepower is equal to 746 watts. What is interesting about these motor parameters is that if you change one of the three variables, the other two are affected. For example, if you increase horsepower while keeping speed constant, torque increases.

Blogger Buzz: Blogger integrates with Amazon Associates

Basic Operation The 555 Timer IC

The 555 timer IC is an amazingly simple yet versatile device. It has been around now for many years and has been reworked into a number of different technologies. The two primary versions today are the original bipolar design and the more recent CMOS equivalent. These differences primarily affect the amount of power they require and their maximum frequency of operation; they are pin-compatible and functionally interchangeable.

Shift Register (S to P)/Serial to Parallel

The term register can be used in a variety of specific applications, but in all cases it refers to a group of flip-flops operating as a coherent unit to hold data. This is different from a counter, which is a group of flip-flops operating to generate new data by tabulating it.
In this context, a counter can be viewed as a specialized kind of register, which counts events and thereby generates data, rather than just holding the data or changing the way it is handled. More commonly, however, counters are treated separately from registers. The two are then handled as separate concepts which work together in many applications, and which have some features in common.
The demonstration circuit below is known as a shift register because data is shifted through it, from flip-flop to flip-flop. If you apply one byte (8 bits) of data to the initial data input one bit at a time, and apply one clock pulse to the circuit after setting each bit of data, you will find the entire byte present at the flip-flop outputs in parallel format. Therefore, this circuit is known as a serial-in, parallel-out shift register. It is also known sometimes as a shift-in register, or as a serial-to-parallel shift register.
By standardized convention, the least significant bit (LSB) of the byte is shifted in first.

---

---

As you would no doubt expect, the counterpart to the shift register above is the parallel-in, serial-out shift register, somtimes called a shift-out register. That circuit is a bit more complex that the shift-in register shown above, but generally operates in a very similar fashion, as we'll see on the next page.

4-Bit Digital Counter

One common requirement in digital circuits is counting, both forward and backward. Digital clocks and watches are everywhere, timers are found in a range of appliances from microwave ovens to VCRs, and counters for other reasons are found in everything from automobiles to test equipment.
Although we will see many variations on the basic counter, they are all fundamentally very similar. The demonstration below shows the most basic kind of binary counting circuit.

D Flip-Flop

This circuit utilizes three interconnected RS latch circuits, as shown. This example uses NOR gates, but NAND gates can easily be used to perform the same function.
The two input latch circuits essentially store the D and D' signals separately, and apply those stored signals to the output latch. While the CLK input is a logic 0, changes to the D input can only affect the state of the lower gate of the lower input latch circuit. The other gates are locked into their output states by their other interconnections.
When CLK goes to logic 1, it inherently forces the outputs of the two middle input gates to logic 0. This effectively isolates the output latch from any input changes. Note that at this time, one or the other of the two input latches will be in an illegal state, depending on the state of the D input. This illegal state overrides the latching action of that input circuit.
Now, when CLK falls to logic 0, whichever input latch was in an illegal state will abruptly resume its latching action, and will at once control the state of the output latch. In this manner, the circuit is still an edge-triggered flip-flop that will take on the state of the D input at the moment of the falling clock edge.

RS NOR Latch

While most of our demonstration circuits use NAND gates, the same functions can also be performed using NOR gates. A few adjustments must be made to allow for the difference in the logic function, but the logic involved is quite similar.
The circuit shown below is a basic NOR latch. The inputs are generally designated "S" and "R" for "Set" and "Reset" respectively. Because the NOR inputs must normally be logic 0 to avoid overriding the latching action, the inputs are not inverted in this circuit. The NOR-based latch circuit is:

For the NOR latch circuit, both inputs should normally be at a logic 0 level. Changing an input to a logic 1 level will force that output to a logic 0. The same logic 0 will also be applied to the second input of the other NOR gate, allowing that output to rise to a logic 1 level. This in turn feeds back to the second input of the original gate, forcing its output to remain at logic 0 even after the external input is removed. Applying another logic 1 input to the same gate will have no further effect on this circuit. However, applying a logic 1 to the other gate will cause the same reaction in the other direction, thus changing the state of the latch circuit the other way. Note that it is forbidden to have both inputs at a logic 1 level at the same time. That state will force both outputs to a logic 0, overriding the feedback latching action. In this condition, whichever input goes to logic 0 first will lose control, while the other input (still at logic 1) controls the resulting state of the latch. If both inputs go to logic 0 simultaneously, the result is a "race" condition, and the final state of the latch cannot be determined ahead of time.

---

RS NAND Latch

n order for a logical circuit to "remember" and retain its logical state even after the controlling input signal(s) have been removed, it is necessary for the circuit to include some form of feedback. We might start with a pair of inverters, each having its input connected to the other's output. The two outputs will always have opposite logic levels.
The problem with this is that we don't have any additional inputs that we can use to change the logic states if we want. We can solve this problem by replacing the inverters with NAND or NOR gates, and using the extra input lines to control the circuit.
The circuit shown below is a basic NAND latch. The inputs are generally designated "S" and "R" for "Set" and "Reset" respectively. Because the NAND inputs must normally be logic 1 to avoid affecting the latching action, the inputs are considered to be inverted in this circuit.
The outputs of any single-bit latch or memory are traditionally designated Q and Q'. In a commercial latch circuit, either or both of these may be available for use by other circuits. In any case, the circuit itself is:

Multiplexer and Demultiplexer/Decoder

One circuit I've received a number of requests for is the multiplexer circuit. This is a digital circuit with multiple signal inputs, one of which is selected by separate address inputs to be sent to the single output. It's not easy to describe without the logic diagram, but is easy to understand when the diagram is available.
A two-input multiplexer is shown below.

The multiplexer circuit is typically used to combine two or more digital signals onto a single line, by placing them there at different times. Technically, this is known as time-division multiplexing. Input A is the addressing input, which controls which of the two data inputs, X0 or X1, will be transmitted to the output. If the A input switches back and forth at a frequency more than double the frequency of either digital signal, both signals will be accurately reproduced, and can be separated again by a demultiplexer circuit synchronized to the multiplexer. This is not as difficult as it may seem at first glance; the telephone network combines multiple audio signals onto a single pair of wires using exactly this technique, and is readily able to separate many telephone conversations so that everyone's voice goes only to the intended recipient. With the growth of the Internet and the World Wide Web, most people have heard about T1 telephone lines. A T1 line can transmit up to 24 individual telephone conversations by multiplexing them in this manner.

The Algebra of Boolean

One of the primary requirements when dealing with digital circuits is to find ways to make them as simple as possible. This constantly requires that complex logical expressions be reduced to simpler expressions that nevertheless produce the same results under all possible conditions. The simpler expression can then be implemented with a smaller, simpler circuit, which in turn saves the price of the unnecessary gates, reduces the number of gates needed, and reduces the power and the amount of space required by those gates.
One tool to reduce logical expressions is the mathematics of logical expressions, introduced by George Boole in 1854 and known today as Boolean Algebra. The rules of Boolean Algebra are simple and straight-forward, and can be applied to any logical expression. The resulting reduced expression can then be readily tested with a Truth Table, to verify that the reduction was valid.
The rules of Boolean Algebra are:.

---

AND Operations (·)

0·0 = 0      A·0  = 0
1·0 = 0      A·1  = A
0·1 = 0      A·A  = A
1·1 = 1      A·A' = 0

Combination Logical Gates

While the three basic functions AND, OR, and NOT are sufficient to accomplish all possible logical functions and operations, some combinations are used so commonly that they have been given names and logic symbols of their own.
We will discuss three of these on this page. The first is called NAND, and consists of an AND function followed by a NOT function. The second, as you might expect, is called NOR. This is an OR function followed by NOT. The third is a variation of the OR function, called the Exclusive-OR, or XOR function. As with the three basic logic functions, each of these derived functions has a specific logic symbol and behavior, which we can summarize as follows:

---

		The NAND Gate The NAND gate implements the NAND function, which is exactly inverted from the AND function you already examined. With the gate shown to the left, both inputs must have logic 1 signals applied to them in order for the output to be a logic 0. With either input at logic 0, the output will be held to logic 1. The circle at the output of the NAND gate denotes the logical inversion, just as it did at the output of the inverter. Also in the figure to the left, note that the overbar is a solid bar over both input values at once. This shows that it is the AND function itself that is inverted, rather than each separate input. As with AND, there is no limit to the number of inputs that may be applied to a NAND function, so there is no functional limit to the number of inputs a NAND gate may have. However, for practical reasons, commercial NAND gates are most commonly manufactured with 2, 3, or 4 inputs, to fit in a 14-pin or 16-pin package.

Basic Logical Gate

While each logical element or condition must always have a logic value of either "0" or "1", we also need to have ways to combine different logical signals or conditions to provide a logical result.
For example, consider the logical statement: "If I move the switch on the wall up, the light will turn on." At first glance, this seems to be a correct statement. However, if we look at a few other factors, we realize that there's more to it than this. In this example, a more complete statement would be: "If I move the switch on the wall up and the light bulb is good and the power is on, the light will turn on."
If we look at these two statements as logical expressions and use logical terminology, we can reduce the first statement to:

Light = Switch

This means nothing more than that the light will follow the action of the switch, so that when the switch is up/on/true/1 the light will also be on/true/1. Conversely, if the switch is down/off/false/0 the light will also be off/false/0.

Pyco Hydro-Power Plant

Pico hydro is water power up to 5 kW. It was given the name "pico" by Nigel Smith because it needs some different ways of thinking to micro, mini and larger hydropower. There are thousands of sites where people have a source of falling water but do not have electricity. For these rural communities, pico hydro is the lowest-cost technology for generating electricity. Lighting from this source is cheaper than using kerosene lamps, and safer, too. For more facts and figures on pico hydro for lighting, there is an article in Boiling Point (May 2007). On the News page is an interesting cost comparison, taken from a report prepared for the World Bank. It shows that Pico hydro is potentially the lowest cost technology for off-grid electrification.

This web site includes information on ways to design, manufacture and install the various parts of pico hydro projects. It includes documents that you can download to assist in the process.

The main parts of the system, as shown in the picture, are: intake from stream or river, pipe (known as the penstock), water turbine, electrical generator, electronic controller, electrical distribution system. 

Micro Hydro-Power Plant

Introduction

Water power can be harnessed in many ways; tidal flows can be utilised to produce power by building a barrage across an estuary and releasing water in a controlled manner through a turbine; large dams hold water which can be used to provide large quantities of electricity; wave power is also harnessed in various ways. It is a technology that has been utilised throughout the world, by a diverse range of societies and cultures, for many centuries. Water can be harnessed on a large or a small scale - Table 1, below outlines the categories used to define the power output form hydropower. Micro-hydro power is the small-scale harnessing of energy from falling water; for example, harnessing enough water from a local river to power a small factory or village. This fact sheet will concentrate mainly at micro-hydro power.

Pressure Transducers

What is a pressure transducer? A pressure transducer is a transducer that converts pressure into an analog electrical signal. Although there are various types of pressure transducers, one of the most common is the strain-gage base transducer. The conversion of pressure into an electrical signal is achieved by the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer and wired into a wheatstone bridge configuration. Pressure applied to the pressure transducer produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure.

Digital Signal Transmission for Process Control

Industrial networks that transmit data using digital signals often are an integral part of a data acquisition or process control solution. A basic understanding of the network technologies that are available for various applications is required to make the best implementation decisions-decisions that can have a profound effect on the ability to adapt to ever-changing technologies. 

Analog Signal Transmission for Process Control

Although the microprocessor and digital network technologies have fundamentally reinvented the ways in which today's data acquisition systems handle data, much laboratory and manufacturing information is still communicated the "old" way, via analog electrical signals. And a fundamental understanding of how analog signal transmission works must first begin with a discussion of electrical basics.

Fail - Save and Emergency Design

Logic circuits, whether comprised of electromechanical relays or solid-state gates, can be built in many different ways to perform the same functions. There is usually no one "correct" way to design a complex logic circuit, but there are usually ways that are better than others.

Digital Logic

We can construct simply logic functions for our hypothetical lamp circuit, using multiple contacts, and document these circuits quite easily and understandably with additional rungs to our original "ladder." If we use standard binary notation for the status of the switches and lamp (0 for unactuated or de-energized; 1 for actuated or energized), a truth table can be made to show how the logic works:

AC Motor Control

Main article: AC motor

In 1882, Nikola Tesla discovered the rotating magnetic field, and pioneered the use of a rotary field of force to operate machines. He exploited the principle to design a unique two-phase induction motor in 1883. In 1885, Galileo Ferraris independently researched the concept. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Motor Speed Controllers

1. Introduction

The purpose of a motor speed controller is to take a signal representing the demanded speed, and to drive a motor at that speed. The controller may or may not actually measure the speed of the motor. If it does, it is called a Feedback Speed Controller or Closed Loop Speed Controller, if not it is called an Open Loop Speed Controller. Feedback speed control is better, but more complicated, and may not be required for a simple robot design. Motors come in a variety of forms, and the speed controller's motor drive output will be different dependent on these forms. The speed controller presented here is designed to drive a simple cheap starter motor from a car, which can be purchased from any scrap yard. These motors are generally series wound, which means to reverse them, they must be altered slightly, (see the section on motors).
Below is a simple block diagram of the speed controller. We'll go through the important parts block by block in detail.

PLC- Programable Logic Controller

Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Relays are far from obsolete in modern design, but have been replaced in many of their former roles as logic-level control devices, relegated most often to those applications demanding high current and/or high voltage switching.

DIAGRAMS OF LADDER

Ladder diagrams are specialized schematics commonly used to document industrial control logic systems. They are called "ladder" diagrams because they resemble a ladder, with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are control circuits to represent. If we wanted to draw a simple ladder diagram showing a lamp that is controlled by a hand switch, it would look like this: