Introduction to digital circuits

Engineers generally classify electronic circuits as being either analog or digital in nature.

Whether or not a device is digital depends upon

1. Does it have an alphanumeric (shows letters and numbers) display?
2. Does it have a memory or can it store information?
3. Can the device the programmed?

If the answer to any one of the three questions is yes, then the product probably contains digital circuitry.

Advantages of digital over analog circuits
1. Generally, digital circuits are easier to design using modem integrated circuits (ICs).
2. Information storage is easy to implement with digital.
3. Devices can be made programmable with digital.
4. More accuracy and precision is possible.
5. Digital circuitry is less affected by unwanted electrical interference called noise.

A signal can be defined as useful information transmitted within, to, or from electronics circuits. Signals are commonly represented as a voltage varying with time. However, a signal could be an electrical current that either vanes continuously (analog) or has an on-off characteristic (digital). An analog device is one that has a signal which varies continuously in step with the input. A digital device operates with a digital signal. The digital signal is only at +5 V or at 0 V. The HIGH voltage is ±5 V or commonly called logical 1; the LOW voltage is 0 V or commonly called logical 0. Circuits that handle only HIGH and LOW signals are called digital circuits. An analog signal assumes a continuous range of values: A digital signal assumes discrete (isolated, separate) values as there are two permitted values

Digital signals are composed of two well- defined voltage levels. Most of the voltage levels used in this class will be about +3 V to +5 V for HIGH and near 0 V (GND) for LOW. These are commonly called TTL voltage levels because they are used with the transistor- transistor logic family of ICs. Logic levels are different for various digital logic families, such as TTL and CMOS. These logic levels are commonly referred to as HIGH. LOW, and undefined.

Limitations of digital circuits

1. Most ‘real-world” events are analog in nature.
2. Analog processing is usually simpler and faster.

Digital circuits are appearing in more and more products primarily because of low-cost, reliable digital lCs. Other reasons for their growing popularity are accuracy, added stability, computer compatibility, memory, ease of use, simplicity of design and compatibility with alphanumeri displays.

Example of digital and analog device

A multimeter can measure continuity, resistance, voltage and sometimes even current, capacitance, temperature. The standard volt-ohm-millimeter (VOM) is an example of an analog measuring device. As the voltage, resistance, or current being measured by the VOM increases, the needle gradually and continuously moves up the scale. A digital multimeter (DMM) is an example of a digital measuring device. As the current, resistance, or voltage being measured by the DMM increases, the display jumps upward in small steps. The DMM is an example of digital circuitry taking over tasks previously performed only by analog devices. This turn toward digital circuitry is growing.

Interpolation

Interpolation is a technique of finding new data points within the range of known data points. Linear interpolation is the simplest method of getting values at positions in between the data points. The points are simply joined by straight line segments.

It is basically the process of estimating the outcomes in between sampled data points. In linear interpolation a line connecting two points is used to estimate intermediate values

Interpolation formula

In the graph above you have got the values of x1, x3, y1 and y3. Now you want to interpolate the value of y2 at any given value of x2 so you can use the simple interpolation formula given below

Y2 = (x2-x1) (y3-y1) + y1

(x3-x1)

Application of interpolation

A spring is an elastic object used to store mechanical energy. In case of mechanical spring there is a spring load and deflection graph. The deflection is in millimeters and the load is measured in newton. The deflection is plotted on the x-axis and the corresponding load in newton on y-axis. Often we have to find the values between the two sets of values (load vs. deflection). Hence interpolation is the technique used to find the unknown values. The graph is usually plotted in excel.

mm	Min force	Max force
7.5	157.31	178.69
10	128.6643192	146.5743338
14.33	79.05	90.95

The figure above shows the spring deflection at values of 7.5 and 14.33mm and the corresponding load. One is the graph for minimum load values shown in blue (157.31 and 79.05). The other graph shown in red is for the maximum force values (178.69 and 90.95). Now we want to find the load values at deflection of 10mm which is done using excel 2010 and using interpolation formula given above.

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ULTRASONIC CLEANING

Ultrasonic cleaning

Ultrasonic cleaning is a method of cleaning that uses an ultrasonic device to clean delicate objects or items with intricate parts. Ultrasonic is the science of sound waves above the limits of human audibility. The ultrasonic device uses a cleaning solution which can be fuel (hydrocarbon) and high frequency sound waves to clean the items.

Ultrasound ranges in frequency from about 20 to 400 kHz. The most commonly used frequencies for industrial cleaning are those between 20 KHz and 50KHz. Ultrasonic cleaning is employed in a variety of industries especially aeronautical and can be used on many materials and objects of varying sizes and shapes. The mechanical effect of ultrasonic energy can be helpful in both speeding dissolution and displacing particles.

Ultrasonic Equipment

To introduce ultrasonic energy into a cleaning system requires an ultrasonic transducer and an ultrasonic power supply or "generator." The generator supplies electrical energy at the desired ultrasonic frequency. The ultrasonic transducer converts the electrical energy from the ultrasonic generator into mechanical vibrations. As these vibrating sound waves travel through water, microscopic bubbles form and repeatedly implode upon a given surface. This powerful action removes visible and even microscopic dirt particles making a dirty mini-blind or any other object cleaner than alternative methods. Not only is this ultrasonic cleaning method completely user-friendly and extremely effective, but it is fast, safe, and gentle.

The ultrasonic power supply (generator) converts 50/60 Hz voltage to high frequency 20 or 40 kHz (20,000/40,000 cycles per second) electrical energy. This electrical energy is transmitted to the piezoelectric transducer within the converter, where it is changed to high-frequency mechanical vibration. The vibrations from the converter are amplified by the probe (horn), creating pressure waves in the liquid.

Ultrasonic Transducers

There are two general types of ultrasonic transducers commonly used Magnetostrictive and piezoelectric. Both accomplish the same task of converting alternating electrical energy to vibratory mechanical energy but do it through the use of different means.

Magnetostrictive transducers utilize the principle of magnetostriction in which certain materials expand and contract when placed in an alternating magnetic field. Piezoelectric transducers convert alternating electrical energy directly to mechanical energy through use of the piezoelectric effect in which certain materials change dimension when an electrical charge is applied to them.

HONING

These are some operations by which a product receives the final machining stage or finishing operation before final dispatch. These processes or methods remove a very small amount of metal, and hence the surface finish obtained is specified in the ranges of micro finishes. Honing is one of these operations.

HONING

The process of finishing ground surfaces to a high degree of accuracy and smoothness with abrasive blocks applied to the surface under a light controlled pressure and with a combination of rotary and reciprocating motions.

Honing is a controlled, low-speed sizing and surface finishing process in which stock is abraded by the shearing action of a bonded abrasive honing stick. In honing, simultaneous rotating and reciprocating action of the stick results in a characteristic cross-hatch lay pattern. Because honing is a low-speed operation, metal is removed without the increased temperature that accompanies grinding and thus any surface damage caused by heat is avoided. Honing uses a special tool, called a honing stone or a hone, to achieve a precision surface. The hone is a composed of abrasive grains that are bound together with an adhesive

In addition to removing stock, honing involves the correction of errors from previous machining operations. These errors include

• Geometrical errors such as out-of-roundness, waviness, bell mouth, barrel, taper, rainbow, and reamer chatter
• Dimensional inaccuracies
• Surface character (roughness, lay pattern, and integrity)

Honing corrects all of these errors with the least possible amount of material removal; however, it cannot correct hole location or perpendicularity errors. The most frequent application of honing is the finishing of internal cylindrical holes. However, numerous outside surfaces also can be honed. Gear teeth, valve components, and races for antifriction bearings are typical applications of external honing.

Process

The tool reciprocates through the bore, the pressure and the resulting penetration of grit is greatest at high spots and consequently the waviness crests are abraded, making the bore straight and round. After leveling high spots, each section of the bore receives equal abrading action. The hole axis is usually in the vertical position to eliminate gravity effects on the honing process; however, for long parts the axis may be horizontal

Advantages of Honing

1. It is characterized by rapid and economical stock removal with a minimum of heat and distortion.
2. It generates round and straight holes by correcting form errors caused by previous operations.
3. It achieves high surface quality and accuracy.

Honing Machines

For the production of few parts, honing may be performed on drill presses or engine lathes on which arrangements can be made for simultaneous rotating and reciprocating motions. The stroking can be done manually or powered depending on the equipment capabilities. On the other hand, the production honing is done with machines built for the purpose. These vertical machines are available in a wide range of sizes and designs. Some horizontal machines operate by manual stroking. In power stroking, the WP is usually held stationary in a rigid fixture, while the hone is rotated and hydraulically powered for stroking, which is considered beneficial for heavier WPs.

Machining Parameters

Parameters affecting the performance of honing process are:

1. Rotation speed the choice of the optimum surface speeds is influenced by:
2. Material being honed—higher speed can be used for metals that shear easily.
3. Material hardness—harder material requires lower speed.
4. Surface roughness—rougher surfaces that mechanically dress the abrasive stick permit higher speed.
5. Number and width of sticks in the hone—speed should be decreased as the area of abrasive per unit area to hone increases.
6. Finish requirement—higher speed usually results in finer surface finish

Tool Specifications
In addition to the variables involved in the honing process, the most important influence on the work result are the specifications of the honing stones, i.e. grain type, grain size, type of bond, hardness and treatment.

The Turbine Flow Meter and its Calibration

The Turbine Flow Meter and its Calibration

Turbine Meters

A turbine meter consists of a practically friction-free rotor pivoted along the axis of the meter tube and designed in such a way that the rate of rotation of the rotor is proportional to the rate of flow of fluid through the meter. This rotational speed is sensed by means of an electric pick-off coil fitted to the outside of the meter housing.

The only moving component in the meter is the rotor, and the only component subject to wear is the rotor bearing assembly. However, with careful choice of materials (e.g., tungsten carbide for bearings) the meter should be capable of operating for up to five years without failure.

There are several characteristics of turbine flow meters that make them an excellent choice for some applications. The flow sensing element is very compact and light weight compared to various other technologies. This can be advantageous in applications where space is a premium

Primary Vs. Secondary Standards

A primary standard calibration is one that is based on measurements of natural physical parameters (i.e., mass, distance, and time). This calibration procedure assures the best possible precision error, and through traceability, minimizes bias or systematic error.

A secondary standard calibration is not based on natural, physical measurements. It often involves calibrating the user's flow meter against another flow meter, known as a "master meter," that has been calibrated itself on a primary standard.

Calibration

"To calibrate" means "to standardize (as a measuring instrument) by determining the deviation from a standard so as to determine the proper correction factors." There are two key elements to this definition: determining the deviation from a standard, and ascertaining the proper correction factors.

Flow meters need periodic calibration. This can be done by using another calibrated meter as a reference or by using a known flow rate. Accuracy can vary over the range of the instrument and with temperature and specific weight changes in the fluid, which may all have to be taken into account. Thus, the meter should be calibrated over temperature as well as range, so that the appropriate corrections can be made to the readings. A turbine meter should be calibrated at the samekinematic viscosity at which it will be operated in service. This is true for fluid states, liquid and gas.

Master Meter

A master meter is a flowmeter that has been calibrated to a very high degree of accuracy. Types of flowmeters used as master meters include turbine meters, positive displacement meters, venturi meters, and Coriolis meters. The meter to be calibrated and the master meter are connected in series and are therefore subject to the same flow regime. To ensure consistent accurate calibration, the master meter itself must be subject to periodic recalibration

Gravimetric Method

This is the weight method, where the flow of liquid through the meter being calibrated is diverted into a vessel that can be weighed either continuously or after a predetermined time. The weight is usually measured with the help of load cells. The weight of the liquid is then compared with the registered reading of the flow meter being calibrated

Volumetric Method

In this technique, flow of liquid through the meter being calibrated is diverted into a tank of known volume. The time to displace the known volume is recorded to get the volumetric flow rate eg gallons per minute. This flow rate can then be compared to the turbine flow meter readings

K-Factor.

“K” is a letter used to denote the pulses per gallon factor of a flowmeter.

Repeatability.

The maximum deviation from the corresponding data points taken from repeated tests under identical conditions.

Positive Displacement Calibrators:

Some of the most dramatic improvements in flow calibrator technology involve the evolution of Positive Displacement calibrators. PD systems are Primary Standard calibrators, which take into account the varying conditions under which flowmeters operate. These calibrators are able to compensate for temperature, density, viscosity and other variables that can shift a meter’s output.it utilizes a precision machined measurement chamber, or flow tube, that houses a piston. This piston acts as a moving barrier between the calibration fluid and the pressurizing media used to move the piston. Attached to the piston is a shaft that keeps the piston moving in a true path and provides the link between the piston and the translator. The translator converts the linear movement of the piston through the precision flow chamber into electrical pulses that are directly related to the displaced volume. Calibrators of this style can be directly traceable to the National Institute of Standards and Technology via water draw validation. Total accuracy of this type of calibrator is conservatively specified at 0.05%

Flow Transfer Standards:

Unlike primary flow standards, whose most important characteristics are their traceability to primary physical measurements (resulting in the minimization of absolute uncertainties, with less concern for usability or cost issues), the key criteria for secondary Flow Transfer Standards are portability, low cost and the ability to calibrate the flowmeter in the physical piping configuration it lives in.

Instead of removing flowmeters from service for recalibration, FTS devices allow users to “bring the calibrator to the flowmeter.” These portable, documenting field flow calibrators are intended for in-line calibration and validation of meters using the actual process conditions for gas or liquid. Advanced FTS systems incorporate hand-held electronics with built-in signal conditioners, thus eliminating bulky interface boxes and the need to carry a laptop computer into the field. High-quality Flow Transfer Standards also have the capability of measuring and correcting the influences of line pressure and temperature effects on flow.

Operation of a portable Flow Transfer Standard requires that a master meter be installed in series with the flowmeter under test. The readings from these instruments are compared at various flow rates or flow totals. A technician can install the master meter in the same system as the test meter, perform the calibration, and note any changes in performance. New calibration data might cause rescaling or new data points to be programmed into a flowmeter’s computer to align the measurement with the current flow calibration data.

Typical Calibration Techniques

Most flowmeter calibration service suppliers provide a choice of calibration techniques to accommodate different applications and flow measurement requirements. One of the most common techniques is the single-viscosity calibration, which consists of running 10 evenly spaced calibration points at a specified liquid viscosity. Single-viscosity calibrations are recommended when the viscosity of the liquid being measured is constant. If a higher degree of accuracy is needed, again, the more data points taken the better defined the meter calibration curve will be

Strouhal Number/Roshko Number

The best, and only completely correct way to present the data for a Turbine Meter is Strouhal Number as a function of Roshko Number, i.e., through the use of two dimensionless parameters. The St vs. Ro presentation takes into account all of the secondary effects to which the meter is sensitive. This presentation or correlation is correct for both liquids and gases. It is almost a must for gas calibrations since the density and kinematic viscosity are a function of both temperature and pressure

Your Calibrated Flowmeter

Once your flowmeter is calibrated, it may still read exactly the same under the same flow conditions as it did before it was calibrated. The difference is that you will know exactly how close those values are to the true values, and you will have a formula to use to calculate the true values from the actual values read by your flowmeter. You can have a correction factor obtained from calibratiob which you can apply to the flow meter readings to obtain the correct or true flowmeter readings. K-factor ignores the effects of changing temperature on the meter body since the meter will change diameter when the temperature changes. The use of Strouhal Number instead of simple K-Factor will account for this temperature effect.