Measuring Instruments PDF

Measuring Instruments Definition of Instruments o An instrument is a device in which we can determine the magnitude or value of the quantity to be measured. The measuring quantity can be voltage, current, power, and energy, etc. Instrumentation re...

Measuring Instruments Definition of Instruments o An instrument is a device in which we can determine the magnitude or value of the quantity to be measured. The measuring quantity can be voltage, current, power, and energy, etc. Instrumentation refers to the application of instruments to measure and monitor physical quantities. Control involves using the information from these instruments to regulate or manipulate a process. o Measuring Instruments ▪ Generally, instruments are classified into two categories. Absolute Instrument o It determines the magnitude of the quantity to be measured in terms of the instrument parameter. This instrument is really used, because each time the value of the measuring quantities varies. Secondary Instrument o It determines the value of the quantity to be measured directly. Generally, these instruments are calibrated by comparing with another standard secondary instrument. o Indicating Instruments ▪ Instruments that indicate the instantaneous value of the electrical quantity being measured at the time at which it is being measured. ▪ This instrument uses a dial and pointer to determine the value of measuring quantity. The pointer indication gives the magnitude of measuring quantity. ▪ Examples are Electrical Measurements and Process Control ▪ Electrical Measurements: Ammeters: Measure electric current. Voltmeters: Measure voltage. Wattmeters: Measure electric power. Ohmmeters: Measure resistance.  ▪ Process Control: Pressure gauges: Measure pressure. Temperature gauges: Measure temperature. Flow meters: Measure flow rate. Level indicators: Measure liquid or solid levels. o Recording Instruments ▪ Instruments that give the variations of such a quantity over a selected period of time. ▪ This type of instrument records the magnitude of the quantity to be measured continuously over a specified period of time. ▪ ▪ Examples are Chart Recorders and Data Loggers: ▪ Chart Recorders: Strip chart recorders: Produce a continuous graph of the measured variable on a paper strip. Circular chart recorders: Record data on a rotating circular chart.  ▪ Data Loggers Digital data loggers: Store data electronically, often in a non-volatile memory. Analog data loggers: Convert analog signals to digital form for storage. o Integrating Instruments ▪ Instruments which measure and register by a set of dials and pointers either the total quantity of electricity or the total amount of electrical energy supplied to a circuit in a given time. ▪ This type of instrument gives the total amount of the quantity to be measured over a specified period of time. ▪ Examples are: Watt-hour meters: Measure electrical energy consumption. Ampere-hour meters: Measure the total electrical charge passing through a circuit. Flow meters: Measure the total fluid flow over a period. Gas meters: Measure the total gas consumption. Totalizing counters: Count events or occurrences over time. o Electromechanically Indicating Instruments ▪ For satisfactory operation electromechanical indicating instrument, three forces are necessary. They are: Deflecting force o When there is no input signal to the instrument, the pointer will be at its zero position. To deflect the pointer from its zero position, a force is necessary which is known as deflecting force.  o It is a force that causes an object to change its direction of motion. It can act on an object in various ways. o Deflecting Torque ▪ Produced by utilizing one or other effects; magnetic, electrostatic, electrodynamic, or thermal. Controlling force o To make the measurement indicated by the pointer definite (constant), a force is necessary which will be acting in the opposite direction to the deflecting force. This force is known as controlling force. o A system which produces this force is known as a controlled system. o Controlling Torque ▪ Oppose the deflecting torque and increases with the deflection of the moving system. Damping force o a special type of force that are used to slow down or stop a motion.  Basic Concept of Control System Automatic Control System o It plays an important role in the development and the advancement of modern civilization and technology. Automation control system is widely used in numerous industrial applications such as manufacturing automation space technology, transportation, robotics, etc. First Control System o The first control system called the fly ball governor was invented by James Watt. In 1788, to control the speed of his steam engine by regulating the supply of steam to the engine. o Basic Concept of Control System is shown below. o  Types of Control System o The two different types of control systems are open loop control system or non- feedback control system and the closed loop control system or feedback control system. o Open Loop Control System ▪ The block diagram here represents the open loop control system or non- feedback control system. ▪ ▪ An input signal or command is applied to the controller whose output acts as the actuating signal that regulates the control process and drive the control variable to the desired value. ▪ Advantages: Simple construction and design Cost is less Maintenance is easy No problem of instability Convenient to measure when the output is difficult to measure ▪ Disadvantages: These are less accurate, and their accuracy depend on the calibration. Inaccurate result is obtained with parameter variations within the system.  o Closed Loop Control System ▪ It is more accurate and adaptable control with a link of feedback from the output of the system to the input of the controller. In order to obtain more accurate control, the control process must be fed back and compared with the reference input then the controller will decide how much activating signal is required to achieve the desired output. The controller will actuate the signal till the error become negligible. ▪ ▪ Advantages: More accurate Non-linear distortions are less The output is less sensitive to parameter changes within the system Bandwidth increases ▪ Disadvantages: Design is complicated More expensive May become unstable, if there are malfunction in the feedback. Key Benefits of an Open and Closed Control System o Open Loop Control System ▪ Open loop control systems have several advantages that make them suitable for various applications. Simplicity: Open-loop systems are typically less complicated than closed control loop systems. Their design and operations are straightforward, making them easier to understand and implement. Stability: Due to the absence of a feedback loop, open-loop systems remain unaffected by potential disturbances in the feedback, ensuring consistent performance. Cost Efficiency: Constructing and maintaining an open loop system is usually more cost-effective due to its simple structure.  Speed: Without the need to process feedback information, open-loop systems can often operate more swiftly than their closed-loop counterparts. Immunity to Feedback Issues: Open loop systems are immune to problems that may arise in the feedback loop, such as noise interference or feedback stability issues. This makes them a reliable choice in certain applications. However, while these benefits make open-loop systems appealing in certain scenarios, their lack of a feedback mechanism could be a limiting factor when a high level of accuracy or adaptability is required. o Closed Loop Control System ▪ Closed-loop control systems present several advantages, making them an ideal choice for certain applications. Accuracy: Due to the feedback mechanism, closed-loop systems can provide more accurate control than open-loop systems. The system continuously monitors its output and makes necessary adjustments to achieve the desired output. Adaptability: Closed loop systems can adapt to changes in the operating environment or process conditions due to their feedback loop. This makes them suitable for applications where the conditions may vary and require continuous adjustments. Stability: Despite potential disturbances, a well-designed closed-loop system can maintain stability in its output. The feedback mechanism enables the system to correct itself and prevent deviation from the set point. Automation: With their self-regulating nature, closed-loop systems can operate without much human intervention, making them ideal for automated processes. Efficiency: By continuously