Lec. 05-06 Force Measurements PDF

ENM 4239 Design of Applied Measuring Systems 2024-2025 Prof. Mohamed Essa, Assoc. Prof. of Automatic Control Systems  Force Measurement Engineering Measurements By 11/23/2024 2 Dr....

ENM 4239 Design of Applied Measuring Systems 2024-2025 Prof. Mohamed Essa, Assoc. Prof. of Automatic Control Systems  Force Measurement Engineering Measurements By 11/23/2024 2 Dr. Mohamed Essa  Introduction Force: It is defined as the reaction between the two bodies or components. The reaction can be either tensile force (Pull) or it can be Compressive force (Push). Measurement of force can be done by any two methods: – Direct Method: This involves a direct comparison with a known gravitational force on a standard mass. Example: Physical Balance. – Indirect Method: This involves the measurement of effect of force on a body. E.g. Force is calculated from acceleration due to gravity and the mass of the component. Engineering Measurements By 11/23/2024 3 Dr. Mohamed Essa  Direct Method: Analytical Balance (Equal arm balance) Engineering Measurements By 11/23/2024 4 Dr. Mohamed Essa  Unequal arm balance: Engineering Measurements By 11/23/2024 5 Dr. Mohamed Essa  Unequal arm balance: Engineering Measurements By 11/23/2024 6 Dr. Mohamed Essa  Unequal arm balance: For balance of moments, Ft * a = Fg * b or test force, Ft = Fg * (b / a) Therefore, the test force is proportional to the distance ‘b’ of the mass from the pivot. Engineering Measurements By 11/23/2024 7 Dr. Mohamed Essa  Proving ring The proving ring is a device used to measure force. It consists of an elastic ring of known diameter with a measuring device located in the center of the ring. They are made of a steel alloy. manufactured according to design specifications established in 1946 by the National Bureau of Standards (NBS). Proving rings can be designed to measure either compression or tension forces. Engineering Measurements By 11/23/2024 8 Dr. Mohamed Essa  Proving ring Standard for calibrating material testing machine. Capacity 1000 N to 1000 kN. Deflection is used as the measure of applied load. This deflection is measured by a precision micrometer. P = force or load M = Bending moment R = Radius of proving ring Engineering Measurements By 11/23/2024 9 Dr. Mohamed Essa  Proving ring A ring used for calibrating tensile testing machines. It works on the principle of LVDT which senses the displacement caused by the force resulting in a proportional voltage. It is provided with the projection lugs for loading. An LVDT is attached with the integral internal bosses C and D for sensing the displacement caused by application of force. When the forces are applied through the integral external bosses A and B, the diameter of ring changes depending upon the application which is known as ring deflection. Engineering Measurements By 11/23/2024 10 Dr. Mohamed Essa  Proving ring The resulting deflection of the ring is measured by LVDT which converts the ring deflection or displacement in to voltage signal. An external amplifier may be connected to provide direct current to drive the indicators or the measured value of force. In place of LVDT micrometer can also be provided for accurate measurement of force or deflection, which is given by formula Where, F is the force, E is the young's modulus, I is the moment of inertia about the centroidal axis, D is the outer diameter of the ring and y is the deflection. Engineering Measurements By 11/23/2024 11 Dr. Mohamed Essa  Components of Measuring System Engineering Measurements By 11/23/2024 12 Dr. Mohamed Essa  Measuring Strain (Strain Gages) What is Strain? Strain is the amount of deformation of a body due to an applied force. More specifically, strain is defined as the fractional change in length. When a force is applied to a structure, the components of the structure change slightly in their dimensions and are said to be strained. Devices to measure these small changes in dimensions are called strain gages. Engineering Measurements By 11/23/2024 13 Dr. Mohamed Essa  Measuring Strain (Strain Gages) What devices can be used to measure strain? Electrical Resistance Strain Gage The ideal sensor for the measurement of strain would - Have good spatial resolution, implying that the sensor would measure strain at a point - Be unaffected by changes in ambient conditions - Have a high-frequency response for dynamic strain measurements. A device that closely meets these characteristics is the resistance strain gage. Engineering Measurements By 11/23/2024 14 Dr. Mohamed Essa  What is Strain Gage? A strain gauge, a device whose electrical resistance varies in proportion to the amount of strain in the device. The most widely used gage is the bonded metallic strain gage. Engineering Measurements By 11/23/2024 15 Dr. Mohamed Essa  Engineering Measurements By 11/23/2024 16 Dr. Mohamed Essa  Engineering Measurements By 11/23/2024 17 Dr. Mohamed Essa  Engineering Measurements By 11/23/2024 18 Dr. Mohamed Essa  Load Cell Load cell is an electromechanical device. A load cell is a force transducer that converts force or weight acting on it into an electrical signal. Load cell can be used to measure force, torque and pressure. Load cell can measure a wide range of force, from 25grams to 3,000,000lbs.  CLASSIFICATION AND APPLICATION Mechanical type load cell Hydraulic Pneumatic Electrical type load cell Resistance based (strain gauge load cell) Capacitance based Inductance based (LVDT load cell) Among the many kinds of load cell the most common type is strain gauge load cell.  Mechanical Load Cell Hydraulic load cells are force balance-devices, measuring weight as a change in pressure of the internal filling fluid. It is ideal for use in hazardous areas as there are no any electrical component in it. Continued…. Pneumatic Load Cells also operate on the force- balance principle. These devices use multiple dampener chambers to provide higher accuracy than hydraulic load cell. Pneumatic load cells are often used to measure relatively small weights in industries where cleanliness and safety of prime concern.  Electrical Load Cell Strain Gauge Load Cell A strain gauge is a device used to measure the strain of an object and convert the load acting on them into electrical signals. Due to application of load, strain changes the electrical resistance of the gauge in proportion to the applied load.  Strain gauge shows a very high accuracy of 0.03%.  Theory behind electrical strain gauge load cell We know that, R=ρ*L/A Where, R= Resistance of the conducting material 𝜌 =Resistivity L= Length A= Cross sectional area From the above formula we can deduce that resistance of an object is directly proportional to its length. ie R∝L Continued…. First term: under strain, wire changes dimension, and thus the resistance changes.(dominant for metals) Second term: change in resistivity due to the change in crystal lattice of the material under strain.(dominant in semiconductors). MaterialResistivity Conductor 10−8 to 10−6 Semi-conductor 1 to 104 Insulator 106 to 1018  Gauge Factor For a given amount of strain(∆𝐿/𝐿),the gauge will undergo a corresponding change in resistance (∆𝑅/𝑅). Gauge Factor is defined as the ratio of (∆𝑅/𝑅) and (∆𝐿/𝐿). i.e, (∆𝑹/𝑹) 𝑭𝒈 = (∆𝑳/𝑳) MaterialGauge Factor Conventional foil gauge 1.5 to 3.5 Constantan strain gauge 1.9 to 2.1 Ni-chrome or platinum-iridium up to 3.5  Strain Gauge Resistance  Strain gauge under tension---Resistance goes up.  Strain gauge under compression---Resistance goes down. Strain gauge How does it work ?  Foils/filaments inside the strain gauge are about 1/1000 inch diameter and made up of basic metal conductors.  The gauge is attached to the object by a suitable adhesive.  As the object is deformed, the foil is deformed causing its electrical resistance to change.  The resistance change is commonly measured using a Wheatstone Bridge.  Wheatstone Bridge What is it?  A Wheatstone bridge is an electrical circuit.  used in a load cell to measure an overall change in resistance.  increases sensitivity and reduces the affects of temperature. Wheatstone Bridge How does it work? 𝑅2 𝑅𝑥 If = then circuit is said to be balanced. 𝑅1 𝑅3 Very small change in 𝑅𝑥 disrupt the balance and can be measured precisely. 𝑅𝑥 𝑅2 𝑉𝐺 = V ( − ) 𝑅𝑥 +𝑅3 𝑅2 +𝑅1  Load cell Implementation Dfdsf  Applied force causes small change in resistance in strain gauge that is measured by change in output voltage across the bridge.  Out put voltage from bridge circuit is typically in the order of a few millivolts and requires amplification by an instrumentation amplifier before it can be used.  It has very low noise and very high common mode rejection ratio.  Load cell types Button Shear S Type Beam Load Cell Comparison Strain Gauge Load Cells Continued……. Mechanical and other load cell LVDT BASED LOAD CELL BENDING BEAM LOAD CELL SHEAR BEAM LOAD CELL  LOAD CELL APPLICATION IN TEXTILES 1. All types of textile testing (Instron). 2. Measurement of weight (Moisture regain testing). 3. Online measurement of tension during various process (like printing).  Instron Tensile Tester  Used to measure the tensile strength, load-elongation curve for the fabrics as well as yarns.  Efficiently detect the strain. strain gauges are bounded to the position on the spring material where strain is largest.  Measurement of weight (Moisture regain tester)  A sample of loose fibre or any textile material which has been subjected to a drying treatment is hung from a cantilever of proper dimensions, to which has been bonded a set of strain gauge.  As the sample changes in weight because of moisture regain, the stress in the beam increases and following through the changes take place in the gage and with the help of this it is possible to record continuously the rate of moisture regain of the material.  Cost of load cell SERIESE OF LOAD SPECIFICATION OF LOAD CELL CAPACITY PRICE (in Dollars) CELL LCFD Miniature low profile 1 kg 750 tension/compression LCGD Miniature low profile compression 1000 lb. 460 LC400 Low profile tension/compression 5000 lb. 730 LC100 S-Beam tension/compression 100 lb. 305 LC500 Cantilever beam 5000lb. 350 tension/compression TWA5, TWA6 Self adjusting weight module with 10000 lb. 615 LC501 load beam included  Report Q1: Prepare a hardware implementation for Force or mass digital device measurement. Take into consideration the following notes: Put your name, section, …etc at the first page. Deadline: After 6 weeks. Engineering Measurements By 11/23/2024 42 Dr. Mohamed Essa