Measuring Weight with Strain Gauges Measuring Weight with Strain Gauges and Load Cells For the past year, I've been working in secret on an improved version of my electric longboard. So far, I implemented a new power switch, a new remote, and a new control circuit that cannot only receive information from the remote wirelessly but also integrates a weight measuring system. This way, if you ever lose control of the electric longboard and must jump off, it recognizes that and brakes automatically. The key component that makes this weight measuring system possible is a so-called strain gauge. A strain gauge is actually pretty common when it comes to measuring masses or forces electrically. So in this video, let's have a closer look at them and find out how we can integrate them in a circuit in order to measure weight/force easily with a microcontroller. On closer inspection, a strain gauge is just a flexible piece of plastic on which a zigzag pattern of resistance wire is secured. By soldering two thin wires to its contacts and measuring the resistance of this particular strain gauge, then we get a value of around 120.4 ohms, which correlates with its data sheets. Besides the 120-ohm value, there also exist 350, 700, or 1000 ohms as standard values. And while my example strain gauge is kind of big, there also exist smaller variations, even sometimes with different patterns. Let's rather find out how a piece of plastic can measure weights, and the answer is actually quite simple. By stretching or compressing the strain gauge, the resistance of its wire pattern increases or decreases slightly. That means the strain of the strain gauge is proportional to its resistance, which logically means we can calculate the mass of an object. So all we have to do is properly glue the strain gauge to the object we want to apply force to and measure its resistance right. Well, it's not that simple since non-extreme weights aka more realistic forces only create a very small, almost not measurable change in the resistance. What we have to utilize instead to fix this problem is a so-called Wheatstone bridge. By replacing R1 with the strain gauge and the rest of the resistors with precise 120-ohm resistors, and applying a 5-volt supply voltage to the circuit, we would get a voltage difference of 0 volts between the resistors if no forces are applied and a voltage proportional to the strain if a force is applied. That means we successfully converted the change in resistance into a voltage that we could now amplify with a differential op-amp configuration with a gain of, for example, 1000 and measure with an analog-to-digital converter. The reason is that the 10-bit ADC of the 8-mega 328p microcontroller features a resolution of 1024 steps which equals voltage steps of 4.9 millivolts. But now we got a much higher resolution, which subsequentially equals smaller voltage steps. This way, we might get more noise in the readings, but we can measure way smaller forces. And with that being said, you should now be familiar with the basics of strain gauge/load cells and how to use them to measure weight easily. Strain Gauges A strain gauge is a sensor whose resistance varies with applied force; It converts mechanical energy into electrical energy. Background The concept of strain gauges dates back to the 1930s, when they were first used to measure stress in aircraft structures. The development of modern strain gauges is attributed to the work of Simmons and Ruge in the 1940s. Initially, strain gauges were made from metal wires or foils, which were bonded to a substrate material using adhesives. The introduction of semiconductor materials in the 1950s led to the development of piezoresistive strain gauges. Today, strain gauges are widely used in various fields such as aerospace, automotive, biomedical, and civil engineering for measuring strain, stress, and pressure. Foil Strain Gauges: These are the most common type of strain gauge and consist of a thin foil grid attached to a flexible backing material. Semiconductor Strain Gauges: These gauges use semiconductor materials such as silicon or germanium to measure strain. Piezoelectric Strain Gauges: These gauges use piezoelectric materials that generate an electric charge in response to mechanical stress. Weighing Scales: Strain gauges are widely used in weighing scales for measuring weight accurately. Crane and Hoist Systems: Strain gauges are used to measure the weight of loads lifted by cranes and hoists. Aerospace and Defense: Strain gauges are used to measure the weight and stress of aircraft and spacecraft components. High Accuracy: Strain gauges can measure weight with high accuracy and precision. Low Power Consumption: Strain gauges require low power consumption, making them suitable for battery-powered devices. Compact Size: Strain gauges are compact in size, making them suitable for use in confined spaces. Measuring Weight with Strain Gauges Strain gauges are widely used for measuring weight, stress, and pressure in various industries such as aerospace, automotive, and medical. In this article, we will explore the concept of measuring weight with strain gauges, its working principle, types of strain gauges, and their applications. Working Principle Strain gauges measure the deformation or strain of an object when a force is applied to it. When a weight is placed on a surface, it causes a slight deformation in the material, which is measured by the strain gauge. The strain gauge converts this deformation into an electrical signal, which is then processed and displayed as a weight reading. Types of Strain Gauges There are several types of strain gauges available, including: Applications of Weight Measurement with Strain Gauges Weight measurement with strain gauges has a wide range of applications, including: Advantages of Weight Measurement with Strain Gauges The advantages of using strain gauges for weight measurement include: Conclusion In conclusion, measuring weight with strain gauges is a widely used technique that offers high accuracy and precision. The various types of strain gauges available make it suitable for a wide range of applications. With its advantages such as low power consumption and compact size, weight measurement with strain gauges continues to be an essential tool in many industries. Q1: What is the principle behind measuring weight with strain gauges? Strain gauges measure the deformation (strain) of an object when a force (weight) is applied to it. By calibrating the strain gauge output, the weight can be calculated. Q2: What type of strain gauge is commonly used for weight measurement? Foil strain gauges are widely used for weight measurement due to their high sensitivity and stability. Q3: How does the orientation of the strain gauge affect the measurement? The orientation of the strain gauge is critical, as it must be aligned with the principal axis of strain to ensure accurate measurements. Misalignment can lead to incorrect readings. Q4: What is the role of a Wheatstone bridge in strain gauge-based weight measurement? A Wheatstone bridge is used to convert the resistance changes of the strain gauges into a measurable voltage output, which can then be calibrated to represent the applied weight. Q5: How does temperature affect the accuracy of strain gauge-based weight measurements? Temperature changes can cause expansion or contraction of the material being measured, leading to errors in the weight measurement. Temperature compensation techniques must be employed to minimize these effects. Q6: What is the significance of calibration in strain gauge-based weight measurement? Q7: How does the material of the object being measured affect the strain gauge measurement? The material's properties, such as Young's modulus and Poisson's ratio, influence the deformation (strain) caused by the applied weight. The strain gauge must be calibrated for the specific material being measured. Q8: Can strain gauges measure dynamic weights? Q9: How does the number of strain gauges used affect the measurement accuracy? Q10: What are some common applications of strain gauge-based weight measurement? Common applications include industrial automation, medical devices, agricultural weighing systems, and aerospace engineering, among others. Rank Pioneers/Companies Contributions 1 Karl Baxter Fuller Invented the first strain gauge in 1938, which led to the development of weight measurement using strain gauges. 2 Hottinger Baldwin Messtechnik (HBM) Developed the first commercial strain gauge weight transmitters in the 1950s, widely used in industrial applications. 3 Vishay Precision Group (VPG) Produced high-precision strain gauges and weight sensors for various industries, including aerospace and medical. 4 Kistler Group Developed piezoelectric strain gauges for dynamic weight measurement in applications like material testing and drop tests. 5 Pacific Instruments (PACI) Created high-performance strain gauge amplifiers for precise weight measurement in industrial environments. 6 FUTEK Advanced Sensor Technology Manufactured a range of strain gauge-based load cells and weight sensors for industries like aerospace and automotive. 7 Interface, Inc. Developed high-precision load cells and weight transmitters using strain gauge technology for various industrial applications. 8 Mettler Toledo International Inc. Produced high-accuracy weighing systems and instruments, including strain gauge-based weight sensors for laboratory and industrial use. 9 Thermo Fisher Scientific Inc. Developed precision weighing systems, including strain gauge-based weight sensors for laboratory and industrial applications. 10 Sartorius AG Manufactured high-precision weighing systems, including strain gauge-based weight sensors for laboratory and industrial use. Measuring Weight with Strain Gauges: Technical Details Parameter Description Sensor Type Foil strain gauges, typically used for measuring static and dynamic strains in various materials. Strain Gauge Configuration Full bridge (4 active arms) or half bridge (2 active arms) configuration to measure bending or axial strain. Sensor Material Copper-nickel alloy, constantan, or polyimide-based materials for the foil strain gauge. Strain Gauge Adhesive Epoxy resin, cyanoacrylate, or specialized adhesives to bond the strain gauge to the load cell material. Load Cell Material High-strength aluminum alloys (e.g., 7075), stainless steel (e.g., 17-4PH), or advanced materials like carbon fiber reinforced polymers (CFRP). Strain Gauge Excitation DC voltage excitation, typically between 5V to 15V, depending on the strain gauge and amplifier specifications. Amplifier Gain Adjustable gain settings (e.g., 100-1,000) to amplify the microvolt-level signals from the strain gauges to a usable voltage range (e.g., 0-10V). Signal Conditioning Low-pass filtering (e.g., 100 Hz), noise reduction, and temperature compensation to improve signal quality. Data Acquisition Analog-to-digital conversion using a microcontroller or dedicated data acquisition system with sampling rates up to several kilohertz. Calibration and Linearity Linear calibration over the desired weight range, typically ±0.1% to ±1% full-scale output (FSO) non-linearity error. Temperature Compensation Temperature coefficient of resistance (TCR) compensation and/or temperature sensing using thermistors or thermocouples to account for thermal expansion effects. Operating Temperature Range Typically -20°C to +60°C, but can vary depending on the strain gauge, adhesive, and load cell material used.