14. CONTINUOUS SENSORS14.1 INTRODUCTION14.2 INDUSTRIAL SENSORS14.2.1 Angular Displacement14.2.1.1 - Potentiometers14.2.2 Encoders14.2.2.1 - Tachometers14.2.3 Linear Position14.2.3.1 - Potentiometers14.2.3.2 - Linear Variable Differential Transformers (LVDT)14.2.3.3 - Moire Fringes14.2.3.4 - Accelerometers14.2.4 Forces and Moments14.2.4.1 - Strain Gages14.2.4.2 - Piezoelectric14.2.5 Liquids and Gases14.2.5.1 - Pressure14.2.5.2 - Venturi Valves14.2.5.3 - Coriolis Flow Meter14.2.5.4 - Magnetic Flow Meter14.2.5.5 - Ultrasonic Flow Meter14.2.5.6 - Vortex Flow Meter14.2.5.7 - Positive Displacement Meters14.2.5.8 - Pitot Tubes14.2.6 Temperature14.2.6.1 - Resistive Temperature Detectors (RTDs)14.2.6.2 - Thermocouples14.2.6.3 - Thermistors14.2.6.4 - Other Sensors14.2.7 Light14.2.7.1 - Light Dependant Resistors (LDR)14.2.8 Chemical14.2.8.1 - pH14.2.8.2 - Conductivity14.2.9 Others14.3 INPUT ISSUES14.4 SENSOR GLOSSARY14.5 SUMMARY14.6 REFERENCES14.7 PRACTICE PROBLEMS14.8 PRACTICE PROBLEM SOLUTIONS14.9 ASSIGNMENT PROBLEMScontinuous sensors - 14.114. CONTINUOUS SENSORS14.1 INTRODUCTIONContinuous sensors convert physical phenomena to measurable signals, typically voltages or currents. Consider a simple temperature measuring device, there will be an increase in output voltage proportional to a temperature rise. A computer could measure the voltage, and convert it to a temperature. The basic physical phenomena typically mea-sured with sensors include;- angular or linear position- acceleration- temperature- pressure or flow rates- stress, strain or force- light intensity- soundMost of these sensors are based on subtle electrical properties of materials and devices. As a result the signals often require signal conditioners. These are often amplifi-ers that boost currents and voltages to larger voltages.Sensors are also called transducers. This is because they convert an input phenom-ena to an output in a different form. This transformation relies upon a manufactured device with limitations and imperfection. As a result sensor limitations are often charac-Topics:Objectives:• To understand the common continuous sensor types.• To understand interfacing issues.• Continuous sensor issues; accuracy, resolution, etc.• Angular measurement; potentiometers, encoders and tachometers• Linear measurement; potentiometers, LVDTs, Moire fringes and accelerometers• Force measurement; strain gages and piezoelectric• Liquid and fluid measurement; pressure and flow• Temperature measurement; RTDs, thermocouples and thermistors• Other sensors• Continuous signal inputs and wiring• Glossarycontinuous sensors - 14.2terized with;Accuracy - This is the maximum difference between the indicated and actual read-ing. For example, if a sensor reads a force of 100N with a ±1% accuracy, then the force could be anywhere from 99N to 101N.Resolution - Used for systems that step through readings. This is the smallest increment that the sensor can detect, this may also be incorporated into the accuracy value. For example if a sensor measures up to 10 inches of linear dis-placements, and it outputs a number between 0 and 100, then the resolution of the device is 0.1 inches.Repeatability - When a single sensor condition is made and repeated, there will be a small variation for that particular reading. If we take a statistical range for repeated readings (e.g., ±3 standard deviations) this will be the repeatability. For example, if a flow rate sensor has a repeatability of 0.5cfm, readings for an actual flow of 100cfm should rarely be outside 99.5cfm to 100.5cfm.Linearity - In a linear sensor the input phenomenon has a linear relationship with the output signal. In most sensors this is a desirable feature. When the relation-ship is not linear, the conversion from the sensor output (e.g., voltage) to a cal-culated quantity (e.g., force) becomes more complex.Precision - This considers accuracy, resolution and repeatability or one device rel-ative to another.Range - Natural limits for the sensor. For example, a sensor for reading angular rotation may only rotate 200 degrees.Dynamic Response - The frequency range for regular operation of the sensor. Typ-ically sensors will have an upper operation frequency, occasionally there will be lower frequency limits. For example, our ears hear best between 10Hz and 16KHz.Environmental - Sensors all have some limitations over factors such as tempera-ture, humidity, dirt/oil, corrosives and pressures. For example many sensors will work in relative humidities (RH) from 10% to 80%.Calibration - When manufactured or installed, many sensors will need some cali-bration to determine or set the relationship between the input phenomena, and output. For example, a temperature reading sensor may need to be zeroed or adjusted so that the measured temperature matches the actual temperature. This may require special equipment, and need to be performed frequently. Cost - Generally more precision costs more. Some sensors are very inexpensive, but the signal conditioning equipment costs are significant.14.2 INDUSTRIAL SENSORSThis section describes sensors that will be of use for industrial measurements. The sections have been divided by the phenomena to be measured. Where possible details are provided.continuous sensors - 14.314.2.1 Angular Displacement14.2.1.1 - PotentiometersPotentiometers measure the angular position of a shaft using a variable resistor. A potentiometer is shown in Figure 14.1. The potentiometer is resistor, normally made with a thin film of resistive material. A wiper can be moved along the surface of the resistive film. As the wiper moves toward one end there will be a change in resistance proportional to the distance moved. If a voltage is applied across the resistor, the voltage at the wiper interpolate the voltages at the ends of the resistor.Figure 14.1 A PotentiometerThe potentiometer in Figure 14.2 is being used as a voltage divider. As the wiper rotates the output voltage will be proportional to the angle of rotation.schematicphysicalresistivewiperfilmV1V2VwV1VwV2continuous sensors - 14.4Figure 14.2 A Potentiometer as a Voltage DividerPotentiometers are popular because they are inexpensive, and don’t require special signal conditioners. But, they have limited accuracy, normally in the range of 1% and they are subject to mechanical wear.Potentiometers measure absolute position, and they are calibrated by rotating them in their mounting brackets, and then tightening