Chapter 2: Transducers (Sensors)

Chapter 2: Transducers (Sensors)

• Transducers are devices that convert one form of energy into another.
• This chapter focuses on static and dynamic specifications.

Static Specifications

• Accuracy: Error is the difference between true output and actual output of a transducer. Measures accuracy in terms of percentage error of full scale output (FSO) and of true reading, and absolute error of the input quantity.
• Resolution: The smallest change in input that corresponds to a detectable change in the output.
• Repeatability: A measure of how well the output returns to a given value when the same input is applied several times. Repeatability is calculated by taking the difference between the maximum reading and the minimum reading and dividing that difference by the full-scale output. This value is often multiplied by 100 to arrive at a percentage
• Hysteresis: A measurement representing the difference in output between the increasing and decreasing input readings. A curve is often plotted to illustrate this comparison.
• Linearity: The measure of how different the actual characteristic from the ideal one is.
• Example-1: A load cell is a transducer used to measure weight. Calibration data is given with this example. This example includes calculating accuracy in terms of FSO and true reading, and absolute error, given an accuracy of +7.85% FSO.

Solution to Example-1

• Plot the calibration curve.

• Calculate the accuracy in terms of percentage of FSO and true readings, for a load of 20KG in both increasing and decreasing inputs.

• Calculate the absolute error considering +7.85% FSO.

Resolution

• The smallest change in input that produces a corresponding change in output.
• Industrial encoders often provide 100 to 1000 pulses per revolution. This translates to a resolution ranging from 3.6 to 0.36 degrees.
• Example 2 demonstrates a scenario involving a 2.5 meter vane rotating, requiring specific resolution capabilities.

Repeatability

• A measure of how consistently a transducer responds to the same input.
• Calculated as 100 * (max reading - min reading) / full scale output.

Hysteresis

• Hysteresis is the difference in output measurements when the input is increased vs decreased. This is illustrated using a graph. The key concept is the difference between the curves.

Linearity

• Linearity evaluates how closely a transducer's output follows a straight line relationship with its input.
• There are various mathematical methods for determining linearity:
  • Endpoint Linearity
  • Independent Straight Line Linearity
  • Least Square Linearity

Dynamic Specifications

• Focuses on time response characteristics of sensors measuring changing inputs.
• First-order element
• Time constants and dead time
• Second-order element
• Damping ratio and natural frequency/DT

Position Transducers (Potentiometer)

• Potentiometer can act as a displacement sensor.
• Wires wound: resolution is derived using the number of turns in the wire.
• Carbon resistors: advantages include higher resolution, smooth surface, and speed.
• Electrical diagrams are provided explaining common connections of potentiometers for position sensing.
• Example 4. Explains a scenario involving determining the feasibility of a potentiometer to meet position resolution accuracy requirements for a panel movement.

Force Transducers, Load Cells

• Load cells are specially designed to measure force.
• Accurately measured force is important for accurate design of machinery such as cars and spacecraft.
• Specifications: The output voltage at rated capacity (e.g in mV/V) is useful to measure the sensitivity of the load cell.
• Example 11. Provides specifications for a load cell. This example includes calculations for related metrics.

Velocity Transducers

• Different devices include electromagnetic linear transducers, laser Doppler systems, DC tacho-generators and AC induction tachogenerators
• Frequency of the signal changes proportionally to the speed.
• Example 12. Illustrates a DC motor conveyor belt speed measurement scenario and the needed calculations to obtain the pulses per revolutions needed using an optical sensor and other parameters.

Fluid Transducers

• Used in several industry sectors, particularly in oil production.
• The unit for pressure is often expressed as pounds per square inch (psi). Equivalent metric units such as pascal are also discussed.
• Manometric means are used to measure pressure.
• Elastic pressure transducers include Bourdon tubes, bellows, and diaphragms. A drawing will illustrate how each of these devices works.

Flow Transducers

• Different measures of flow rate are discussed (volumetric, mass, and velocity).
• Laminar and turbulent flow is explained.
• Flow measurements using orifice plates, Venturi tubes, and Dall tubes are discussed.
• Diagrams illustrating their functions are provided.
• Rotameter, Cantilever Flow Meter are discussed.

Level Transducers

• Different types of level sensors (discrete and continuous) are discussed (with diagrams illustrating how each one works). Different methods of measuring the level using for example, pressure sensing, float level sensors or capacitive level sensors are discussed.
• Calculations are provided to obtain the height of the liquid level.

Temperature Transducers

• Early and accurate temperature measurement is discussed along with the units of measurement of temperatures (Celsius to Fahrenheit to Kelvin).
• Different methods and diagrams used for temperature measurement such as mercury thermometers or bimetallic thermometers are provided.
• Thermocouples, RTDs and thermistors used for temperature measurement are discussed.
• There is a discussion on various circuits useful for temperature sensing, using for example, thermocouples, RTD or thermistors, including the method to create an ice point reference.

Other Types of Transducers

• Discussions on linear variable differential transformers (LVDT)
• Optical Encoder are are discussed.

This quiz explores the specifications of transducers, focusing on static characteristics such as accuracy, resolution, repeatability, and hysteresis. Understand how these parameters quantify the performance of sensors in converting energy forms. Test your knowledge of key concepts from Chapter 2.