# ME302: Mechatronics

Unit 4: Feedback Devices   In the last unit, you studied the types of actuators used in mechatronic systems. The feedback devices are equally important to the actuators. Feedback devices are effectively the eyes and ears of the control system that enable it to accurately control the mechatronic system. As mentioned in the last unit, it is generally necessary to control position, speed, or both. Thus, it is important to have available feedback devices for position and speed. This unit will introduce you to the different types of feedback devices that are used in mechatronic systems. Feedback devices in mechatronic systems are mainly used to measure the position, speed, or orientation of the system that is being controlled.

In the following three subunits, you will be introduced to three groups of feedback devices that are used in mechatronic systems. In order for a controller to properly control a mechatronic system, it needs to possess accurate and up-to-date information about the velocity and position of the system. This allows the controller to compare the desired velocity (or position) with the actual velocity (or position) and to adjust the actuator outputs accordingly. Providing such information is the role of feedback devices.

This unit should take approximately 4.25 hours to complete.

☐    Subunit 4.1: 1.75 hours

☐    Subunit 4.2: 0.75 hours

☐    Subunit 4.3: 1.75 hours

Unit4 Learning Outcomes
Upon successful completion of this unit, you will be able to:
- explain the need for feedback devices in mechatronic systems; - describe the principle of operation of an incremental shaft encoder; - describe the principle of operation of an incremental encoder; - describe the principle of operation of the linear variable differential transformer (LVDT); and - describe the principle of operation of an accelerometer and a tilt-meter.

4.1 Shaft Encoders: Incremental and Absolute Shaft Encoders   A shaft encoder is a device that is mechanically connected to a rotating shaft, such that it rotates at exactly the same speed of the shaft and attains exactly the same position, and it provides an electrical output that represents the position, speed or both of the mechanical shaft to which it is connected.

Shaft encoders are digital in nature. They provide an output in a digital format in the form of pulses. Shaft encoders are of two types: incremental and absolute. Incremental shaft encoders provide a stream of pulses that are proportional to the rotational speed of the shaft. In effect, incremental shaft encoders are ideally used for speed feedback.

Absolute shaft encoders, on the other hand, provide a digital output that represents the actual rotational position of the shaft. Absolute shaft encoders provide an output in the form of a number of bits (e.g., 12 bit; 14 bit; 18 bit). The angular resolution of the shaft encoder increases as the number of bits increases.

• Reading: Rensselear Polytechnic Institute and Marquette University: Kevin Craig’s Multidisciplinary Mechatronic Innovations: “Optical Encoder and the Arduino” Link: Rensselear Polytechnic Institute and Marquette University: Kevin Craig’s Multidisciplinary Mechatronic Innovations: “Optical Encoder and the Arduino” (PDF)

Instructions: Please click on the link above and select the link titled “Optical Encoder and the Arduino.” While the links are mostly ordered alphabetically, note that this link appears at the top of the list. Read pages 1 to 11 of the presentation by Kevin Craig about optical encoders and the Arduino.

Reading this presentation and the information provided in this subunit should take approximately 1 hour and 30 minutes.

• Reading: The Saylor Foundation’s “Grey Code versus Binary Code” Link: The Saylor Foundation’s “Grey Code versus Binary Code” (PDF)

4.2 Linear Variable Differential Transformer: Principle of Operation   The shaft encoders discussed in the last subunit are rotary in nature. The linear variable differential transformer is the main feedback device used for linear position feedback. It is very widely used in industrial applications.

• Reading: RDP Electronics Ltd.’s “How It Works – LVDT” Link: RDP Electronics Ltd.’s “How It Works – LVDT” (HTML)

Instructions: Read the material on the webpage linked above, and watch the LVDT animation. You will notice that as the ferromagnetic core moves left and right, it changes the coupling between the primary coil and the each of the two secondary coils.

• Reading: InTech: Dr. Lutfi Al-Sharif et al.’s MATLAB for Engineers – Applications in Control, Electrical Engineering, IT, and Robotics: “Chapter 4: Linear Variable Differential Transformer Design and Verification Using MATLAB and Finite Element Analysis” Link: InTech: Dr. Lutfi Al-Sharif et al.’s MATLAB for Engineers – Applications in Control, Electrical Engineering, IT, and Robotics: “Chapter 4: Linear Variable Differential Transformer Design and Verification Using MATLAB and Finite Element Analysis” (PDF)

Instructions: Please click on the link above and select “Download as PDF” to access the text. Read the first two pages of Chapter 4. Notice the details of the construction of the LVDT. In particular, notice the primary coil, secondary coils and the core. The LVDT is the main sensor used nowadays for measuring linear displacement accurately. It has very high sensitivity and can cover very small strokes. Unlike linear potentiometers, it does not suffer from friction.

Reading this chapter should take approximately 30 minutes.

4.3 Accelerometers: Principle of Operation   The feedback devices discussed in the last two subunits measure velocity or displacement. It is also possible to measure acceleration directly by the use of accelerometers. It is common in some applications to measure acceleration and derive velocity by the use of integration. One of the most widely used types of accelerometers is the so-called seismic accelerometer.

• Reading: University of Jordan: Dr. Lutfi Al-Sharif’s “Acceleration, Vibration, and Shock Measurement” Link: University of Jordan: Dr. Lutfi Al-Sharif’s “Acceleration, Vibration, and Shock Measurement” (PDF)

Instructions: Please click on the link above to access the PDF, and read the material on accelerometers (7 pages). Note how a seismic accelerometer device can be used to convert acceleration into displacement.

gyroscope is a device that can store the original orientation of a vehicle or an aircraft. It is based on the principle of conservation of angular momentum. In effect, it resists the change of its orientation.

Reading this chapter should take approximately 1 hour and 30 minutes.