Magnetic Actuators and Sensors

CHAPTER 1

Introduction

Magnetic actuators and sensors use magnetic fields to produce and sense motion. Magnetic actuators enable applied electric voltage or current signals to move objects. To sense the motion with an electric signal produced by magnetic fields, magnetic sensors are often used.

Since computers have inputs and outputs that are electrical signals, magnetic actuators and sensors are ideal for computer control of motion. Hence magnetic actuators and sensors are increasing in popularity. Motion control that was in the past accomplished by manual command is now increasingly carried out by computers with magnetic sensors as their input interface and magnetic actuators as their output interface.

Both magnetic actuators and magnetic sensors are energy conversion devices, using the energy stored in static, transient, or low frequency magnetic fields. This book is focused on these magnetic devices, not on devices using electric fields or high frequency electromagnetic fields.

1.1 OVERVIEW OF MAGNETIC ACTUATORS

Figure 1.1 is a block diagram of a magnetic actuator. Input electrical energy in the form of voltage and current is converted to magnetic energy. The magnetic energy creates a magnetic force, which produces mechanical motion over a limited range, typically along a straight line but sometimes rotating over an arc. Thus magnetic actuators convert input electrical energy into output mechanical energy. As mentioned in the caption of Figure 1.1, the blocks are often nonlinear (output not proportional to input), as will be discussed later in this book.

Figure 1.1 Block diagram of a magnetic actuator. The blocks are not necessarily linear. Both the magnetic circuit block and the force factor block are often nonlinear. The force factor block often produces a force proportional to the square of the magnetic field.

Typical magnetic actuators include the following.

• Contactors, circuit breakers, and relays to control electric motors and circuits.
• Switchgear and relays for electric power transmission and distribution.
• Head positioners for computer disk drives.
• Loudspeakers.
• Fuel injectors in engines of automobiles, trucks, and locomotives.
• Electrohydraulic valves in airplanes, tractors, robots, automobiles, and other mobile or stationary equipment.
• Biomedical prosthetic devices for artificial hearts, limbs, ears, and other organs.
• Magnetic separators that produce forces on magnetic objects large and small, including particles smaller than a micron targeted within the human body for tumors, etc.

Since magnetic actuators produce motion over a limited range, other electromechanical energy converters with large ranges of motion are not discussed in this book. Thus electric motors that produce multiple 360° rotations are not covered here. However, “step motors” which produce only a few degrees of rotary motion are classified as magnetic actuators and are included in this book.

1.2 OVERVIEW OF MAGNETIC SENSORS

A magnetic sensor has the block diagram shown in Figure 1.2. Compared to a magnetic actuator, the energy flow is different, and the amount of energy is often much smaller. The main input is now a mechanical parameter such as position or velocity, although electrical and/or magnetic input energy is usually needed as well. Input energy is converted to magnetic field energy. The output of a magnetic sensor is an electrical signal. In many cases the signal is a voltage with very little current, and thus the output electrical energy is often very small.

Figure 1.2 Block diagram of a magnetic sensor. The blocks are not necessarily linear.

Magnetic devices that output large amounts of electrical energy are not normally classified as sensors. Hence typical generators and alternators are not discussed in this book.

Typical magnetic sensors include the following.

• Proximity sensors to determine presence and location of conducting objects for factory automation, bomb or weapon detection, and petroleum exploration.
• Microphones that sense air motion (sound waves).
• Linear variable differential transformers to determine object position.
• Velocity sensors for antilock brakes and stability control in automobiles.
• Hall effect position or velocity sensors.
• Magnetoresistive position or field sensors.

Design of magnetic actuators and sensors involves analysis of their magnetic fields. The actuator or sensor should have geometry and materials that utilize magnetic fields to produce maximum output for minimum size and cost.

1.3 ACTUATORS AND SENSORS IN MOTION CONTROL SYSTEMS

Motion control systems can use nonmagnetic actuators and/or nonmagnetic sensors. For example, electric field devices called piezoelectrics are sometimes used as sensors instead of magnetic sensors. Other nonmagnetic sensors include global positioning system (GPS) sensors that use high frequency electromagnetic fields, radio frequency identification (RFID) tags, and optical sensors such as television cameras. Nonmagnetic actuators and sensors are not discussed in this book.

An example of a motion control system that uses both a magnetic actuator and a magnetic sensor is the computer disk drive head assembly shown in Figure 1.3. The head assembly is a magnetic sensor that senses (“reads”) not only the computer data magnetically recorded on the hard disk, but also the position (track) on the disk. To position the heads at various radii on the disk, a magnetic actuator called a voice coil actuator is used.

Figure 1.3 Typical computer disk drive head assembly. The actuator coil is the rounded triangle in the upper left. The four heads are all moved inward and outward toward the spindle hub by the magnetic force and torque on the actuator coil. Portions of the actuator and all magnetic disks are removed to allow the coil and heads to be seen.

Often the best way to control motion is to use a feedback control system as shown in Figure 1.4. Its block diagram contains both an actuator and a sensor. The sensor may be a magnetic sensor measuring position or velocity, while the actuator may be a magnetic actuator producing a magnetic force. It is found that accurate control requires an accurate sensor. Control systems books widely used by electrical and mechanical engineers describe how to analyze and design such control systems [1–4]. The system design requires mathematical models of both actuators and sensors, which will be discussed throughout this book.

Figure 1.4 Basic feedback control system which may use both a magnetic actuator and a magnetic sensor.

Another example of a magnetic actuator and a magnetic sensor is shown in Figure 1.5. It shows a tubular magnetic actuator and a magnetic Hall effect sensor packaged together to produce and sense motion along a straight line. This linear motion is accomplished without any gears or chains, thus enabling long maintenance-free life with low friction. Associated electronic controls enable precise motion control.

Figure 1.5 Magnetic actuator with built-in magnetic sensor, producing straight-line motion along its axis. Figure courtesy of Dunkermotoren Linear Systems.

1.4 MAGNETIC ACTUATORS AND SENSORS IN MECHATRONICS

The word “mechatronics” is a blend of the words mechanics and electronics [5]. Mechatronic systems contain both mechanical components and electronics with controlling software. To enable the electronics to control mechanical motion, electromechanical devices are used, often containing magnetic actuators and sensors, as shown in Figures 1.1–1.5.

Figure 1.6 depicts mechatronics as made up of four major overlapping systems [6]. The mechanical systems are controlled by electrical/electronic systems, computer systems, and control systems–-all working together. Note all four major systems have overlaps; one overlap area is called electromechanical systems. Magnetic actuators and sensors are important components of electromechanical systems.

Figure 1.6 Venn diagram showing major engineering areas in mechatronics and how they relate to magnetic actuators and sensors.

Figure 1.6 is actually a simplified picture of the overlapping and multidisciplinary or “multiphysics” nature of mechatronics. This book also deals with additional overlaps not explicitly indicated in Figure 1.6, for example, the use of computer software to analyze and design magnetic actuators and sensors. To understand mechatronic systems containing magnetic actuators and sensors, this book is ordered in parts devoted to Magnetics, then Actuators, then Sensors, and finally to the resulting Systems.

REFERENCES

1. Dorf RC, Bishop RH. Modern Control Systems, 9th ed. Upper Saddle River, NJ: Prentice-Hall Inc.; 2001.

2. Dorsey J. Continuous and Discrete Control Systems, New York: McGraw-Hill; 2002.

3. Phillips CL, Harbor RD. Feedback Control Systems, 4th ed. Upper Saddle River, NJ: Prentice-Hall Inc.; 2000.

4. Lumkes JH Jr. Control Strategies for Dynamic Systems, New York: Marcel Dekker, Inc.; 2002.

5. Cetinkunt S. Mechatronics, Hoboken, NJ: John Wiley & Sons, Inc.; 2007.

6. Kevan T. Mechatronics primer: Reinventing machine design, Desktop...