Voice Coil Michael Ryan ECE 5320 Mechatronics Assignment 1
Outline • Voice coil actuators fall into two main categories based on the type of motion provided. The first and most widely used is rotational. The second is linear. • I will begin with a description of both types, followed by their applications, specifications, advantages, and limitations.
Introduction • Voice coil actuators allow linear movements over a limited range of motion. Originally, they were used in radio loud speakers, after which they are named. Voice coil actuators are direct drive devices based on a permanent magnet field and current-carrying coil windings, and produce a force, which is directly proportional to the applied current. Therefore, they are easy to understand, to build, to apply and to control. In a closed-loop servo system, the position sensor sends feedback signals to the drive electronics enabling high speed, accuracy, and a high degree of precision.
Principles of Operation • A Voice Coils electromechanical energy conversion process is based on the Lorentz Force principle, which is illustrated in the figure to the right. This law expresses that if a current-carrying conductor is placed in a magnetic field, a force will act upon it, which is proportional to the current and the magnetic flux density.
Principles of Operation • Based on the assumptions that the magnetic field between the pole shoes is homogeneous and static, that no fringing exists at the pole shoes, that no flux-leakage occurs, and that no field variations are caused by the DC conductor current, this force can be calculated using the following equation: • F L = ×(i´B) • where: • F : Force vector • I : Current vector • B : Magnetic flux density vector • L : Conductor length in the magnetic field
Principles of Operation • In the case that the conductor moves in a magnetic field, a voltage proportional to its velocity is induced across the conductor, which is illustrated in below. This voltage is known as the induced voltage. Based on the same assumptions made before and the additional assumption that the flux density vector, velocity vector and orientation vector of the conductor are orthogonal, the magnitude of the induced voltage can be calculated by • Ui = L×v×B • where: • Ui : Induced voltage • i : Current • v : Velocity of the conductor • B : Magnetic flux density • L : Conductor length in the magnetic field
Rotational Voice Coil Operation • A Rotary Voice Coil consists of an arm assemble which contains the voice coil at one end and a head at the other. The Coil end of the arm is placed inside a permanent magnet assembly which provides a strong magnetic field. As current is applied to the coil it is attracted to the permanent magnet and a force is induced. The arm is mounded at a pivot point about which the arm assembly will rotate when current is applied to the voice coil.
Rotational Voice Coil Example • Magnet Assembly is the permanent magnet to which the Voice coil is attracted when current is applied to the Voice Coil. • The Actuator Axis forms a leverage point about which the Arm rotates. • As you can see the Magnets physical size restricts how far the arm can rotate.
Linear Voice Coil Operation • In its simplest form shown in, a linear voice coil actuator is a tubular coil of wire placed within a radially orientated magnetic field. The field is generated by a tubular permanent magnet with radial magnetization, which is mounted on the interior of a hollow soft iron cylinder. The magnetic circuit is completed by an inner soft iron core positioned along the centerline of the coil. This core and the permanent magnet are fixed to the soft iron cylinder, jointly forming the shell of the actuator.
Principles of Operation • Applying a current through the coil windings generates an axial force between the coil and the shell. This force also arises from the Lorentz Force principle, as discussed before, and thus, with respect to the same assumptions made before, this force and the induced voltage can be calculated by • F N = ×L×i×B • Ui = N×L×v×B • where: • N : Number of coil windings • L : Length of one coil winding
Linear Voice Coil Example • The picture top right is of a liner voice coil actuator, and the one on the bottom is of a disassembled linear voice coil. • As you can see the cylindrical housing is a fixed, magnet, while the coil is free to move within the housing, on top of the coil is mounted a non magnetic armature. • (1-Soft Iron Core; 2-Soft Iron Cylinder; 3-Permanent Magnet; 4-Coil)
Applications • Hard Drive Head Actuator • Audio Speaker • Robotics • Industrial processes • Medical • Microcircuit manufacturing
Major Specifications • Linear Voice Coil • Stroke Length • 40micron to 100-mm range without comutations • Speed • 20-400 Hz • Force • 5000N • Acceleration • 30-300 G
Major Specifications • Rotary Voice Coil • Stroke Length • 120deg • Speed • 20-400 Hz • Force • 120 N-m • Acceleration • 30-300 G
Advantages • Zero Hysteresis • Advantagous when rapid change in direction is required, such as in hard drive head positioning, and in many automation applications • High acceleration • Higher acceleration rates than servo, and stepper motors, yet lower than can be achieved with pezoelectric • Produce little heat. • What heat is produced is as a factor of the resistance of the coil, and a small amount due to friction.
Direct Drive. • There are no gears, cogs, or screws, so there is no backlash. • Low Acoustic Noise • The lack of gear backlash results in less noise than geared systems • Inexpensive • Relative to traditional servo motors. • Long Life • Limited number of moving parts ensures long life. • High force • When compared to comperable stepper motors, and servomotors
High precision • The armature of a Voice Coil Actuator can be positioned as precisely as can be measured. • Linearity • Near linear relationship between force and current • Light weight • Light wieght relative to stepper and servo motor systems.
Limitations • Limited Range of Motion • Rotational Voice Coils can’t rotate beyond about 120deg, limited by the fixed magnet. • Linear voice Coils are limited in stroke length by force of friction, and load on actuator arm. • Limited Accuracy • Accuracy is limited to feedback sensor accuracy. This is true of all voice coils, and of any closed loop system. • Sensitivity to Noise • Sensitive to wide band noise and to electro magnetic fields, should be used in closed loop systems where errors can be reduced.
Power Limitations • Power levels are mainly limited by the fact that low noise power amplifiers must be used, and they are mainly at low power levels.
Reference List • “About Voice Coil Motors” GlobalSpec. http://electric-motors.globalspec.com/LearnMore/Motion_Controls/Motors/Voice_Coil_Motors • Babinski, Alex, and Tsao Tsu-Chin. “Accelerated Feedback Design for Voice Coil Actuated Direct Drive” Proceedings of the American Control Conference. California June 1999 • Bishop, Robert. The Mechatronics Handbook. New York: CRC Press, 2002. • Curtiss, Steve. “Meso-Machine Tool Research at the Universityof ILLinois” Department of Mechanical and Industrial Engineering June 19, 2001.
“Head Actuator” The PC Guide. April 7 2001 http://www.pcguide.com/ref/hdd/op/actActuator-c.html • “Inprovement of voice coil applications design using FEM Flux Software” CERDRAT June 28 2004 http://www.cedrat.com/applications/software/doc/VoiceCoil2/index.htm • “Where do voice-coil actuators fit in?” Machine Design. http://www.machinedesign.com/ASP/strArticleID/56123/strSite/MDSite/viewSelectedArticle.asp