In motion platforms, voice coil motors are commonly used to drive small movements, such as 1-5 mm or even smaller strokes.
Because they have significant characteristics such as small size, small moving mass, and high acceleration.
Whether in the industrial field, it is widely used in medical endoscopes, ventilators, and camera focusing.
For example, small displacement suction heads are commonly driven by flat voice coil motors and matched with LVDT absolute displacement sensors to detect the position of the motion axis.
For another example, in a mobile phone camera, a cylindrical voice coil motor is used to drive the lens movement and achieve focus adjustment.
Coil motion single-axis voice coil motor drive platform
Voice coil motor used for micro camera focusing
Different forms of voice coil motor driving platforms and focusing platforms and their parameters
So, what is voice coil motor (VCM=Voice Coil Motor)? What is its principle of motion?
1. What is a voice coil?
First, what is a voice coil?
The word voice coil actually comes from the speaker.
It's essentially a coil.
In the speaker, it is used to generate (display) sound signals, so it is called the voice coil. Thanks to it, we can hear wonderful music.
The underlying logic of the speaker is actually that the charged wire undergoes regular movement under the Lorentz force in the magnetic field, causing the relevant components to vibrate, thereby emitting sound.
Let’s take a closer look.
The voice coil (coil) is wound around the cone body. The north and south poles of the permanent magnet are as shown in (a) on the left above. The inner ring is the north pole and the outer ring is the south pole. In the cross-sectional view (b), the magnetic field radiates outward through the voice coil wire. , when the wire passes a sound signal (current), the voice coil is forced in the left and right directions and moves, driving the cone to vibrate and diffusing the sound to the outside through the air.
The principle of voice coil used in speakers
When power is applied, the voice coil moves back and forth
2. Voice coil motor structure and principle
The principle of the voice coil motor is exactly the same as mentioned above: the coil is energized and forced in the permanent magnetic field, and moves. However, the voice coil motor coil is not connected to the amplification component, but to other components that need to be driven.
Schematic diagram of the internal structure of a cylindrical voice coil motor with moving magnets: the voice coil is fixed, the magnet is connected to the shaft, and there are flexible guide structures at both ends of the shaft. When the coil is energized, an axial force is generated between the coil and the magnet, driving the shaft to move.
Cross-sectional view of a moving coil voice coil motor. The current in the upper coil is perpendicular to the screen and goes outward. The current in the lower part is perpendicular to the screen and goes inward. The magnetic field is perpendicular to the current in the gap. Use the left-hand rule (let the magnetic induction lines pass through the palm of your hand and point the four fingers toward the current. direction, the direction pointed by the thumb is the direction of force on the coil), it is easy to know that the coil moves in the left and right directions under force.
This picture is very similar to the axial linear motor. You only need to copy, arrange and fix the magnets, and then release the coil. Isn't it the axial linear motor?
The force on the coil is called the Lorentz force and is determined by the product of the current and the magnetic flux:
F = k * B * L * I * N
in,
F = Force (N)
k = force constant
B = Magnetic flux density (Tesla)
L = line length (m)
I = current (amps)
N = number of conductors
For a given voice coil, all parameters are fixed except current. Therefore, the force generated is proportional to the input current. The direction of the force is perpendicular to both the direction of the magnetic flux and the direction of the current. Changing the direction of the current will change the direction of the force.
This is the principle of voice coil motor.
Its core is that the energized coil is subject to the Lorentz force in the magnetic field.
In practical applications, the force generated by the voice coil pushes the load, and the moving part can be a coil or a magnet (the forces are mutual).
Coil motion is much lower mass than magnet motion and can drive higher loads.
However, the coil generates heat, and if the load is sensitive to temperature fluctuations, it is better to use magnets to move it.
Of course, as a complete actuator, the voice coil motor, like the linear motor, requires a guide mechanism and a position feedback device.
The guide mechanism can use flexible structures (generally used for small movements), slider guides, air bearings, or crossed roller guides, etc.
Feedback devices can use grating rulers and reading heads, or LVDT displacement sensors, Hall sensors, etc. Together with servo controllers, they provide a closed-loop system that enables extremely precise position and speed control.
Even without feedback devices, voice coil motors have good force control because the force they produce is proportional to the applied current.
Single-axis and dual-axis voice coil motor drive platforms, the bottom picture is a dual-axis motion platform, and the bottom motion axis is driven by dual motors.
2-axis voice coil motor drive platform
3-axis voice coil motor drive platform
3. How is the voice coil motor used in lens focusing?
Let's take a closer look at how the voice coil motor is used to adjust the lens focus.
Look at both flat and cylindrical shapes.
Flat voice coil motor is used to drive the micro lens to focus: As shown in Figure (b), the lens and permanent magnet are fixed on the frame, and then passed through the two guide rods on the diagonal line in Figure (c). As shown in Figure (c), the guide rod, coil and Hall sensor are fixed on the base. When the coil is energized, it can drive the lens to move up and down to achieve focus adjustment.
The cylindrical voice coil motor drives the lens to move up and down to achieve focusing: the lens and coil are fixed on the blue lens holder, and there are yellow flexible guide structures on the top and bottom, which are firmly connected to the lens holder. The cylindrical magnet provides a magnetic field on the outside, and the radiation direction of the magnetic field is perpendicular to the coil. It is easy to judge that the coil is stressed in the up and down direction, driving the lens to move, thereby achieving focus adjustment.
At this point, we understand the working principle of the voice coil motor.
Because of this principle, voice coil motors have the following advantages: no need for commutation, low to no hysteresis, zero cogging, fewer mechanical parts compared to "screw" power actuators, linear control characteristics, and high power-to-mass ratio , high power to volume ratio, unlimited position (limited only by encoder), high acceleration, etc.
Therefore, voice coil motors are very suitable for small displacement linear motion. If your design has this requirement, you may wish to consider using voice coil motors.