Since highly accurate machines have been introduced, the detection of gear rotation with high resolution and high accuracy has been more and more requested. Here, some configuration examples to detect the gear rotation with high accuracy are introduced. Such a configuration uses the element in which the resistance value changes as the magnetic field changes (semiconductor magnetoresistive element).

Detection of gear rotation using semiconductor magnetoresistive element

• High speed rotation detectable
• Highly resistant to contamination such as dust, dirt, or oil, because of detection of magnetism
• High noise-resistance due to high output performance
• Easy to correspond to shaft diameter
• No dark current

Outline of detection of gear rotation using semiconductor magnetoresistive element

For the detection of gear, a 4-terminal magnetoresistive element with the phases A and B outputs in full bridge structure as shown in the figure.
* Currently, all our products have this structure. Contact us for any other structures.

A semiconductor magnetoresistive element and a bias magnet are combined for use. As shown in the figure, a bias magnet is placed at the back face of the semiconductor magnetoresistive element. This combination shall be used for the range where the rate of resistance change is linear and rather larger comparing to the magnetic flux density change, setting the operation point to 0.4T or more using the bias magnet.
In this configuration, if the gear (a magnetic element) rotates in parallel with the semiconductor magnetoresistive element surface, the magnetic flux density on the semiconductor magnetoresistive element changes. The change in the magnetic flux density is taken out as a voltage change.

The figure at the right indicates the relative positions of the gear and four magnetoresistive elements. Two magnetoresistive elements in the bridge structure (two magnetoresistive elements sandwiching A or B) are placed with a space of p (pitch)/2, and these two bridges are offset by p/4. This configuration allows the phases A and B outputs to have a phase difference of 90 degrees.
Currently, we provide the magnetoresistive elements corresponding to three types of gears complying with the JIS, such as the gear modules m = 0.8 (pitch = 0.8 p ), m = 0.4 (pitch = 0.4 p ), and m = 0.2 (pitch = 0.2 p ).
Contact us for any other pitches.

The figure indicates the output voltages of the magnetoresistive elements in phases A and B when the gear rotates under the conditions below.
Under the conditions below, it is understandable that both the output voltages in the phases A and B are a pseudo-sine wave of approx. Vp - p » 500 mV (neutral voltage: 2.5 V ± 15 mV), and the phase difference between the phases A and B is p/4 (90 degrees).

<Measurement conditions>

• Gear: Module m = 0.8 (pitch = 0.8 p )
• Gear radius: 51.2 mm
• Product No. of our semiconductor magnetoresistive element: MS-0080
• Distance between package surface and gear tip of MS-0080: 0.5 mm
• Magnet size: 5.5 x 4.5 x t5.0 mm (in magnetization direction: t5.0 mm)
• Magnet material: Sm2Co17 (samarium-cobalt magnet)
• Control voltage of semiconductor magnetoresistive element Vc: 5 V

The figure indicates the ambient temperature characteristics of the output voltage amplitude Vp-p of the semiconductor magnetoresistive element when the gear rotates under the conditions below. Owing to Asahi Kasei's unique semiconductor film technology, the semiconductor magnetoresistive element with much more excellent temperature characteristics comparing to those of the conventional semiconductor magnetoresistive element can be provided. The output voltage amplitude shows almost no change in a temperature range between -40 and 100 °C.
*A samarium-cobalt magnet with excellent temperature characteristics is used as a bias magnet.

<Measurement conditions>

• Gear: Module m = 0.8 (pitch = 0.8 p )
• Gear radius: 51.2 mm
• Product No. of our semiconductor magnetoresistive element: MS-0080
• Distance between package surface and gear tip of MS-0080: 0.5 mm
• Magnet size: 5.5 x 4.5 x t5.0 mm (in magnetization direction: t5.0 mm)
• Magnet material: Sm2Co17 (samarium-cobalt magnet)
• Control voltage of semiconductor magnetoresistive element Vc: 5 V

The figure indicates the dependence of the output voltage amplitude Vp-p of the semiconductor magnetoresistive element on the gap (the distance between the package surface and gear tip) under the conditions below.

<Measurement conditions>

• Gear: Module m = 0.8 (pitch = 0.8 p )
• Gear radius: 51.2 mm
• Product No. of our semiconductor magnetoresistive element: MS-0080
• Magnet size: 5.5 x 4.5 x t5.0 mm (in magnetization direction: t5.0 mm)
• Magnet material: Sm2Co17 (samarium-cobalt magnet)
• Control voltage of semiconductor magnetoresistive element Vc: 5 V

<Measurement conditions>

• Gear: Module m = 0.8 (pitch = 0.8 p )
• Gear radius: 51.2 mm
• Product No. of our semiconductor magnetoresistive element: MS-0080
• Distance between package surface and gear tip of MS-0080: 0.5 mm
• Magnet size: 5.5 x 4.5 x t5.0 mm (in magnetization direction: t5.0 mm)
• Magnet material: Sm2Co17 (samarium-cobalt magnet)
• Control voltage of semiconductor magnetoresistive element Vc: 5 V

The output voltage of the semiconductor magnetoresistive element obtained by the rotating gear is very close to the ideal sine wave with quite little waveform distortion. The figure indicates the FFT analysis result of the output voltage of the semiconductor magnetoresistive element measured when the gear is rotated under certain conditions. As shown in this figure, the output voltage from the semiconductor magnetoresistive element has a superimposed harmonic content even though it is small, resulting in a deviation from the ideal sine wave. In general, the distortion factor is used as an index indicating a degree of this deviation. The distortion factor K (%) indicates how much the output voltage waveform is distorted comparing to the ideal sine wave, and can be calculated with the expression below after obtaining the amplitude for each discrete frequency.

Where
E1: Amplitude of fundamental wave
E2: Amplitude of secondary harmonic component
E3: Amplitude of tertiary harmonic component
E4: Amplitude of quaternary harmonic component

The figure indicates the dependence of the distortion factor of the output voltage from the semiconductor magnetoresistive element on the gap under the conditions below. If the gear module m = 0.4 is used, the distortion factor shows a very small figure of 0.5% or less. The distortion factor tends to be smaller when the gap becomes larger. If the gap becomes larger, the output voltage amplitude becomes smaller, therefore the gap shall be determine considering the balance of them.

<Measurement conditions>

• Gear: Module m = 0.4 (pitch = 0.4 p )
• Gear radius: 51.2 mm
• Product No. of our semiconductor magnetoresistive element: MS-0040
• Magnet size: 4.4 x 4.4 x t5.0 mm (in magnetization direction: t5.0 mm)
• Magnet material: Sm2Co17 (samarium-cobalt magnet)
• Drive voltage of semiconductor magnetoresistive element: 5 V

Applications

The detection of rotation in various fields and applications, including machine tools in which high accuracy is required

• Machine tools: Spindle control
• Elevator and escalator control
• Electric injection molding machine spindle control

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