Non-Contact Sensor For Servo Track Writer.

New sensor increases accuracy and allows servo track writing without the use of clean room.

The increase in the write density of Hard Disk Drives (HDDs) has given rise to the necessity of non-contact control of the head position at the time of writing onto the servo track. Some manufacturers of encoders have already proposed a double-servo sensor system that combines an ordinary encoder and non-contact edge sensor. This type of system uses a rotary encoder to control the position of an edge sensor with a large mass that direct1y detects the HDD's arm position. The arm is aligned with the position of the edge sensor by means of feedback control. Thus, position control at high speed is difficult and the sensitivity of the edge sensor is lower than that of the encoder.

Sony's non-contact sensor is designed to overcome these draw-backs. Featuring higher resolution, excellent stability, and greater accuracy than competitive techniques, it enables the HDD manufacturer to satisfy his increasing technical demand to comply with the higher TPI of his products. When it is used in a combination of an arm head and a clock head, it allows servo track writing without the use of a clean room.

Composition

The Sony system is composed of a scale sheet provided for each HDD (which may be allowed to stay in place after writing), a sensor head, and a processing circuit. The user first pastes/glues the scale sheet onto the arm of the HDD. The external sensor head reads the scale position while the arm position is controlled according to the position data generated by the processing circuit, writing the servo signal. A circular scale sheet may be attached to the spindle motor to obtain the clock signal.

Detecting Principle

The scale unit consists of relief-type 1m-pitch diffraction grating with a radial recording. The sensor head detects the scale position from a distance of 19.6mm using the principle of a grating interferometer.

The light coming from the semiconductor laser is split into two beams, enters the scale, and is diffracted. At the time of diffraction, the scale's grating causes the phase of diffracted light beams to change. The light beams that were diffracted in the opposite direction are superimposed to cause them to interfere with each other. This interference results in a very accurate sine wave signal with a pitch that is 1/4 times the scale pitch.

The sensor's optical system is characterized by the fact that it does not detect the change in wavelength of the light source (semiconductor)and the scale's Z-axis vibration. The unique interferometer not only allows for easy adjustment, but it also enables a signal with a 0.25m period (four times the scale's resolution) to be obtained. The fact that the pitch is approximately one-twentieth the signal's pitch of competitive encoders results in a very high resolution and reduces the vulnerability to disturbed air by almost 20 times. The optical system outputs sine and cosine waves with DC components canceled out. These waves are interpolated to 1/500 to obtain a 32-bit parallel binary output with 0.5nm resolution.

Scales

The scale for detecting the arm position is available in several distances (Rs): in a range from 10mm and 20mm (distance R is from the center of the arm's rotation to the optical head detecting position). The scale's resolution is 0.07sec with an R of 14mm, and 0.05sec with an R of 19.6mm. Response speed is 200mm/sec.

The scale used as a clock generator is also available in two Rs: 8mm and 20mm. The output of 123,200 or 246,400 pulses will be produced without interpolation using the scale with R=4.9mm or R=9.8mm, respectively. These outputs have a frequency of 10MHz and 20MHz, respectively, even at a spindle speed of 5,000rpm. The signal's DC components are removed at the clock side. Response speed is 45MHz.

Change required according to the scale configuration and the position in which the scale is attached can be made. Generally speaking, the scale's insensitivity to vibrations in the direction of height coupled with its high basic resolution is outstanding.

Scale material is polyethylene terephthalate or metal. Dimensions are as specified by the user; thickness is 60 to 150m.

A compact servo track writer without moving parts is now available. Use of the Sony system allows for very high speed and simple control of the HDD's arm position up to the limit of the arm's capabilities. It is also capable of high resolution that matches the ever higher write density of the HDDs. In addition, by providing a transparent window in the HDD, writing to an HDD is now possible outside a clean room.

One relatively minor draw-back of the Sony system is the fact that the disposable scale adds to the running costs. However, the scale is expected to cost only about 1/100th (1%) the price of an HDD head.

Larry Sato is the president of Sony Precision Technology America (Lake Forest, CA).