Encoder and attachment method

An attachment method of attaching an encoder to a detection target,

• wherein the encoder includes:
• an emitter that emits light;
• a rotary plate that rotates and includes an annular region that reflects or transmits the light emitted from the emitter, the annular region being provided surrounding a rotation axis of the rotary plate;
• and
• a light receiver that receives light emitted from the emitter and arriving via the annular region,
• the light receiver includes:
• a first set that includes a first light-receiving region and a second light-receiving region that are arranged side by side in a first direction intersecting a rotation direction of the rotary plate;
• and a second set that includes a third light-receiving region and a fourth light-receiving region that are arranged side by side in a second direction intersecting the rotation direction and that is provided side by side with the first set in the rotation direction, and
• the encoder further includes:
• an outputter that outputs a signal indicating a value of X expressed as below and a signal indicating a value of Y expressed as below:
• X
• =
• (
• A
• ⁢
• 1
• +
• B
• ⁢
• 2
• )
• -
• (
• B
• ⁢
• 1
• +
• A
• ⁢
• 2
• )
• Y
• =
• (
• A
• ⁢
• 1
• +
• B
• ⁢
• 1
• )
• -
• (
• A
• ⁢
• 2
• +
• B
• ⁢
• 2
• )
• where A1 denotes a signal value corresponding to light received by the first light-receiving region,
• A2 denotes a signal value corresponding to light received by the second light-receiving region,
• B1 denotes a signal value corresponding to light received by the third light-receiving region, and
• B2 denotes a signal value corresponding to light received by the fourth light-receiving region,
• the attachment method comprising:
• attaching the rotary plate to the detection target;
• and
• after the attaching, causing the outputter to output the signal indicating the value of X and the signal indicating the value of Y,
• wherein in the attaching, the rotary plate is attached to the detection target at a position where the value of X indicated by the signal output by the outputter becomes 0 and the value of Y indicated by the signal output by the outputter becomes 0.