Encoder readhead apparatus

1. An encoder readhead, comprising:

a housing having an opening for capturing an encoder scale comprising an elongate rod having a substantially circular cross-section, the opening defining an axial direction along which a captured encoder scale can slide back and forth; and

a scale reader for reading a portion of an encoder scale located in the opening,

wherein the housing includes an access portion that allows the encoder scale to be inserted into, and/or removed from, the opening by a relative movement between the housing and the encoder scale in a non-axial direction.

2. An encoder readhead according to claim 1 , wherein the access portion allows the encoder scale to be inserted into, and/or removed from, the opening by a relative movement between the housing and the encoder scale in a direction substantially perpendicular to the axial direction.

3. An encoder readhead according to claim 1 , wherein the access portion comprises a gap formed in a side wall of the housing, the gap providing side access to the opening that allows the scale member to inserted into, and/or removed from, the opening through the gap.

4. An encoder readhead according to claim 3 , wherein the housing defines a gap having at least one elastically deformable wall portion that permits the gap to expand thereby allowing the encoder scale to pass therethrough.

5. An encoder readhead according to claim 4 , wherein the gap comprises a fluted entrance that causes the gap to expand when pressed into engagement with a encoder scale.

6. An encoder readhead according to claim 4 , wherein when the encoder scale is forced into engagement with the gap it passes through the gap and is captured in the opening by a snap-fit action.

7. An encoder readhead according to claim 1 , wherein the housing defines an opening having at least one elastically deformable wall portion, wherein the at least one elastically deformable wall portion of the opening is biased into engagement with an encoder scale located in the opening thereby capturing the encoder scale.

8. An encoder readhead according to claim 7 , comprising a leaf spring and a an inner slider having one or more surfaces that define the opening, wherein the leaf spring is arranged to bias the one or more surfaces into contact with an encoder scale thereby capturing said encoder scale in the opening.

9. An encoder readhead according to claim 1 , wherein a captured encoder scale fits snugly within the opening and is held against one or more surfaces having a fixed position relative to the scale reader thereby maintaining a substantially constant ride-height between the scale reader and the associated encoder scale as they slide back and forth relative to one another.

10. An encoder readhead according to claim 1 , wherein the opening comprises at least one surface for engaging a captured encoder scale, wherein said at least one surface comprises a material having a low coefficient of friction.

11. An encoder readhead according to claim 1 , wherein the opening only partially surrounds a captured encoder scale.

12. An encoder readhead according to claim 1 , wherein the scale reader comprises at least one magnetic sensor for reading an encoder scale comprising a series of magnetic markings.

13. An encoder readhead according to claim 1 , comprising a mounting for attaching the encoder readhead to a device, wherein the mounting is substantially stiff in a direction parallel to the axial direction and compliant in one or more directions non-parallel to the axial direction.

14. Encoder apparatus comprising; an encoder readhead according to claim 1 and an encoder scale comprising an elongate rod having a substantially circular cross-section.

15. A machine comprising encoder apparatus according to claim 14 , the machine comprising a piston rod that forms the encoder scale, wherein, in use, the piston rod also carries a mechanical load.