Portable coordinate measurement machine

A portable coordinate measurement machine (CMM) for measuring the position of an object in a selected volume, comprising:

• an articulated arm having opposed first and second ends, said arm including at least five rotary joints such that said articulated arm has at least five degrees of freedom, each degree of freedom corresponding to a rotational axis such that said articulated arm travels through at least five rotational axes;
• a measurement probe attached to a first end of said articulated arm;
• an electronic circuit which receives the position signals from transducers in said arm and provides a digital coordinate corresponding to the position of the probe in a selected volume;
• and
• wherein at least one of said rotary joints further comprise;
• a periodic pattern of a measurable characteristic;
• at least two read heads spaced from and in communication with said pattern;
• and
• said pattern and said at least two read heads being positioned within said rotary joint so as to be rotatable with respect to each other and wherein said at least two read heads cooperate to decrease error in rotary measurement which is due to applied loads affecting the relative position between said read heads and said pattern.