The PTB angle comparator:
primary standard for angle measurement

Angle-measuring tables or angular index tables in which the round angle (360°) is made use of as a natural primary standard on the basis of the circular scale, are the most important angle comparators. PTB Working Group 5.23 has at its disposal three precision angle-measuring tables of this kind with incorporated circular gratings and photoelectric and incremental read-out of the values measured. The newest angle-measuring table (made by Heidenhain, Traunreut) was installed at the PTB's Clean Room Centre in 1994. At present, this angle comparator is the most accurate standard measuring device for the plane angle and allows an uncertainty of measurement of 0,005" (k = 2) to be reached.

The figure below shows a schematic view of the angle comparator in a sectional drawing.

The comparator stands on a solid, vibration-isolated granite plate. It comprises an air bearing rotor with axial plane bearing and cylindrical radial bearing which ensure high load capacity and rotational accuracy. A wheel and disk tangential drive free from mechanical play acts on the circumference of the rotor's driving bell and allows rotating speeds between 7,5 rpm and 7,5"/min to be reached in dynamic measurements. In static measurements, an additional piezo drive makes possible exact positioning in a small angular range. The angle-measuring system of the rotor consists of a ring-shaped index disc of glass with a radial reflected-light phase grating with 2¹⁷ = 131 072 graduation periods on a circle approx. 400 mm in diameter. Eight scanning heads uniformly distributed over the circumference of the graduation are used for scanning this graduation. 2¹⁸ = 262 144 signal periods per second are formed in each scanning head, which corresponds to an angular period of approx. 5". Digital interpolation of the signal period with the factor 2¹² = 4096 finally furnishes 2³⁰ = 1 073 741 824 measurement steps per 360°, which corresponds to an angle-measuring step of approx. 0,0012" per scanning head. The angle value measured is finally obtained by averaging over all scanning heads. The upper side of the rotor bears a support which allows fine adjustment and takes up the test system to be calibrated.
A mounting ring around the support may be used for setting up measuring arrangements.

The following figures show two measuring arrangements set up on the angle comparator for the calibration of a precision polygon and an incremental angle-measuring system.

Measuring arrangement used for the calibration of a precision polygon with an autocollimator

Measuring arrangement used for the calibration of an incremental angle-measuring system

As shown in the above schematic view, the inner rotor part together with the support can be decoupled from the outer rotor part through an axial air gap and rotated in relation to the outer rotor part into any angular position desired. This equipment allows the divided circle of the comparator to be calibrated against a test system or against the second angle-measuring system connected with the internal rotor, applying the so-called rosette measuring method.
Self-calibration of the comparator is possible as well. In this case, eight additional calibration scanning heads are read out in addition to the eight measuring scanning heads. These additional heads are arranged so that all partial angles of 360°/2ⁿ (with n = 1 to 7) are formed on the divided circle.

Both calibration methods furnished a uniform value of U = 0,005"
(k = 2) for the comparator's uncertainty of measurement.
Due to errors in the transfer of dimension, a greater uncertainty of measurement is obtained for the calibration of an angle standard or an angle-measuring system. At present, U = 0,05" (k = 2) is achieved as the smallest calibration uncertainty for high-quality precision polygons or exact incremental angle-measuring systems.

---

© Physikalisch-Technische Bundesanstalt Page created: 2001-08-28 , last update: 2006-05-05, D. Schulz