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AR-G2/AR 2000ex/AR 1500ex Getting Started Guide
Motor
The requirement that the bearing used on the rheometer should be low in friction applies equally to the
motor. The AR rheometers use a non-contact “drag cup” motor. A thin-walled metal cup is mounted on the
rotating spindle of the rheometer. A magnetic field rotating at thousands of revolutions per minute is gener-
ated by continuously varying the current supplied to stationary pole pieces surrounding the cup. This pro-
duces an eddy current in the cup, which generates a second magnetic field. The two fields oppose each
other, in accordance with Lenz's law, and the cup field is forced to follow the rotating field. Hence, the cup
is “dragged” round by the rotating field, and a torque is generated whether the cup moves or not.
Drag cup motors have many desirable characteristics besides their low friction. Since they have no fixed
magnets, the torque produced is independent of the angular position. Futhermore, the torque is approxi-
mately proportional to the square of the current, which means that a wide torque range is produced by a rel-
atively narrow current range. The rotating components of the motor can be very low in inertia—the limit is
the thinness to which the cup walls can be machined. Low inertia is important whenever the angular veloc-
ity of the moving parts is changed, for example, during transient or dynamic experiments, or steady
changes in torque.
The motor on the AR-G2 rheometer incorporates a patented drag cup temperature sensor. For the first time
in any rheometer design, the temperature of the drag cup is measured, ensuring the most accurate possible
torque output.
The low friction and inertia of the motor, together with sophisticated modern electronics allow close con-
trol of the motor, both in its native, controlled torque, mode, and through feedback in controlled displace-
ment or angular velocity mode. Although designed according to the principles of traditional controlled-
stress rheometers, the AR rheometers are better regarded as both controlled-stress and controlled-strain
rheometers.
Optical Encoder
The transducer used to determine the angular position of the rotating spindle should have high resolution,
low friction (
i.e
., non-contact), low inertia, and a rapid linear response. These criteria are met by the optical
encoder used on the AR rheometers. This consists of a non-contacting light source and photocell, arranged
on either side of a transparent disc mounted on the rheometer spindle. At the edge of this disc are
extremely fine, photographically etched radial lines, which form a diffraction grating. A stationary seg-
ment of a similar disc is also mounted between the light source and the photocell, and the diffraction pat-
tern formed by the light transmitted through the gratings is detected by the photocell. As the spindle
rotates, the diffraction pattern changes. The associated electronic circuitry interpolates and digitizes the
resulting signal, to produce digital high resolution, angular position data.
The angular velocity of the rotating spindle is calculated from successive readings of the angular position,
and since this is done at electronic processor speed, the encoder effectively has two outputs, the angular
position and the angular velocity.
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