DHR Series Getting Started Guide Page 23
Radial Air Bearings
Radial bearings provide stiffness and support in the radial direction. The Discovery Series rheometers are
designed with two porous carbon radial bearings, located above and below the motor.
Motor
The requirement that the bearing used on the rheometer should be low in friction applies equally to the
motor. The Discovery Series 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 generated by continuously varying the current supplied to stationary pole pieces surrounding the
cup. This produces 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 have a very low moment of inertia—
the limit is the thinness to which the cup walls can be machined. Low inertia is important whenever the
angular velocity of the moving parts is changed, for example, during transient or dynamic experiments, or
steady changes in torque.
Drag cup motors can get hot during use, and the torque output will vary with temperature. The motor
incorporates a patented drag cup temperature sensor. The temperature of the drag cup is measured, and the
input to the motor corrected, 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 Discovery Series 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 moment of inertia, and a rapid linear response. These criteria are met
by an optical encoder. 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, photo-
graphically etched radial lines, which form a diffraction grating. A stationary segment of a similar disc is
also mounted between the light source and the photocell, and the diffraction pattern 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 digi-
tal 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.
In the DHR-3, the performance of the encoder is enhanced by using a dual reader configuration. This pat-
ent pending design improves the basic resolution and noise level of the displacement signal as well as
removing drift and improving phase resolution.
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