52
THEOR
Y
5. Operating Principles
5.1
SPEED
Inside the dynamometer enclosure is an optical speed transducer. A high-speed optical switch,
consisting of an Infrared (IR) LED and IR receiver, senses the passage of light through a slotted disk
that is attached to the end of the dynamometer shaft. Light passing through a slot in the disk causes
the speed output signal to go to a logic low (near 0 VDC). As a solid section passes in front of the
IR receiver, the output signal switches to a logic high (near 5 VDC). Rotation of the disk results in
the optical switch generating a pulse frequency of 60 bits per shaft revolution. For schematic, see
Section B.2 – Speed Sensor Board
in Appendix B.
5.2
TORQUE
Torsional force from the hysteresis brake assembly is measured by a load cell. The load cell consists
of a flexing beam with four strain gauges. The strain gauges are in a bridge configuration, producing
an analog signal proportional to torque.
The load cell schematic in Appendix B shows voltage levels and connection identifications. The
mechanical diagram below illustrates how the assembly clamps onto the rear support member of
the brake.
LOAD CELL PC BD.
WIRE COLORS --
BLUE GREEN WHITE RED
LOAD CELL
ASSEMBLY
PIVOT ASSEMBLY
STRAIN GAUGES
FINE BALANCE
LOCKING SCREW
NOTE: A coarse balancing
resistor, located on the
torque signal amplification
board, may be connected
between the blue or green
and white wire terminals.
TEMP COMP
Figure 5–1 Mechanical Layout of a Load Cell
