Page 434 - The Mechatronics Handbook

0066_Frame_C19 Page 56 Wednesday, January 9, 2002 5:27 PM

capacity, strain gage transducer with either slip rings or rotary transformers and integral speed sensor,
specified nonlinearity and hysteresis each within ±0.1%, costs about $4000 (1997). Compatible instru-
mentation prividing transducer excitation, conditioning, and analog output with digital display of torque
and speed costs about $2000. A comparable magnetoelastic transducer with ±0.5% accuracy costs about
$1300. High-capacity transducers for extreme speed service with appropriate lubrication options can
cost more than $50,000. Type 2 magnetoelastic transducers, mass produced for automotive power steering
applications, cost approximately $10.

Apparatus for Power Measurement

Rotating machinery exists in specific types without limit and can operate at power levels from fractions
8
of a watt to some tens of megawatts, a range spanning more than 10 . Apparatus for power measurement
exists in a similarly wide range of types and sizes. Mechanical power flows from a driver to a load. This
power can be determined directly by application of Eq. (19.64), simply by measuring, in addition to ω,
the output torque of the driver or the input torque to the load, whichever is the device under test (DUT).
When the DUT is a driver, measurements are usually required over its full service range of speed and
torque. The test apparatus therefore must act as a controllable load and be able to absorb the delivered
power. Similarly, when the DUT is a pump or fan or other type of load, or one whose function is simply
to alter speed and torque (e.g., a gear box), the test apparatus must include a driver capable of supplying
power over the DUT’s full rated range of torque and speed. Mechanical power can also be determined
indirectly by conversion into (or from) another form of energy (e.g., heat or electricity) and measuring
the relevant calorimetric or electrical quantities. In view of the wide range of readily available methods
and apparatus for accurately measuring both torque and speed, indirect methods need only be considered
when special circumstances make direct methods difficult.
Dynamometer is the special name given to the power-measuring apparatus that includes absorbing
and/or driving means and wherein torque is determined by the reaction forces on a stationary part (the
stator). An effective dynamometer is conveniently assembled by mounting the DUT in such a manner
as to allow measurement of the reaction torque on its frame. Figure 19.49 shows a device designed to
facilitate such measurements. Commercial models (Torque Table® [12]) rated to support DUTs weighing
222–4900 N are available with torque capacities from 1.3 to 226 Nm. “Torque tubes” [4] or other DUT
mounting arrangements are also used. Other than for possible rotational/elastic resonances, these systems

Flexure straps with
strain gauges
Center of rotation (4)

Table for mounting
device under test

Electrical
connector
Base

FIGURE 19.49 Support system for measuring the reaction torque of a rotating machine. The axis of the machine
must be accurately set on the “center of rotation.” The holes and keyway in the table facilitate machine mounting
and alignment. Holes in the front upright provide for attaching a lever arm from which calibrating weights may be
hung [4,11].

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