The retrofit of a 1/4-W split-Stirling cooler with a linear driven on the displacer was achieved and its performance characterized. The objective of this work was to demonstrate that a small linear motor could be designed to meet the existing envelope specifications of the cooler and that an electric linear drive on the displacer could improve the cooler's reliability and performance. The paper describes the characteristics of this motor and presents cooler test results

Ackermann, R. A.

Bhate, S. K.

Byrne, D. V.

English

NASA Technical Reports Server

SPLIT-STIRLING-CYCLE DISPLACER LINEAR-ELECTRIC DRIVE
R.A. Ackermann, S.K. Bhate, and D.V. Byrne
MECHANICAL TECHNOLOGY INCORPORATED
ABSTRACT
The retrofit of a I/4-W split-Stirling cooler with a linear drive on the
displacer was achieved and its performance characterized. The objective of this
work was to demonstrate that a small linear motor could be designed to meet the
existing envelope specifications of the cooler and that an electric linear drive
on the displacer could improve the cooler's reliability and performance. The
paper describes the characteristics of this motor and presents cooler test
results.
INTRODUCTION
A recent in-house program at Mechanical Technology Incorporated (MTI) was
directed toward retrofitting a I/4-W split-Stirling-cooler displacer with a
linear-electric drive. The cooler was supplied to MTI by the U.S. Army Chemical
Systems Laboratory and the Honeywell Corporation with the objective of demon-
strating the feasibility of adding a linear drive to the cooler. The in-house
program involved:
• Designing a small linear motor to fit into the displacer bounce space
• Integrating the motor with the displacer
• Developing the control circuitry and logic to control displacer motion
• Testing the displacer with a standard I/4-W cooler compressor.
COOLER DESCRIPTION
The I/4-W split-Stirling cooler (Figure I) was developed at the Night
Vision Electro-Optics Laboratory. The retrofit to the cooler was made by adding
a linear-electric drive to the displacer. The drive was designed to be added
without changing the thermodynamics of the cooler, and to use all existing hard-
ware. To achieve this, the linear drive was added to the displacer bounce space
and attached to the displacer rod.
A layout of the retrofitted displacer is shown in Figure 2. This retrofit
was assembled with existing components (shown to the left of line A-A) and the
linear drive components (shown to the right of A-A). The drive components
consist of a permanent-magnet linear motor, a housing, and a displacer rod
connection. The linear motor has a radially energized, moving permanent-magnet
plunger (which is attached to the displacer), and a laminated stator containing
several circular coils. The housing, which provides a hermetic case for the
motor, was designed for effective cooling of the motor, and to provide a rear
bearing for radial support of the plunger.
231
https://ntrs.nasa.gov/search.jsp?R=19840007275 2020-03-22T08:11:50+00:00Z
In this retrofitted design, the motor was used to control the displacer
motion by monitoring the pressure wave and triggering the displacer motion from
this control parameter. The flexibility of the control schemeis shown in
Figure 3. Control consisted of the ability to vary the phasing between the
pressure wave and displacer motion (#d), the duration between intake and exhaust
strokes (*d), pulse width (_), and the amplitude of the displacer forcing func-
tion (Im). Through these four parameters, the effect of displacer phasing and
motion on cooler performance can be evaluated. The control circuit (Figure 4)
consisted of a pressure transducer mountedat the compressoroutlet, the phasing
circuitry, and a motor powersupply.
MOTOR CHARACTERISTICS
The retrofit displacer motor is one of a family of permanent-magnet (SmCo 5)
linear motors under development at MTI*. Figure 5 schematically shows a section
of the motor with the plunger in the center position. The motor consists of
stationary outer electrical coils wound on a steel bobbin and an inner
permanent-magnet plunger. The magnets are radially magnetized with alternate
magnets having north polarity on the outside diameter and south polarity on the
inside diameter. The design characteristics for the motor are given below.
Frequency: 25.0 Hz
Stroke: 1.9 mm (0.075 in.)
Size: 25-ram dia x 25-mm long (i in. x I in.)
Weight:
Static Force at 0.8 A:
Electrical Input Power:
0.07 kg (0.15 ib )
m
10.2 N (2.3 Ibf)
<3.0 W
RESULTS OF MOTOR BENCH TESTING
STATIC RESPONSE TEST RESULTS
As a first step in characterizing the performance of the MTI displacer
linear motor, static force tests were conducted. The purpose of these tests was
to determine available static force capability of the motor as a function of
input current and plunger position. This information is particularly relevant
to the operation of the motor to overcome friction effects as cooler wear and
contamination progress.
*Bhate, S.K., "Linear-Oscillating Electric Machine with Permanent-Magnetic
Excitation," U.S. Patent: 4,349,757, September 14, 1982, assigned to MTI,
Latham, New York.
232
These tests were conductedwith the apparatus shownin Figure 6. With this
apparatus, the motor force is measuredwith a piezoelectric force transducer as
a function of displacement measuredwith an LVDTand motor current. A plot of
the motor force is given in Figure 7 for currents up to 1.6 A. An important
characteristic defined by the force plot is the extraordinary force capability
of this miniature motor. Themotor can produce several poundsof force through-
out the required stroke range, which is more than adequate to control the motion
of the displacer.
DYNAMICRESPONSET STRESULTS
A displacement test apparatus was used to measurethe dynamic response of
the motor to an alternating step-current input. This type of test is important
because it determines the ability of the motor to quickly travel from one
extreme stroke position to another. The data also provide the controls design
team with dynamic response information necessary for the establishment of
control system time constants and parameters. Figure 8 is a sketch of oscillo-
scope traces of a typical dynamic motor response test at 25 Hz. The plunger
travels the stroke of 1.7 mm(0.065 in.) in approximately 3 ms*.
COOLERTESTRESULTS
Retrofitting of the cooler was recently completed. To determine cooler
performance, preliminary tests with the following parameters were conducted:
• Operation of the cooler at a reduced stroke of 1.7 mm(0.065 in.) to
determine control parameter settings
• Cool-downrate to 80 K
• Motor input power.
The oscillograph tracing presented in Figure 9 shows the system pressure
motor current time history. For the run depicted, phasing betweenthe pressure
wave and displacer intake motion (positive current pulse) was i0 ms (approxi-
mately 48°), pulse width was 2 ms, and current amplitude was0.8 A. Becauseof
the limited testing conducted to date, sufficient data has not been taken to
verify that these are the optimum control settings. However, the cooler did
achieve the cool-down specification of 80 K in less than i0 min even at a reduced
stroke. It is anticipated that in future testing at a design stroke of 1.9 mm
(0.075 in.), the motor will enable the cooler to showa significant improvement
over the cool-down specification.
The following analysis presents the motor power for the operating pointdescribed.
*Comparing favorably with a typical travel time of 6-8 ms in a pneumatically
driven system.
233
Assumptions:
Im - Displacer Motor Current = 0.8 A
R - Motor Coil Resistance = 27
Ff - Plunger Friction Force = 1.78 N (0.4 ibf)
Ax - Motor Stroke = 1.7 mm(0.065 in.)
At - Plunger Travel Time = 0.002 s
FA - Acceleration Force = 4.89 N (i.i ibf) (Assume_ constant over travel)
PeakMotor PowerCalculation:
Ppeak = Heating Loss + Friction Loss + Acceleration Power
= Im2R+ (Ff x Ax)/At + (FA x Ax)/At
( ( 0.065
= (0.82 x 27) + 0.4 + 1.1) x
0.002
i ft
12 in. lw I]0.74 J- ibf/sec
(0.82
= x 27) + [(0.4 + 1.1)(3.67)] = 17.3 + 5.5 = 22.8 W
Average Motor Power Calculation:
Pavg Ppeak x Duty Cycle* = (22.8 W)(0.13) = 3.0 W
This analysis shows that because the duty cycle is low, average displacer
motor power is approximately 3 W (about 10%) of the total power consumed by a
typical I/4-W cooler system. Ongoing tests indicate that further reduction in
the power required to drive the displacer will be achieved by implementation of
optimum control parameters. A displacer motor power reduction to less than 2 W
in this prototype, and to approximately i W in an advanced design that elimi-
nates the frictional rear bearing, is anticipated. Further, the flexibility
gained in displacer control should lead to performance improvements through
better selection of regenerator materials (now limited by displacer dynamics),
displacer motion control, vibration elimination, and stroke modulation.
CONCLUSION
The conclusion reached from this work is that a small linear electric
displacer drive can be developed for the 1/4 W split-Stirling cooler that will
fit into the present cooler envelope. The benefits that will be gained from a
linear drive are improvements in performance and reliability. Future work at
MTI will be directed towards advancing the state of the art of linear coolers by
building a linear drive compressor and coupling it with our retrofitted expander
to demonstrate the feasibility of a total linear split-Stirling cooler.
*Calculated from Figure 9.
234
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Split-Stirling-cycle displacer linear-electric drive

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