Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-03-31
2001-09-11
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S323020, C310S323030
Reexamination Certificate
active
06288473
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
(Not Applicable)
BACKGROUND OF THE INVENTION
Piezoelectric (piezo) rotary motors have found their way into industrial applications where high torque, low rpm requirements are needed in small volumes. Magnetic flux motors are typically large, have a high rpm, and require gear reduction mechanisms to reduce speed. However, Shinsei Corporation produces 30 and 60 mm diameter piezoelectric or ultrasonic motors for testing and product applications. Other Japanese piezo motor manufacturers have incorporated 60 mm diameter motors in automobile seat and steering column adjustments while some have been used in window blind motorized mechanisms. These applications all require high torque, low speed outputs within a small volume.
Honeywell Federal Manufacturing and Technologies (H-FM&T), Kansas City, Mo., has developed and marketed a 17 mm diameter motor for military testing, evaluation, and applications. More recently, H-FM&T has developed jointly with Sandia National Laboratories, Albuquerque, N. Mex., an 8 mm diameter rotary piezo motor. Early evaluation of this 8 mm motor showed that the drive frequency required to operate the motor at its highest output torque and speed is the most critical variable associated with its operation. Experimental evaluation showed that many other variables also effect the operating frequency. Most importantly for many applications, temperature has the largest effect on the operating frequency. Since the operating frequency shifts as a function of temperature, the drive electronics design must be able to shift the drive frequency to maintain optimum performance from the motor.
8 mm DIAMETER ROTARY PIEZOELECTRIC MOTOR
FIG. 1
shows an exploded view of the H-FM&T 8 mm diameter rotary motor
10
which consists of a piezo ring
12
that is bonded to a circular stator
14
. The other side of the piezo element is metallized with two half circle metal contacts (not shown) electrically isolated from each other. The piezo element is permanently poled in alternating sections around the ring circumference prior to bonding to the stator. The piezo material sandwiched between stator
14
and each metallized contact of ring
12
can be thought of as two electrically independent piezo elements.
Stator
14
is one continuous piece of metal in the shape of a wheel about 8 mm in diameter, thicker around the outside circumference
15
. Radial cuts are made in this thicker part
15
of stator
14
opposite bonded piezo elements on ring
12
prior to bonding the elements. The radial cuts have the appearance of teeth
16
radially cut along the motor circumference. It is the action of these stator teeth- that produce motion and cause a piezo motor to operate.
Electrical contacts
17
-
19
at one end of a ribbon cable
20
are connected at the other end of cable
20
, respectively, to each element of ring
12
and to stator
14
. When an ac voltage is applied between one element's electrical contact and the stator, the poled ceramic piezo effect causes a standing wave to develop around the stator circumference. Applying two 90° out phase signals to the two contacts creates two standing waves 90° out phase which is the equivalent of a traveling wave around the motor circumference. For the 8 mm motor
10
, the traveling wave is three-wave, or it has three minima and three maxima while traveling around the stator circumference. This is the desired mode although other vibrational modes exist.
The traveling wave imparts an elliptical motion on the top surface of each tooth
16
from the traveling wave maxima and minima passing along the stator circumference. The top surface of a stator tooth
16
at a traveling wave peak has a maximum vertical displacement and its direction of travel is parallel to the top surface of the stator. Something is placed in contact with the top surface of tooth
16
will be moved in the same direction as the tooth. Typically in a rotary motor a rotor is pressed against the stator and the teeth move the rotor.
The rotor
22
shown in
FIG. 1
has machined gear teeth
24
around its circumference. The rotor
22
is the part of motor
10
that turns and causes mechanical action in an application. The rotor
22
is held in place with a shaft
24
, bearing
27
, and spring washer
28
. When motor
10
is assembled, the spring
28
forces a surface of rotor
22
onto the stator teeth
16
. When the traveling wave is induced in the stator
14
, the stator teeth
16
move in the elliptical motion described above and they push the rotor
22
along by making contact between the stator and rotor at three locations corresponding to the three maxima of the traveling wave. To reduce wear on the metal surfaces, a friction liner
26
is placed between the stator teeth
16
and the rotor
22
.
The amount of force that spring
28
applies to hold the rotor on the stator is called the motor preload. When the preload applied is very low, the motor will have a high rpm and very low torque. When the preload is very high the motor will have low rpm and high torque, and may not turn at all. Somewhere between these two values of preload is a point where the motor has optimum speed and torque.
A data point on the torque-speed curve represents the preload setting for the motor optimum performance, which is also a function of operating frequency. The data point is determined experimentally by setting the preload, sweeping through a range of frequencies, and measuring torque and speed. Next the preload is increased and the curve is generated again. This procedure is repeated until the optimum torque-speed is found at a particular preload and at a specific frequency, f
1
.
The peak motor performance is also effected by the ac voltage magnitude applied to the two piezo elements. At lower voltages the stator teeth
16
have smaller elliptical motion and therefore less rotor movement, and at higher drive voltages the piezo element can break down electrically or be stressed to the point of fracture. The peak motor performance data point is dependant on the motor preload, the drive voltage amplitude, and the drive voltage frequency.
With all of these variables taken into account, one can empirically determine the operating frequency the 8 mm rotary piezo electric motor requires for its peak performance. However, because of manufacturing variables, it is difficult to make motors with identical, predictable, performance. Even with the best fabrication process on a group of motors processed at one time: 1) all the piezo elements will be somewhat different in thickness, shape, poling, etc. due to tolerances and material characteristics 2) all the springs will not have the same force for the same deflection, 3) all the piezo elements will not be bonded in the precisely the same location on all the stators, 4) all the stators will not be machined in precisely the same shape, 5) all the bearings may not have the same friction properties, etc. All of these imperfections and tolerance variations in fabrication can and do have an effect on the motor performance and also the motor operating frequency.
When one factors in variations in performance that also may result from changes in temperature and voltage, it becomes obvious that because f
1
varies both with time in a particular motor and across a sample of motors, a constant frequency motor drive circuit is not an optimal solution for operating these motors. Another method of controlling the operating frequency must be implemented that is automatic for all variables mentioned above.
FIG. 2
shows the equivalent circuit for the 8 mm rotary piezo electric motor
10
. It is made up of two one-element circuits with a common node. The com contact in
FIG. 2
is the common connection between the two piezo elements and is also the connection to the stator web. The sin and cos inputs are the metallized surfaces of the two half circle piezo elements. The three conductor cable
20
in the motor exploded view in
FIG. 1
allows access to the sin, cos, an com nodes and is used to apply the drive voltages to the pi
Dougherty Thomas M.
Libman George H
Sandia Corporation
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