Patent Application
20050068989
The present invention relates to microelectromechanical systems (MEMS) and in particular, to a MEMS device used to measure strain.
Conventional strain gauges are made of wire or foil (metal) or a semiconductor material that is mounted to the surface of the object. As the underlying object is strained, the strain is transferred to the strain gauge material and a change in electrical resistance is exhibited proportional to the deformation (i.e. strain) of the strain gauge material. In addition to resistance, capacitance, inductance or reluctance may also be used to establish a measure of strain. Some semiconductor strain gauges are made of a piezoresistive material such as silicon. As the piezoresistive material is stressed, the fundamental materials properties of the material change causing the resistance to change. This change in electrical resistance is measured to provide an indication of the stress in the piezoresistive material and therefore also an indication of the strain in the underlying object.
MEMS devices are extremely small machines fabricated using integrated circuit techniques or the like. The small size of MEMS devices makes possible the mass-production of high-speed, low-power, high sensitivity, and high-reliability mechanisms that could not be realized on a larger scale.
It would thus be desirable to provide a MEMS device that can be mounted onto an underlying object that is operable to provide an indication of strain experienced at the object. It would furthermore be desirable to measure the strain at a plurality of locations over an area.
In one aspect of the invention, a microelectromechanical system (MEMS) strain gauge providing measurement of strain of an object is provided. The strain gauge includes a substrate having a surface that is attachable to the object. The strain gauge further includes a longitudinally extending beam. At least one flexible arm has first and second ends attached to the substrate, and also has a middle portion supporting the beam above the substrate. A detector communicates with the beam for detecting a frequency of vibration of the beam to provide a measure of strain of the object.
The above aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, and not limitation, a preferred embodiment of the invention. Such embodiment does not define the scope of the invention and reference must be made therefore to the claims for this purpose.
Referring now to
Pylon pairs 34 and 36 support the ends of the flexible arms 28 and 30, respectively, extending upward from the substrate 12 with the flexible arms 28 and 30 directly attached at their tops or attached to the pylon pairs 34 and 36 through temperature compensating structures as is described in U.S. Pat. No. 6,617,750 issued Sep. 9, 2003, assigned to the assignee of the present invention and hereby incorporated by reference. Temperature compensating structures reduce the expansion and contraction that can be induced in the substrate 12 resulting from temperature fluctuations. While the temperature compensating structure would thus reduce the sensitivity of strain gauge 10, it may sometimes be desirable to minimize temperature-induced strain. Alternatively, the sensitivity of strain gauge 10 can be increased, and the strain gauge output can include temperature-induced strain, by connecting pylon pairs 34 and 36 to the substrate 12 without implementing temperature compensating structures.
The flexible arms 28 and 30 enable the beam 24 to translate along a longitudinal axis 40 perpendicular to the strain axis 22 that extends generally parallel to the flexible arms 28 and 30.
The attachment of the flexible arms 28 and 30 to the substrate 12 causes the flexible arms 28 and 30 to be stretched with any strain of the substrate 12 along strain axis 22. If it is anticipated that it will be desirable to measure compression (i.e. negative strain) of the underlying object, then it will be necessary to pre-stress the flexible arms during fabrication, so that the compression will result in a reduced stress in the arms. The amount of total strain in the substrate controls the amount of stress in the arms. The beam 24 thus forms a spring mass oscillator with the flexible arms 28 and 30 whose natural frequency of oscillation will depend according to well-known formulas in part on the stress of the flexible arms 28 and 30. The stress in the arms will change the stiffness, i.e. the spring constant. The value of the natural frequency, fn, is given by:
fn=(k/m)1/2 (1)
where k is the spring constant of the structure and m is the mass of the structure. It should be noted that the resonant frequency of the system, fn, observed experimentally is slightly lower than the natural frequency, given in Eq. 1, and is related to fn by:
fr=fn(1½Q2)1/2 (2)
where Q is the quality factor of the second order mechanical system.
For this reason, the resonant frequency of oscillation of beam 24 will depend on the strain of substrate 12. Accordingly, when substrate 12 is attached to an underlying object, the stress passed to the flexible arms will reflect the strain in the underlying object.
The beam 24 and flexible arms 28 and 30 may be constructed, for example, of silicon etched from the substrate using techniques described in U.S. patent applications Ser. No. 09/843,563 filed Apr. 26, 2001 and Ser. No. 09/963,936 filed Sep. 26, 2001 and Ser. No. 10/002,725 filed Oct. 25, 2001, assigned to the assignee of the present invention and hereby incorporated by reference. If desired, an optional insulating bridge 33 of silicon dioxide, or other insulating material, may be formed into the beam 24 dividing the beam 24 into two ends, each communicating with only one of the flexible arms 28 and 30. An upper metallization layer may be applied to the beam 24 and flexible arms 28 and 30, and the pylons 34 and 36. If insulating bridge 33 is implemented, the upper metallization layer would not be applied to the bridge in order to provide electrical isolation to each of the divided segments of the beam 24.
Referring still to
Attached to a rightmost end of the beam 24 may be a detector 52 comprised of similar movable capacitor plates 56 and opposing stationary capacitor plates 54, the former attached to either side of the beam 24 at its rightmost end and the latter attached to pylons 58 and 62 flanking the beam 24 and attached in turn to the substrate 12. Again, the opposed plates may have interdigitated fingers 60 to increase the capacitance surface area so as to linearize their operation with movement of the beam 24. If beam 24 includes insulating portion 33, the insulator would isolate detector 52 from voltages originating at actuator 42 that might otherwise travel along beam 24.
Unlike the capacitor plates 48 and 44, the movable capacitor plate 56 extending on the upper side of the beam 24 is to the right of its corresponding stationary capacitor plate 54 while the movable capacitor plate 56 extending on the lower side of the beam 24 is to the left of its corresponding stationary capacitor plate 54. It will be understood, therefore, that capacitances formed between these plates 54 and 56 on different sides change in opposite directions with motion 26 of the beam 24. In particular, a leftmost movement of beam 24 decreases the capacitance formed by the upper plates 54 and 56 and increases the capacitance formed by the lower plates 54 and 56, and vice versa with rightward movement of beam 24. Connection to the capacitors formed by the upper capacitor plates may be made through terminals formed by pylon 58 and pylon 36, while connections to the capacitors formed by the lower capacitor plates may be made through terminals formed by pylon 62 and pylon 36. The relative value of these capacitors may be measured by a detector incorporating capacitance measurement circuitry disclosed in U.S. Pat. No. 6,501,282 issued Dec. 31, 2002 hereby incorporated by reference to provide an indication of the position of the beam 24.
Referring now also to
In particular, a set of movable capacitor plates 44′ is attached to beam 24 between transverse actuator 42 and detector 52 extending transversely out therefrom on either side. The capacitor plates 44′ present longitudinal interdigitating fingers 46′ to an opposed pair of stationary capacitor plates 48′ having corresponding interdigitating fingers 46′ and positioned adjacent to the left of the movable capacitor plates 44′. The stationary plates 48′ in turn connect the pylons 50′ attached to the substrate. The capacitor plates 44′ and 48′ are metallized and electrical connection to each of plates 44′ and 48′ may be had via pylons 34 and 50′, respectively. A voltage placed across pylons 50 and 34 thus produces an attractive electrostatic force across the plates 44 and 48 and thus a longitudinal force on beam 24, as described above. It should be appreciated that capacitor plates 44′ and 48′ are oriented opposite to capacitor plates 44 and 48 of actuator 42. Accordingly, a voltage placed across pylons 50′ and 34, actuator 42′ provides an electrostatic force in a direction opposite to that of actuator 42.
Actuators 42 and 42′ thus can operate in tandem to translate beam 24 in both longitudinal directions. It may be desirable, for instance, to initially calibrate MEMS device 10 and position the upper and lower sets of capacitor plates 54 and 56 equally, thereby maximizing the range of beam movement during operation. The calibration is achieved by applying a voltage to pylons 34, and further selectively applying a voltage to either pylons 50 or 50′, depending on the desired direction of beam movement. The beam position can be adjusted until the desired position is attained by obtaining a substantially equal capacitance formed by the upper capacitor plates 54 and 56 and the lower capacitor plates 56 and 54. During operation, only one actuator 42 would be used to vibrate beam 24.
While actuator 42′ is illustrated as being positioned between actuator 42 and detector 52 (and electrically isolated from detector 52 via insulating portion 33), it should be appreciated that actuator 42′ can alternatively be positioned anywhere on beam 24.
As noted, the structure formed by the beam 24 and flexible arms 28 and 30 has a mechanical resonant frequency that will be dependent upon the stress in the flexible arms which is indicative of the underlying strain along strain axis 22. This resonant frequency may then be measured as follows: A voltage impulse is applied to actuator 42 which results in a mechanical impulse being applied to the structure, i.e. the structure is quickly moved to one side and released. It will then vibrate back and forth at its resonant frequency and with decreasing amplitude until the motion 26 dies out. The motion 26 is monitored by detecting changes in the capacitances formed by plates 54 and 56. Such motion may also occur from naturally occurring environmental shocks to the MEMS strain gauge.
Referring now to
Microprocessor 71 may preferably includes a capacitance sense circuit that receives and senses capacitance output 66 from the detector 52, and produces a digital output 68 that can be read by other system components to ultimately provide an output to the user. These other system components may be either integrated with the MEMS device or be discrete components electrically connected to the MEMS device, or some combination thereof. Output 68 can alternatively be an analog signal throughout this disclosure. Alternatively, device 10 can include a separate capacitance sense circuit, connected between detector 52 and microprocessor 71, that receives capacitance output 66. As illustrated, the analysis of the capacitance values takes place in the microprocessor 71. Microprocessor 71 can further communicate with a memory (not shown) to store and later retrieve statistical data accumulated during operation.
It should be appreciated that microprocessor 71 can be local, as illustrated, or alternatively remote from MEMS device 10 and receive signals from detector 52 as described above. In the case where the frequency detection is simply a counter enabled for a predetermined period of time, a digital frequency value 68 is produced which may be read by the rest of the electronic system components. In accordance with the preferred embodiment, the actuator 42 provides a series of pulses whose period is such that the motion 26 of beam 24 will decay to zero before the next pulse is applied
An empirically derived calibration curve may be used to convert this frequency into an absolute strain measurement of the underlying object or relative strain may be deduced being simply a change in frequency.
Referring now to
Resonant frequency can be attained by feedback circuitry 72 sweeping a wide range of frequencies by outputting a sinusoidal waveform covering a wide frequency range, including the resonant frequency of the structure. Once the resonant frequency has been determined, feedback circuitry can sweep the sinusoidal waveform over a narrower frequency range about the previously determined resonant frequency to continuously determine the resonant frequency of the structure. The narrower sweep range would account for possible drift or shift in the resonance frequency as the structure undergoes strain.
In an additional alternative embodiment, the generator 64 can provide a continual ac (e.g., sinusoidal) driving force to the structure at a predetermined frequency. Monitoring of the amplitude of the motion 26 due to this driving force is another monitor of the stress in the flexible arms and therefore of the strain in the substrate.
Referring again to
It should be appreciated that a communication module 69 can optionally be integrated onto MEMS device 10 illustrated in
Referring now to
The present invention recognizes that MEMS device 10 can detect strain that may be a function of environmental changes, such as temperature, component drift, and other effects, as opposed to strain in the underlying device. The environmental effects can be negated by providing a second dummy MEMS strain gauge constructed similar to strain gauge 10 illustrated in accordance with any of the embodiments above. One of the pylons 34 would not be connected to the underlying substrate, and thus would not produce an output as a function of underlying strain. Rather, the output of the dummy MEMS device would reflect only environmental changes. Because the environmental changes in the dummy device would also be experienced by the real-time sensor 10, strain calculated in the dummy device can be subtracted from the strain experienced in the real-time sensor 10 to provide a measurement that is a representation of only the strain of the underlying object. The concept of a dummy device to negate environmental effects is further described in U.S. Pat. No. 6,617,750, issued on Sep. 9, 2003, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein.
Referring now to
Arm 128 further includes a plurality of capacitor fingers 153 extending to the right of arm 128 that are interdigitated with stationary capacitor fingers 154 of a fixed capacitor plate 156. Accordingly, a detector 152 can measure the capacitance formed between arm 128 and plate 156. Microprocessor 171 can then determine the strain of the underlying device by counting the number of crossings at a predetermined capacitance value over a period of time. The predetermined capacitance value can be, for example, a calibrated value when fingers 153 and 154 are disposed in a neutral position. The calculated oscillation frequency can be used in accordance with any of the methods described above to determine strain of the underlying object.
Strain gauge 110 can be further simplified by removing one of the capacitor plates 146 and 153 along with the corresponding capacitor plate 144 and 156. Accordingly, for example, only fingers 146, 148, and capacitor plate 144 would remain. Plate 144 could then function as both an actuator and detector by first applying a pulse to arm 128 in the manner described above. After the applied pulse, the capacitance between plate 144 and arm 128 can be sensed as arm 128 oscillates and the strain of the underlying device can be calculated in the manner described above until the oscillation decays to zero, at which time a pulse would again be applied to plate 144.
Referring to
A link 16 being, for example, a machined aluminum plate, may be attached to the substrate 12 by epoxy 18 or the like so that stress on the link 16 is transferred in part to the substrate 12. The link 16 may include mounting provisions, such as holes 20, aligned with a longitudinal or strain axis 22 along which stress will be exerted on the link 16 causing a corresponding strain in the link 16 and substrate 12. Alternatively, the MEMS strain gauge 10 may be mounted directly on a device to be placed under stress such as a beam, cable, wire, rope, motor stator winding, or the like.
When the link 16 is used, it may be incorporated into a device such as a load cell or fastened to another object from which strain will be obtained. The attachment may be according to methods well known in the art for attaching resistive strain gauges. Alternatively, substrate 12 could be attached directly onto the device.
The cover 14 may include indicia 15 indicating the direction of sensitivity of the internal structure of the MEMS strain gauge 10 and aligned with the strain axis 22.
Referring now to
It should furthermore be appreciated that, when strain gauge 10 is equipped with communication module 69, all strain gauges can communicate with a central processor which could either replace, or be provided in addition to, local microprocessor 71. The centralized processor can receive strain levels from each connected strain gauge 10 and plot strain levels throughout a device, and furthermore can time synchronize the strain at various locations on the device. The centralized processor could further connect to strain gauges of multiple devices.
It should further be appreciated that a plurality of MEMS strain gauges 10 can be dispersed either on one or multiple links 16, and attached to an underlying device to measure strain in multiple directions at a plurality of locations on the device. For instance, the strain gauge(s) 10 can be mounted onto various surfaces of a device that undergo strain, or alternatively can be disposed in a beam, cable, wire, rope, or the like.
The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. In order to apprise the public of the scope of the present invention, the following claims are provided.
