TWI452819B - The stator of the ultrasonic motor and the ultrasonic motor using the stator - Google Patents

Description

Ultrasonic motor stator and ultrasonic motor using the same

The present invention relates to a stator, and more particularly to a stator of an ultrasonic motor and an ultrasonic motor using the same.

With the development of industrial technology, ultra-precision positioning systems are becoming more and more widely used in high-precision industrial fields such as semiconductor industry, optical communication industry, ultra-precision processing, etc., among them, micro-position stage ), Inchworm motors, and Ultrasonic motors are more common.

The micro-positioning platform is made up of a piezoelectric material with a flexure structure, which uses a voltage to cause a slight deformation of the piezoelectric material, and then cooperates with the elastic deformation of the flexible structure to achieve high-precision positioning, but the micro-positioning platform The short stroke is inconvenient in application; while the ruler motor uses the applied voltage to make the piezoelectric material expand and contract, so as to carry out long-distance movement, and it is inconvenient to improve the short positioning of the micro-positioning platform. However, the ruler motor The movement speed is slow, which makes the working time increase.

The ultrasonic motor is precisely positioned by the resonant frequency of the piezoelectric material, and a high-resolution displacement can be obtained when a low-frequency voltage is applied, and a high moving speed can be obtained when a high-frequency voltage is applied.

Referring to Figures 1 and 2, the inventors previously applied for a new patent of the "Supersonic Motor" of the Republic of China Announcement No. M286995, comprising a motor base 11, an elliptical stator 12, a supporting pre-compression mechanism 13, and an object 14 . The elliptical stator 12 includes an elliptical elastic body 121, two piezoelectric actuating elements 122 correspondingly disposed in the elliptical elastic body 121, and a fixing disposed between the piezoelectric actuating elements 122. Column 123. The piezoelectric actuator element 122 is subjected to a periodic micro-deformation motion by applying a voltage, thereby squeezing the elliptical elastic body 121 to deform, so that the action block 124 of the elliptical elastic body 121 generates an elliptical motion trajectory of sufficient size to The object 14 is driven.

However, the ultrasonic motor still has the following disadvantages to be improved:

1. Low strength: Since the elastic body 121 is in an elliptical ring shape, the fixing post 123 must be coupled to the support base 131 supporting the pre-compression mechanism 13, but the elastic body 121 is not in contact with the support base 131. The structural strength of the elastic body 121 is low, and deformation is easily caused to affect the accuracy of driving the object 14.

2. Complicated structure: Since the elastic body 121 of the elliptical stator 12 is in an elliptical ring shape, the support base 131 must be additionally used to position the elliptical stator 12 on the motor base 11, and therefore, The manufacturing assembly is more complicated and cumbersome, and the manufacturing assembly cost is relatively increased.

3. Poor accuracy: Since the above is complicated and complicated in manufacturing and assembly, assembly errors are relatively easy to occur, and the strength of the elastic body 121 is low as described above, so that the accuracy of the ultrasonic motor is poor.

Accordingly, it is an object of the present invention to provide a stator of an ultrasonic motor having a high strength and a simple structure.

Another object of the present invention is to provide an ultrasonic motor which has high strength, simple structure and high precision.

Therefore, the stator of the ultrasonic motor of the present invention comprises a fixing base and a driving unit integrally connected to the fixing base.

The driving unit includes a driving mechanism integrally formed with the fixing seat and having a substantially semi-elliptical ring shape, and two piezoelectric actuators disposed between the fixing seat and the driving mechanism, wherein each piezoelectric device The actuator has a first end that abuts against the drive mechanism and a second end that abuts against the first end against the fixed seat.

The ultrasonic motor of the present invention comprises a base, a passive member movably disposed on the base, at least one stator detachably disposed on the base, and a pushing unit pushed against the stator.

The stator includes a fixing base detachably disposed on the base, and a driving unit integrally connected with the fixing base, wherein the driving unit includes a semi-elliptical ring integrally formed with the fixing base a driving mechanism, and two piezoelectric actuators disposed between the fixing base and the driving mechanism, wherein each piezoelectric actuator has a first end that abuts against the driving mechanism, and a Conversely opposite the first end and abutting against the second end of the mount.

The pushing unit includes a fixing block detachably disposed on the base, and a resilient pushing mechanism movably inserted into the fixing block and pushed against the fixing seat. The piezoelectric actuator energizes the drive mechanism to deform, and drives the passive member to move relative to the fixed seat.

The invention has the advantages that the design of the driving mechanism of the semi-elliptical ring shape and the fixing base can improve the strength of the driving mechanism, avoid deformation due to external force, and simplify the structure of the stator. In order to reduce the complexity and cost of manufacturing assembly, and reduce the assembly error, thereby improving the accuracy of the driving mechanism to drive the passive component.

The above and other technical contents, features and advantages of the present invention will be apparent from the following detailed description of FIG.

Before the present invention is described in detail, it is noted that in the following description, similar elements are denoted by the same reference numerals.

Referring to FIG. 3, a preferred embodiment of the stator 2 of the ultrasonic motor of the present invention includes a mounting base 21 and a drive unit 22 integrally coupled to the mounting base 21.

The fixing base 21 has a mounting portion 211 and a positioning portion 212 protruding outward from the mounting portion 211, so that the fixing base 21 has a convex shape as a whole.

The driving unit 22 includes a driving mechanism 221 integrally formed with the fixing base 21 and having a substantially semi-elliptical ring shape, and two piezoelectric actuators 227 interposed between the fixing base 21 and the driving mechanism 221 . .

The fixing base 21 and the driving mechanism 221 may be integrally formed of a metal material such as aluminum alloy or stainless steel. In the preferred embodiment, the fixing base 21 and the driving mechanism 221 are integrally formed of an aluminum alloy. to make.

The two ends of the driving mechanism 221 are integrally connected to the mounting portion 211, and the driving mechanism 221 has an inner annular surface 222 and an outer annular surface 223 spaced apart from each other, and a protruding portion of the outer annular surface 223 Contact 224. Moreover, the contact member 224 has a connecting portion 225 integrally connected to the outer ring surface 223, and a wear portion 226 disposed on the connecting portion 225. Of course, the contact 224 may not be needed in practical applications, and the same effect can still be achieved.

Referring to FIG. 5, each of the piezoelectric actuators 227 has a first end 228 that abuts against the inner annular surface 222 of the driving mechanism 221, and a top end 228 opposite to the fixed end. The second end 229 on the positioning portion 212 of FIG.

Referring to FIG. 3, since the semi-elliptical annular driving mechanism 221 and the fixing base 21 are integrally formed, the strength of the driving mechanism 221 of the stator 2 can be improved, and deformation due to external force can be avoided. The overall structure of the stator 2 can be simplified to reduce the complexity and cost of manufacturing assembly and reduce the occurrence of assembly errors.

Referring to FIG. 4, a first preferred embodiment of the ultrasonic motor of the present invention includes a base 3, a passive member 4 movably disposed on the base 3, and a detachably disposed on the base 3. The stator 2, and a pushing unit 5 that pushes against the stator 2. In the preferred embodiment, the passive member 4 is a platform that is linearly movable relative to the stator 2.

Since the structure of the stator 2 of the present embodiment, that is, the preferred embodiment of the stator 2 in FIG. 3, is not described herein, wherein the mounting portion 211 of the fixing base 21 of the stator 2 is detachably disposed therein. The locating portion 212 protrudes from the mounting portion 211 toward the passive member 4, and the contact member 224 is in contact with the side edge of the passive member 4.

The pushing unit 5 includes a fixing block 51 detachably disposed on the base 3, and a resilient pushing mechanism 52 movably inserted into the fixing block 51 and pushed against the fixing seat 21, wherein The elastic pushing mechanism 52 has an outer cylinder 521 which is spirally inserted into the fixing block 51, a push rod 522 which is inserted into the outer cylinder 521 and is pushed up to the mounting portion 211 of the fixing base 21, and a push rod 522 The two ends are respectively abutted against the outer tube 521 and the compression spring 523 on the push rod 522.

Since the semi-elliptical annular driving mechanism 221 and the fixing base 21 are integrally formed, the ultrasonic motor including the stator 2 can also achieve the aforementioned complexity and cost of manufacturing assembly and reduce assembly error. The efficiency is also improved, and since the assembly error is small and the strength of the stator 2 is high, the positioning accuracy of the passive member 4 of the ultrasonic motor can be effectively improved.

When the piezoelectric actuator 227 is energized, the driving mechanism 221 can be deformed, and the passive member 4 is driven to move relative to the fixing base 21 of the stator 2. Hereinafter, the operation of driving the ultrasonic motor will be described. Referring to Figures 5, 6, and 7, the semi-elliptical annular drive mechanism 221 has a normal vibration mode and a tangential vibration mode perpendicular to each other as shown in Figures 5 and 6 (the imaginary line is the drive mechanism 221). Original location). Moreover, as can be seen from FIG. 7, the normal vibration frequency of the semi-elliptical annular driving mechanism 221 is similar to the natural frequency of the tangential vibration mode. Therefore, the piezoelectric actuator 227 can be applied with the same frequency and different phases. The driving voltage is applied while exciting the normal and tangential vibrations of the drive mechanism 221 to drive the contact 224 to operate.

Referring to FIG. 8 and reviewing FIG. 3, and the inventors have found that when the width and thickness of the driving mechanism 221 are fixed, the natural frequency of the tangential and normal vibration dynamics is as shown in FIG. The ratio of the lengths of the long and short axes L1 and L2 of the ring-shaped driving mechanism 221 changes, wherein the natural frequency of the tangential vibration mode rapidly rises as the ratio of the long and short axes L1 and L2 increases, and the normal vibration mode The natural frequency is reduced, and the natural frequency of the tangential vibration mode is the closest to the normal vibration mode when the ratio of the long and short axes L1 and L2 is about 2.0.

In order to enable the driving voltage of the driving piezoelectric actuator 227 to simultaneously excite the tangential vibration mode and the normal vibration mode of the driving mechanism 221 to perform one periodic vibration motion in one cycle, the driving mechanism 221 must The tangential vibration mode and the normal vibration mode have the same natural frequency. Therefore, the ratio of the long axis L1 to the short axis L2 of the semi-elliptical annular drive mechanism 221 is between 1.9 and 2.1. In the preferred embodiment, the ratio of the major axis L1 to the minor axis L2 is two.

Referring to FIGS. 9 and 10, when a sinusoidal driving voltage having the same frequency as shown in FIG. 9 but having a phase difference of 90° is applied to the piezoelectric actuator 227, the contact member 224 of the driving mechanism 221 is caused. Correspondingly, an elliptical motion trajectory as shown in FIG. 10 is generated. In order to facilitate the comparison between the driving voltage and the elliptical motion trajectory of the contact member 224 of the driving mechanism 221, the voltage waveform of FIG. 9 is divided into one cycle, A, B, C, D, E, F, G. The time point of H, etc. is explained, and the imaginary line in FIG. 10 is the starting position of the drive mechanism 221 when the driving voltage is not applied to the piezoelectric actuator 227.

At point A, the piezoelectric actuator 227 on the right side of FIG. 10(A) is shortened by a voltage of -10 volts, and the piezoelectric actuator 227 on the left side is not deformed by the voltage, so that the contact 224 is as shown in FIG. 10(A) is slightly deformed to the upper left; at point B, the piezoelectric actuator 227 on the right side of FIG. 10(B) is shortened by a voltage of -5 √ 2 volts, and the piezoelectric actuator 227 on the left side is subjected to +5√2 volts is elongated, so that the contact 224 is deformed to the left as shown in Fig. 10(B); when it is point C, the piezoelectric actuator 227 on the right side of Fig. 10(C) is not subjected to voltage Deformation, while the piezoelectric actuator 227 on the left side is elongated by a voltage of +10 volts, so that the contact 224 is slightly deformed to the lower left as shown in Fig. 10(C); when it is point D, the pressure on the right side of Fig. 10(D) Both the electric actuator 227 and the piezoelectric actuator 227 on the left side are elongated by a voltage of +5 √ 2 volts, so that the contact 224 is deformed downward as shown in Fig. 10(D); when it is point E, Fig. 10 ( E) The piezoelectric actuator 227 on the right side is elongated by a voltage of +10 volts, and the piezoelectric actuator 227 on the left side is not deformed by the voltage, so that the contact 224 is slightly rightward as shown in Fig. 10(E). Deformation below; at point F The piezoelectric actuator 227 on the right side of Fig. 10(F) is elongated by a voltage of +5 √ 2 volts, and the piezoelectric actuator 227 on the left side is shortened by a voltage of -5 √ 2 volts, so that the contact 224 is as shown in Fig. 10. (F) is deformed to the right; at the G point, the piezoelectric actuator 227 on the right side of FIG. 10(G) is not subjected to voltage without deformation, and the piezoelectric actuator 227 on the left side is subjected to -10 volts. The shortening is such that the contact member 224 is deformed to the upper right as shown in FIG. 10(G); at the H point, the piezoelectric actuator 227 on the right side of FIG. 10(H) and the piezoelectric actuator 227 on the left side are both subjected to - The voltage of 5 √ 2 volts is shortened, so that the contact 224 is deformed upward as shown in FIG. 10(H), and after periodic changes of A, B, C, D, E, F, G, and H, the Contact 224 forms an elliptical motion trajectory. Specifically, FIG. 9 and FIG. 10 show that the driving voltage of the piezoelectric actuator 227 on the left side leads the driving voltage of the piezoelectric actuator 227 on the right side by a phase angle of 90°. Therefore, the contact 224 is used. The elliptical motion trajectory is operated in a counterclockwise direction. If the driving voltage of the piezoelectric actuator 227 on the right side leads the driving voltage of the piezoelectric actuator 227 on the left side by a phase angle of 90°, the elliptical shape of the contact member 224 The motion trajectory is operated in a clockwise direction. Therefore, the elliptical motion trajectory of the contact member 224 can be controlled by changing the phase difference between the driving voltages of the piezoelectric actuator 227 on the left side and the piezoelectric actuator 227 on the right side. Further, as shown in FIG. 11 , the contact member 224 is driven to drive the passive member 4 to reciprocate relative to the fixed seat 211 as indicated by an arrow in FIG. 11 , and the elastic restoring force of the compression spring 523 of the pushing unit 5 is used to push the push rod. 522 (shown in FIG. 4) enables the contact member 224 of the drive mechanism 221 to reliably contact the passive member 4, and the wear portion 226 of the contact member 224 can reduce contact wear.

In addition to the two-phase high-frequency driving voltage applied to the piezoelectric actuator 227, as shown in FIG. 10, the contact member 224 may have an elliptical motion trajectory, as shown in FIG. 12 or FIG. The same effect can be achieved by applying a single-phase high-frequency driving voltage to one of the piezoelectric actuators 227.

When a sine wave driving voltage is applied to the piezoelectric actuator 227 on the right side of FIG. 12, the piezoelectric actuator 227 on the right side can be periodically stretched and contracted, so that the contact member 224 is as shown in FIG. An elliptical motion locus of an oblique angle indicated by an arrow; when a sine wave driving voltage is applied to the piezoelectric actuator 227 on the left side of FIG. 13, the piezoelectric actuator 227 on the left side can be periodically stretched and contracted. The contact 224 is rendered in an elliptical motion trajectory at an oblique angle as indicated by the arrow in FIG.

Applying the two-phase high-frequency driving voltage as shown in FIG. 10 to the piezoelectric actuator 227 or the single-phase high-frequency driving voltage shown in FIGS. 12 and 13 can generate a higher driving speed. When higher positioning accuracy is required, the low frequency driving method as shown in Figs.

Referring to Figures 14 and 15, when a low frequency driving voltage as shown in Fig. 14 is applied to the piezoelectric actuator 227, the contact 224 of the driving mechanism 221 is caused to correspondingly generate the motion as shown in Fig. 15. Track. When the position of the (a) point of FIG. 14 is that the piezoelectric actuator 227 is subjected to a driving voltage of zero, the contact member 224 of the driving mechanism 221 is not deformed as shown in FIG. 15(a); With the position from the (a) point to the (b) point of FIG. 14, the piezoelectric actuator 227 receives the same negative driving voltage, and generates the same amount of deformation, so that the contact 224 of the driving mechanism 221 is as 15(b) is outwardly convex; and when from the point (b) to the position (c) of FIG. 14, the left piezoelectric actuator 227 in FIG. 15(b) continues to receive a negative driving voltage. The negative driving voltage of the right piezoelectric actuator 227 in FIG. 15(b) is gradually reduced to 0, so that the contact member 224 of the driving mechanism 221 protrudes upward to the right as shown in FIG. 15(c), and the driving diagram is pushed. The passive member 4 in 11 moves to the right for a small step; finally, from the position (c) of Fig. 14 to the position of (a), the negative drive of the left piezoelectric actuator 227 in Fig. 15(c). The voltage is gradually reduced to 0, and the right piezoelectric actuator 227 in Fig. 15(c) maintains a voltage of 0, causing the contact 224 of the drive mechanism 221 to return to the position shown in Fig. 15(a). After the timing change of the driving voltages of (a), (b), (c), and (a) of FIG. 14, the passive member 4 can be moved at a low moving speed and high displacement resolution like a ruler motor. .

Referring to Figure 16, the second preferred embodiment of the ultrasonic motor of the present invention is substantially the same as the first preferred embodiment, except that the passive member 4 is a rotor rotatable relative to the stator 2 due to The preferred embodiment is substantially the same as the first preferred embodiment, and therefore, in addition to the effect of the first preferred embodiment, the two-phase high frequency drive is applied to the piezoelectric actuator 227 as described above. The voltage, the single-phase high-frequency driving voltage, or the low-frequency driving voltage can drive the passive member 4 to rotate according to the direction of the arrow in FIG. 16 by the trajectory of the contact member 224 of the driving mechanism 221 .

Referring to Figure 17, a third preferred embodiment of the ultrasonic motor of the present invention is substantially the same as the first preferred embodiment. The difference is that the ultrasonic motor includes four stators 2, and the passive member 4 is a The rotating ring surrounding the stator 2 can be utilized by applying a two-phase high-frequency driving voltage, a single-phase high-frequency driving voltage, or a low-frequency driving voltage to the piezoelectric actuator 227 of the stator 2 as described above. The trajectory of the contact 224 of the drive mechanism 221 drives the rotor to rotate in accordance with the direction of the arrow in FIG.

In summary, the stator 2 of the ultrasonic motor of the present invention and the ultrasonic motor using the stator 2 can be improved by the design of the semi-elliptical annular drive mechanism 221 and the fixed seat 21 The strength of the mechanism 221 avoids deformation due to external force, and the structure of the stator 2 can be simplified, thereby reducing the complexity and cost of manufacturing assembly and reducing the assembly error, thereby improving the accuracy of the driving mechanism 221 driving the passive component 4. Therefore, the object of the present invention can be achieved.

The above is only the preferred embodiment of the present invention, and the scope of the invention is not limited thereto, that is, the simple equivalent changes and modifications made by the scope of the invention and the description of the invention are All remain within the scope of the invention patent.

2. . . stator

twenty one. . . Fixed seat

211. . . Installation department

212. . . Positioning department

twenty two. . . Drive unit

221. . . Drive mechanism

222. . . Inner annulus

223. . . Outer torus

224. . . Contact

225. . . Connection

226. . . Wear part

227. . . Piezoelectric actuator

228. . . First end

229. . . Second end

3. . . Pedestal

4. . . Passive part

5. . . Push unit

51. . . Fixed block

52. . . Elastic push mechanism

521. . . Outer tube

522. . . Putt

523. . . compressed spring

L1. . . Long axis

L2. . . Short axis

Figure 1 is an exploded perspective view showing the new patent of the "Supersonic Motor" of the Republic of China Announcement No. M286995;

Figure 2 is a schematic view, assistance to illustrate Figure 1;

Figure 3 is a perspective view showing a preferred embodiment of the stator of the ultrasonic motor of the present invention;

Figure 4 is a perspective view showing a first preferred embodiment of the ultrasonic motor of the present invention;

Figure 5 is a schematic view showing the normal vibration mode of the stator;

Figure 6 is a schematic view showing the tangential vibration mode of the stator;

Figure 7 is a response diagram illustrating the relationship between the displacement of the stator operating point and the natural frequency;

Figure 8 is a graph illustrating the relationship between the natural frequency of the normal vibration mode and the tangential vibration mode of the stator and the ratio of the long and short axes of the drive structure;

Figure 9 is a schematic view showing the sequence of deformation trajectories of the stator when subjected to a two-phase high-frequency driving voltage;

Figure 10 is a schematic view showing the sequence of operations when the stator is subjected to a biphasic high frequency driving voltage;

Figure 11 is a schematic view showing the equivalent structure of the ultrasonic motor;

Figure 12 is a schematic view showing the sequence of operations when one of the piezoelectric actuators of the stator is subjected to a single-phase high-frequency driving voltage;

Figure 13 is a schematic view showing the sequence of operations when another piezoelectric actuator of the stator is subjected to a single-phase high-frequency driving voltage;

Figure 14 is a signal diagram showing the mode of a low frequency driving voltage applied to the piezoelectric actuator of the stator;

Figure 15 is a schematic view showing the sequence of operations when the stator is subjected to a low frequency driving voltage;

Figure 16 is a schematic view showing a second preferred embodiment of the ultrasonic motor of the present invention; and

Figure 17 is a schematic view showing a third preferred embodiment of the ultrasonic motor of the present invention.

2. . . stator

twenty one. . . Fixed seat

211. . . Installation department

212. . . Positioning department

twenty two. . . Drive unit

221. . . Drive mechanism

222. . . Inner annulus

223. . . Outer torus

224. . . Contact

225. . . Connection

226. . . Wear part

227. . . Piezoelectric actuator

228. . . First end

229. . . Second end

L1. . . Long axis

L2. . . Short axis

Claims (9)

A stator of an ultrasonic motor comprising: a fixing base; and a driving unit comprising a driving mechanism integrally formed with the fixing seat and having a substantially semi-elliptical ring shape, and two spacers disposed on the fixing base and the driving mechanism Piezoelectric actuators, wherein each piezoelectric actuator has a first end that abuts against the drive mechanism, and a second opposite the first end that abuts against the mount end. The stator of the ultrasonic motor according to the first aspect of the invention, wherein the fixing base has a mounting portion and a positioning portion protruding outward from the mounting portion, and the two ends of the driving mechanism are integrally connected On the mounting portion, each of the first ends of the piezoelectric actuators is abutted against the driving mechanism, and each of the second ends is abutted against the positioning portion. The stator of the ultrasonic motor according to the second aspect of the invention, wherein the driving mechanism has an inner annular surface and an outer annular surface, and a contact protruding from the outer annular surface, wherein Each of the first ends of the piezoelectric actuators is abutted against an inner annular surface of the drive mechanism. The stator of the ultrasonic motor according to claim 3, wherein the contact member of the driving mechanism has a connecting portion integrally connected to the outer ring surface, and a wear portion disposed on the connecting portion . The stator of the ultrasonic motor according to any one of claims 1 to 4, wherein the ratio of the major axis to the minor axis of the substantially semi-elliptical ring drive mechanism is between 1.9 and 2.1. An ultrasonic motor comprising: a base; a passive member movably disposed on the base; at least a stator, including a fixed seat detachably disposed on the base, and a fixed seat An integrated driving unit, wherein the driving unit comprises a driving mechanism integrally formed with the fixing seat and having a substantially semi-elliptical ring shape, and two piezoelectric plates spaced apart between the fixing base and the driving mechanism The piezoelectric actuator has a first end abutting against the driving mechanism, and a second end opposite to the first end against the fixing seat; and a pushing unit The utility model comprises a fixing block detachably disposed on the base, and an elastic pushing mechanism movably inserted in the fixing block and pushing against the fixing seat; the piezoelectric actuator can be energized The drive mechanism is deformed to drive the passive member to move relative to the fixed seat. The ultrasonic motor according to claim 6, wherein the fixing base has a mounting portion detachably disposed on the base, and a positioning portion protruding from the mounting portion toward the passive member And the two ends of the driving mechanism are integrally connected to the mounting portion and have an inner annular surface and an outer annular surface, and a protruding from the outer annular surface and contacting the passive member And a contact member, each of the first ends of the piezoelectric actuator is abutted against an inner annular surface of the driving member, and each of the second ends is abutted against the positioning portion. The ultrasonic motor according to claim 7, wherein the ratio of the major axis to the minor axis of the semi-elliptical ring-shaped driving mechanism is between 1.9 and 2.1, and the contact of the driving mechanism The utility model has a connecting portion integrally connected to the outer ring surface, and a wear portion disposed on the connecting portion and contacting the passive member. The ultrasonic motor of claim 8, wherein the elastic pushing mechanism of the pushing unit has an outer cylinder that is threaded into the fixing block, is placed in the outer cylinder, and is pushed into the outer cylinder. The push rod of the mounting portion of the fixing seat and a pair of two ends respectively abut against the outer tube and the compression spring on the push rod.