Abstract:
An electric field sensor comprising: a substrate having a hole; a shielding electrode and a sensing electrode, disposed in the hole of the substrate; a piezoelectric bar having one end connected to the center of the shielding electrode, the other end fixed on the substrate. Present invention provides several electric field sensors, which have the same feature of utilizing electrodes interleaving vibration to modulate external electric field. They have IC-compatible operation voltage and small volume.

Description:
FIELD OF THE INVENTION 
     This invention relates to an electric field sensor, particularly to the electric field sensor with an electrode interleaving vibration. 
     BACKGROUND OF THE INVENTION 
     Electric field sensors (EFSs) have significant importance for many applications. For example, accurate measurements made in different altitude by Electric Field Sensors provide important information in the study of weather phenomena such as thunderstorms. The EFSs are also used to monitor the electric field generated by power line. The EFSs widely used today are based on traditional mechanical technology, which have the advantages of high precision, but with large volume and high power consumption. There are also some miniature EFSs using laterally electrostatic comb-drive, which have small volume but need high driving voltage. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an electric field sensor with electrode interleaving vibration which can increase the output signal of the sensor. 
     In order to accomplish the above object, an electric field sensor comprising: 
     a substrate having a hole; 
     a shielding electrode and a sensing electrode, disposed in the hole of the substrate; 
     a piezoelectric bar having one end connected to the center of the shielding electrode, the other end fixed on the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the working principle of EFS with electrode interleaving vibration. 
         FIG. 2  shows front view and back view of an example of EFS using piezoelectric bar. 
         FIG. 3  is schematic view of the first example of EFS using thermal actuation. 
         FIG. 4  shows schematic view of the second example of EFS using thermal actuation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows the working principle of the EFS with electrode interleaving vibration.  FIG. 2  shows an electric field sensor with piezoelectric driving structure.  FIG. 3  and  FIG. 4  show an electric field sensor with thermal driving structures. As shown in  FIG. 1 , the EFS utilizes the interleaving vibration between the shielding electrode and sensing electrode to realize external electric field modulation. In a certain external electric field E, the conductive sensing electrode can induce the charges. As the grounded shielding electrode move vertically, the amount of charges on sensing electrode are varying. The amount of the charge is reduced when top surface of the shielding electrode is higher than the top surface of the sensing electrode and, the shielding electrode is moving upwards. The amount of the charge is increased while the top surface of the shielding electrode is lower than the top surface of the sensing electrode, and the shielding electrode is moving downwards. With the periodical vibration of shielding electrode, an AC output current signal i from sensing electrode is formed, which is used to measure the external electric field intensity. 
     The electric field sensor can be fabricated by Micro-electromechanical System (EMS) technology, or MEMS technology combined with precise mechanical fabrication technology. To be operated under Integrated Circuit (IC)-compatible voltage, suitable driving method may be chosen such as piezoelectric actuation or thermal actuation. Three typical EFSs are described as examples in this invention: one is the piezoelectric bar, and other ones are thermal actuators. 
       FIG. 2  shows an example of the EFS using piezoelectric bar. The EFS mainly includes shielding electrode  1 , sensing electrode  2 , piezoelectric bar  3  and substrate  7 . The substrate  7  has a rectangle hole  8 , which allows the shielding electrode  1  and the sensing electrode  2  to be placed in. The hole exposes the shielding electrode  1  and sensing electrode  2  to external electric field and provides a space for vibrating shielding electrode  1 . The sensing electrode  2  has a rectangle rim with comb fingers inside. The shielding electrode  1  is designed to have a comb shape with its comb fingers alternatively arranging with comb fingers of sensing electrode  2 . The slim piezoelectric bar  3  is utilized to actuate the shielding electrode  1 . One end of piezoelectric bar  3  is connected with the center of shielding electrode  1 , and the other end of piezoelectric bar  3  is fixed on the substrate by the colloidal material  116 . 
     The piezoelectric bar  3  has the advantage of achieving large displacement under low driving voltage. As the piezoelectric bar  3  is applied with AC voltage, it can drive the shielding electrode  1  to vibrate up and down along z-direction, which results in the interleaving vibration between shielding electrode  1  and sensing electrode  2 . 
     The substrate  7  can be designed to have any kinds of shapes such as rectangle, round, echelon and so forth. The substrate can be made from glass or other nonconductive materials. If substrate  7  is made from conductor such as metal, it ought to be grounded and be insulated from sensing electrode  2 . 
     Both shielding electrode  1  and sensing electrode  2  are made from conductors or other conductive materials such as doping silicon. The piezoelectric bar  3  can be fixed on the substrate by colloidal material  116 , and also can use other fixed methods such as bonding as long as piezoelectric bar  3  can be fixed. 
     Other materials such as shape memory alloy may also be adopted for substituting the piezoelectric bar  3 . 
       FIG. 3  shows an example of the EFS with thermal actuation. The EFS mainly includes shielding electrode  21 , sensing electrode  22 , driving structure  24 , substrate  27 , and anchors  25 ,  26 . There are many actuators can be used as driving structure in EFS, we present two typical designs in  FIG. 3  and  FIG. 4  respectively. We use clamp-clamp beams as thermal actuators for EFS in  FIG. 3  and U-shaped thermal actuators in  FIG. 4 . 
     In  FIG. 3 , There is an isolating layer on the surface of the substrate  27  and all of the anchors  25 ,  26  are fixed on the isolating layer. Driving structure  24  is clamp-clamp beam and there is a hump at the center of the beam  24 . The ends of clamp-clamp beam are integrated with anchors  25 ,  26 . The Shielding electrode  21  is comb shape and its two ends are respectively fixed on the center hump of two clamp-clamp beams  24 . Sensing electrode  22  is comb array and is arranged alternately with the comb fingers of shielding electrode  21 . 
     As a driving voltage is applied to the anchor  25  and anchor  26 , Ohmic heating causes the clamp-clamp beam  24  to expand due to a positive thermal coefficient of expansion. The clamp-clamp beam  24  buckles upwards because there is a hump at the center of the beam  24 . As the driving voltage is stopped, the clamp-clamp beam  24  could shrink and recover to original shape. 
     The top surface of shielding electrode  21  is designed to be a little lower than the top surface of the sensing electrode  22 , which is to form interleaving vibration. As a rectangle-wave voltage U is applied to the anchor  25  and −U is applied to the anchor  26 , the center of clamp-clamp beam  24  is set to zero electric potential since the driving voltage U and −U are equivalent in amplitude and opposite in sine. The clamp-clamp beams  24  actuates the shielding electrode  21  to vibrate up and down along z-direction, which leads to the interleaving vibration between the shielding electrode  21  and the sensing electrode  22 . It is also doable to substitute the rectangle-wave voltage with sine voltage. 
     The substrate  27  can be made from silicon with an insulating layer such as Silicon Nitride, or be made from other insulating materials. The shielding electrode  21 , sensing electrode  22 , and the clamp-clamp beam  24  can be made from polysilicon or other conductive materials. 
     There is no limitation for the shape of anchors  25 ,  26 , which can be square, rectangle, round, triangle and so forth. On the surface of the anchor, there is a layer of metal for bonding. 
     In order to increase the vibration frequency of the clamp-clamp beam, which results in an increase of current signal at the sensing electrode, it is recommended not to design the beam with large width and thickness. 
     The  FIG. 3  ( b ) shows another design of the shielding electrode  1 , which incorporates with clamp-clamp beam  4 . 
     There are also other options for the number of anchors such as 1) in  FIG. 3(   c ), Only two anchors are utilized to connect with clamp-clamp beams (other parts of EFS are not shown) 2) we can add one or more clamp-clamp beams to support the shielding electrode, which are parallel to the existed clamp-clamp beams (the added beams are not shown) 
       FIG. 4  shows another example of the EFS with thermal actuation. This design is different from above thermal actuation in the structure of actuator. As shown in  FIG. 4(   a ), a U-shaped thermal actuator consists of top layer  38 , bottom layer  39  and connector  310 . If a voltage is applied across anchor  312  and anchor  313 , the current only goes through the bottom layer  39 , which will expand due to the increase of its temperature. The temperature difference between top layer  38  and bottom layer  39  causes the end of actuator deflecting upwards. 
     As anchors  314  are grounded, anchor  312  and  313  are respectively applied across driving voltage U and −U, the connector keeps zero electric potential due to 1) the average value of U and −U is always zero. 2) the anchors  314  are grounded. As a result, the end of U-shaped thermal actuator, where the top layer and bottom layer are connected, vibrates up and down periodically. 
     If the vibrating track of end of U-shaped thermal actuator is concerned, the vibrating track isn&#39;t a straight line but an arc. Although this nonstraight line track is acceptable for EFSs, we also can make some modify to obtain straight line track. We use a pair of U-shaped thermal actuators connected with a beam  315 , as shown in  FIG. 4  ( b ), the beam  315  is integrated with top layers of two U-shaped thermal actuators. Due to the symmetry of structure, the center of beam  315  is vibrating up and down in a straight line along z-direction. This is one kind of method to obtain straight track in the vibration direction. There are also other methods such as 1) after beam  315  is integrated, we can also add another beam under the beam  315 , which is integrated with bottom layer  39 . 2) deleting the beam  315 , and integrating a new beam with bottom layer  39 . 
     In  FIG. 4(   b ), the complete EFS includes shielding electrode  31 , sensing electrode  32 , two pairs of U-shaped thermal actuators  311 , substrate  37 , and anchors  312 ,  313 ,  314 . There is an isolating layer on the surface of the substrate  37 , on which the sensing electrode  32  and all of the anchors  312 ,  313 ,  314  are fixed. The comb fingers of shielding electrode  31  are arranged alternately with sensing electrode  32 . 
     On the substrate, we can also add some pairs of U-shaped thermal actuators to support the shielding electrode, which are parallel to the existed U-shaped thermal actuators (the added actuators are not shown) 
     For above EFSs with thermal actuation of  FIG. 3  and  FIG. 4 , they have the same features in substrate, shielding electrode, and sensing electrode except thermal actuator. It is certainly feasible to use other thermal actuator as long as it can provide vertical actuation. If other thermal actuator is adopted in EFS, it is also needed to consider the formation of straight line track for shielding electrode. If the motion track of shielding electrode is not straight line, it will also be acceptable, since the sensing electrode can also induce the variable amount of charges. 
     It is also feasible to use electromagnetic actuation with external magnetic field. 
     It is also doable to use other actuation structures such as piezoelectric actuation structure, electrostatic actuation structures or other actuation structures to substitute the thermal actuation.