Abstract:
A method and apparatus for controlling a gap between conductors in an electro-mechanical device by controlled displacement of a displaceable conductor within the electro-mechanical device is provided. The apparatus includes a current controller configured to generate a controlled current output for the electro-mechanical device in response to a control signal, the current controller selectively routing a voltage to an array element including control circuitry and the electro-mechanical device.

Description:
BACKGROUND OF THE INVENTION 
   Mircro-electromechanical systems (MEMS) exist which combine mechanical devices, such as mirrors and actuators, with electronic control circuitry for controlling the mechanical devices. One such device is referred to as a diffractive light device (DLD), and includes a variable capacitor composed of a fixed reflective ground plate and a semi-transparent, electrostatically movable second plate. The variable gap between the plates produces a desired interference or diffraction of light passing therein, which can be used for spatial light modulation in high resolution displays and for wavelength management in optical communication systems. 
   Conventional control systems for controlling the variable gap in DLDs and other MEMs devices, however, have been shown to have a non-linear relationship between the voltages generated to control the gap size versus plate displacement for achieving a desired gap size. This non-linear relationship limits precise control of plate movement to less than one third of the total gap distance before the plate “snaps down” to mechanical stops. This “snap down” phenomenon is also known as a pull-in characteristic in the art. 
   Techniques for increasing the controllable distance often require large control circuit footprints due in part to the presence of switching elements and the like, which correspondingly increases the footprint of the controller and prevents implementation in applications requiring relatively small control circuit sizes (e.g., not greater than 20 u 2  per MEMs device). Other techniques for increasing the controllable distance suffer from parasitic drops in control lines (particularly in arrayed DLD applications), which causes a variation in power to DLDs across the array. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a control system according to an embodiment of the present invention. 
       FIG. 2  is an exemplary 2×2 array using the control system of  FIG. 1 . 
       FIG. 3  is a graph demonstrating gap as a function of ideal charge control. 
       FIG. 4  is a theoretical timing diagram for the control system of  FIG. 1 , assuming typical characteristic behaviors of implemented components. 
       FIG. 5  is a circuit diagram of a control system according to yet another embodiment of the present invention. 
       FIG. 6  is an exemplary 2×2 array using the control system of  FIG. 5 . 
       FIG. 7  depicts a method of controlling a gap between at least one fixed plate and an electrostatically movable plate in a MEMs device according to an embodiment of the present invention. 
       FIG. 8  depicts circuit diagram of a control system according to yet another embodiment of the present invention. 
       FIG. 9  is a schematic diagram of a control system according to an embodiment of the present invention. 
       FIG. 10  is a graph demonstrating gap as a function of ideal voltage control. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 10  is a graph of voltage control which illustrates gap as a function of ideal voltage control for a MEMS device. The voltage is incremented in small steps, and the gap is allowed to settle to an equilibrium state for each increment. Even with ideal voltage control, gap range “snaps down” after ⅓ gap travel (initial gap in this simulation was 3000 Ang, and snap down occurs at 2000 Ang). This is known as the pull-in effect and the ⅓ limit is a widely understood phenomenon in published literature. 
     FIG. 3  demonstrates gap as a function of ideal charge control for a MEMS device. Ideal charge control is achieved by coupling an ideal (fully voltage compliant) current source to the MEMS actuator and modulating the on time of the ideal current source (Q=i*dt). Gap is essentially a linear function of charge, and full gap control range is shown. 
   Referring now to  FIG. 9 , a control system for controlling a gap between conductors in a MEMs device  150  according to an embodiment of the present invention is shown schematically. The control system includes a current controller  900  configured to generate a controlled current output for MEMs device  950 . The current controller includes a magnitude input which controls magnitude of output current. A switch element  970  is disposed to couple or decouple the current output of the current controller  900  to the MEMS device  950  accordance with an enable signal on line EN. Thus, charge “written” to MEMS device=magnitude of current*Enable on-time. 
   Referring now to  FIG. 1 , a further embodiment of the present invention is disclosed for controlling a gap between conductors in a MEMs device  150 , which uses realistic and available components. The control system includes a current controller  100  configured to generate a controlled current output for MEMs device  150 . While only one current controller  100  for one MEMs device  150  is shown in  FIG. 1 , it should be appreciated that the current controller  100  and MEMs device  150  may be incorporated into an array (see controllers  101 ,  102 ,  103  and  104  in  FIG. 2 ) to control a gap between conductors in a plurality of MEMs devices  150 . By way of example a typical SVGA display/projector would have an 800×600 array of MEMs devices  150 . In a large array of MEMs devices  150  the per-column column data may be set up first. As will be described in greater detail below, that column data controls the magnitude of the current that the MEMS device  150  in that column sinks. Once that column data is set up, one row is enabled for a particular pulse width. After that row is disabled, the column data for the next row is set up, then the next row is enabled, etc. So once a row has been “written”, capactive elements of the system hold the charge on the MEMS device  150  while other rows are written, and that first row can be written again. Other array configurations are also plausible, as would be readily apparent to one of ordinary skill in the art after reading this disclosure. 
   The current controller  100  includes an input for receiving a control signal (e.g., enabling control signal (EN)), a current mirror having MOS devices  130  and  160 , and a pull-up MOS device  170  coupled to the current mirror and configured to disable the output of the current mirror (e.g., raise a gate voltage of MOS  130 ) when the transmission gate  140  disables the current mirror. An inverter  110  may or may not be provided, depending on the particular type of transmission gate  140  used or if the control signal(s) are generated on the periphery of the array and routed to the array. Thus, the controlled current outputs are variable voltage compliant. Further, it should be appreciated that, while transmission gate  140  is shown as one type of current mirror enabler, other components may also be used, such as a pmos or nmos type device. With the aforementioned structure, however, the current controller  100  may have a footprint not greater than 20 u 2  per MEMs device  150 . Note that enabler device  140  in this embodiment has the purpose of coupling or decoupling the gates of  160  and  130  when the current mirror output (drain of  130 ) is “off”. When the current mirror is off, the gate of device  130  is prevented from floating (to stop current from flowing out of the drain of  130 ), so MOS device  170  is used to fully turn off  130 . Note that devices  130  and  170  are included because, in a large array, there would be one of devices  160  &amp;  120  per column, and one of devices  140 ,  170 , and  130  in each pixel in each column. Since a charge is “written” a row at a time, it is desired to have only one device  130  in a particular column “on” at any given time. Consequently, it is desired that the gate of device  160  and the gate of only ONE device  130  in the column be coupled together at any given time. 
   The current mirror is configured to mirror a reference current onto a controlled current output for MEMs device  150 . In this regard, the reference current may be generated by an external current source coupled to the current controller  100  via coupling  120 , or may be generated by a current source within the current controller  100  provided at coupling  120 . By either configuration, the reference current is precisely controlled to achieve a corresponding gap control in MEMs device  150  when the current mirror is enabled by transmission gate  140 . In this manner, a gap size within the MEMs device  150  can be adjusted for a particular MEMs device  150  in an array of MEMs devices. 
     FIGS. 3 and 4  show exemplary timing diagrams for a control system.  FIG. 3  demonstrates gap as a function of ideal charge control. Idea charge control is achieved by coupling an ideal (fully voltage compliant) current source to the MEMS actuator and modulating the on time of the ideal current source (Q=i*dt). Gap is essentially a linear function of charge, and full gap control range is shown. Current (A) refers to the current mirrored onto a controlled current output by the current mirror. Charge (fC) refers to the charge between the movable plate and the fixed plate within MEMs device  150  (the amount of charge being put on a MEMs device  150  is a function of current magnitude and pulsewidth by Q=I*dt). Capacitance (fF) refers to the capacitance between the movable plate and the fixed plate within MEMs device  150 . Voltage (V) refers to the voltage difference between the movable plate and the fixed plate within MEMs device  150 . Gap (Ang) refers to the distance between the movable plate and the fixed plate within MEMs device  150 . 
     FIG. 4  illustrates one embodiment (using the simple MOS current mirror) of charge control using realistic models of MOS devices with primary and secondary parasitic effects included in the MOS models. The method of  FIG. 3  (time-modulated current pulse) is used to put a controlled amount of charge on the MEMS device. While parasitic effects of the MOS devices do not allow, for this embodiment, full gap control range, gap control remains stable well beyond the ⅓ voltage control limit to at least 45% of the gap. An embodiment for implementing  FIG. 4  is shown in  FIG. 1 . An additional benefit of the  FIG. 1  design (besides the extended gap control range) is that it may be easily incorporated into a large addressable array ( 130 ,  140 , and  170  would be replicated at each pixel, while  110 ,  120 , and  160  would be replicated on a pre-row or per-column basis) with a small per-pixel footprint. 
   A control system for controlling a gap between conductors in a MEMs device  150  according to yet another embodiment of the present invention is shown in the circuit diagram of  FIG. 5 . The control system of this embodiment includes a controller  500  configured to generate a control signal to selectively move the electrostatically movable plate in a MEMs device  150 , and a current digital-to-analog-converter (DAC)  510  per MEMs device  150  configured to generate a controlled current output to move the electrostatically movable plate in a MEMs device  150  coupled thereto. More specifically, the DAC  510  converts a magnitude value (MAG) to an analog output current, the current magnitude being adjusted based on the particular amount of gap desired. Additionally, the pulsewidth of the EN signal can be adjusted based on the particular amount of gap desired. 
   As with preceding embodiment(s) of the present invention, the control system of the present embodiment may be used to control a plurality of MEMs devices. By way of example, see the plurality of controllers  501 ,  502 ,  503 , and  504  in  FIG. 6 , depicted in a 2×2 array. Any number of MEMs devices  150  could thus be controlled in this manner, by simply providing a DAC  510  per MEMs device  150  with appropriate control lines running thereto. 
   This embodiment of the present invention also capable of a greater amount of precise gap control than in conventional voltage control. Additionally, this embodiment can be easily implemented with existing componentry by properly arranging a plurality of DACs. 
   A method of controlling a gap between at least one fixed plate and an electrostatically movable plate in a MEMs device according to another embodiment of the present invention is shown in the flow chart of  FIG. 7 . In step  710 , if desired, a reference current can be adjusted to represent the desired gap between the fixed plate and the electrostatically movable plate. In step  720  a control signal is then time modulated to represent a desired gap between the electrostatically movable plate and the fixed plate. The reference current is then selectively mirrored in step  730  onto a controlled current output coupled to a MEMs device on the basis of the time modulated control signal. This controlled current output then results in displacement of the electrostatically movable plate in step  740 . In this manner, the desired gap size in the MEMs device can be achieved. As with other embodiments, this method may be replicated for use with a plurality of MEMs devices, such as in an array of MEMs devices. 
   A control system for controlling a gap between conductors in a MEMs device  150  according to an embodiment of the present invention is shown in the circuit diagram of  FIG. 8 . As with previously described embodiments, the present embodiment sources current onto MEMs device  150  to control the gap size between plates therein. To further improve current control, however, the present embodiment sets the charge on the MEMs device  150  to a known state (such as about zero) before the charging occurs. Thus, according to this embodiment, an NMOS  810  is used to couple MEMs device  150  to ground. Using the NMOS  810  to clear MEMs device  150 , the MEMs device  150  can be actuated by clearing substantially all of the charge off the MEMs device  150 , setting up a current magnitude signal in a current controller as previously described, and enabling current output of the current controller for a known period of time. Other embodiments of the present invention, such as the DAC technique, may also use an NMOS  810  to clear MEMs device  150  in a like manner, thereby further improving the gap control. 
   The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.