Improvements in or relating to micromechanical devices

Abstract: A method of operating a micromechanical device (12). The device is in a first state. Data for the next state of the device is loaded onto the activation circuitry of the device (18a, 18b) where the next state may be the same state the device is currently in, or a state different from the first state. The equilibrium of the device is shifted away from the next state, by making the data appear complementary to the true data for the next state. When the trapping field is removed or lowered, and a signal to start the transition is provided, the device moves to its new state and the trapping field is reapplied. The data can be made to look complementary by either loading the complements to the true data, or by reversing the polarity of the trapping field.

Description: This invention relates to the operation of micromechanical devices, more particularly to the activation and reset of such devices. BACKGROUND OF THE INVENTION Micromechanical devices typically consist of miniaturized moving parts that are activated by electrical currents or fields. These moving parts range from gears and motors to movable mirrors and shutters. The control of the fields surrounding the parts allows operation of these devices normally involves rather sophisticated underlying electronic circuitry. One of the difficulties faced with micromechanical moving parts lies in the activation and movement of the parts. In the case of continuously moving parts, such as gears and motors, activation occurs when the necessary fields are applied to start the motion, supplied to keep the motion continuous and then removed to stop the motion. In contrast, a far different series of events takes place for those micromechanical devices that have stepped motions, such as the shutters and mirrors mentioned above. These micromechanical devices become activated when they are acted upon by applied fields to start the part moving, the part moves and then it comes to rest. The time that they are transitioning from one state to another is called "flight" time. When operating a large number of these parts within the device, the flight time for each part must coordinate with the flight times of the other parts. Additionally, the reset of each part, where the part is released from its current state to prepare for the next state, must be coordinated among the parts. These processes increase the controllability requirements of the device. Additionally, since each individual part comes to rest in the new state, the necessary field forces required to start its initial movement may be higher. This in turn increases then complexity of the activation circuitry. Both of these problems result in a more complex device that must be controlled very closely. Therefore, some method is needed for the operation and control of micromechanical devices to decrease their flight time and to increase their controllability. SUMMARY OF THE INVENTION The present invention as defined in the appending claims provides an improved method for operating a micromechanical device. In one aspect of the invention either complementary data or the appearance of complementary data is used to assist in the reset process of micromechanical parts in which the parts are released from their current state to prepare for the next state. Fields are briefly set to shift the equilibrium opposite of the next system state. This pumps energy into the system to prepare for the next state transition. Either the activation process occurs with fields complementary to the fields corresponding to true data or the polarity of a trapping field is reversed, making the data appear complementary. The trapping field is then removed or lowered and the part transitions to its next state. The next state may be a state that is the same as the first state or a state different from the first state. It is an advantage of the invention that it increases the departure speed of a part moving to a next state that is different than its first state. It is an advantage of the invention that it decreases the departure speed of a part moving to a next state that is the same as its first state. It is an advantage of the invention that it increases the controllability of arrays of these individual devices. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying Drawings in which: Figures 1a-b show a cross-sectional view of a prior art micromechanical device that moves from one stable state to another depending upon the activation data; Figures 2a-h show a prior art sequence of a micromechanical device transitioning to states dictated by new data; Figures 3a-h show one embodiment of an improved sequence for a micromechanical device transition; Figures 4a-j show a second embodiment of an improved sequence for a micromechanical device transition; and Figures 5a and 5b show examples of waveforms for the above embodiments of the invention. DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS While the techniques described herein may be applied to all micromechanical devices, they are especially applicable to those devices that move in a stepped fashion, transitioning from one resting state to another resting state. These devices typically change states depending upon fields applied corresponding to incoming data. The discussion contained herein will focus on these types of devices, but is in no way intended to limit the scope of the claimed invention. For example, the devices under discussion use electrical signals and electrostatic fields to activate the parts and then trap them. These activation fields and trapping fields in the devices under discussion are referred to as data and bias voltage. However, the use of these terms is in no way intended to eliminate the application of this invention to electrically controlled micromechanical devices in any way. One type of these stepped devices that suffers from several operating constraints are micromechanical spatial light modulators. These types of devices typically comprise arrays of individually addressable elements used to form images. It is in the image-forming applications that these devices suffer the most constraints. In forming images from incoming video data, the modulator elements must be able to switch states at very high speeds. Additionally, in embodiments where these elements consist of mirrors suspended on hinges, they occasionally suffer from a condition called hinge sag, in which the hinges supporting the mirror do not return completely to their resting position, causing stray light to enter the imaging optics. Additionally, if these mirrors are activated such that they actually touch down onto another surface, the combination of sticking and friction (called stiction) can make the release (reset) of these parts difficult. These elements are activated by electrical signals that cause them to assume a particular state by causing electrostatic attraction to build between the element and the activation circuitry. Reset refers to the process of releasing the element from its current state to prepare it to move to its next state. The elements are held in place between transitions from state to state by a trapping field, which in this case is a bias voltage. One example of such a modulator is the digital micromirror device (DMD™), shown in Figures 1a-b. In Figure 1a the modulator element 10 is shown in its unaddressed state. The element comprises a mirror 12, supported over a yoke 14. The yoke 14 has hinges, not shown, that are in the plane coming out of the page in this view. The hinges allow the yoke 14, and therefore the mirror 12, to move about the axis of the hinges in response to activation signals. The activation signals are provided on addressing electrodes 18a and 18b. When these electrodes are addressed, the electrostatic forces between the yoke 14 and one of the address electrodes builds up such that the electrostatic forces are unbalanced. This causes the yoke 14 to be attracted to whichever of the address electrodes is addressed. The result of this attraction is shown in Figure 1b. In this example, the address electrode 18b was addressed to cause the yoke 14 to be attracted to it. The yoke/mirror structure then deflects about the hinges towards address electrode 18b until the yoke comes into contact with landing electrode 16b. Additionally, a mirror stop or landing electrode 20b may be provided. At this point, the mirror has transitioned to a stable state, which in imaging applications is either the ON state, where light reflected off the deflected mirror is directed to the imaging optics, or the OFF state, where light is directed away from the imaging optics. The strength and balance of the electrostatic field is maintained through manipulation of various voltages. A prior art sequence and examples of voltage values that can be used to cause these transitions is shown in Figures 2a-h. In Figure 2a, the mirror is in the state shown in Figure 1b. In this example, this will be assumed to be the beginning of a new segment of data, such as a new frame in video applications. The mirror has a bias voltage upon it, V M , which equals 20 volts. The activated electrode 18b has a voltage, V b , which equals 0 volts. The opposite electrode 18a has a voltage, V a , which equals 6 volts. The voltages V a and V b remain the same in Figure 2b, which is where the new data is being loaded for the next frame. In this case, the new data will require the element to stay in its same state. V a equals 6 volts and V b equals 0 volts. In Figure 2c, the bias voltage, V M , is dropped momentarily to 6 volts to allow the element to transition to its new state. Since the data is the same as that for the previous frame, the voltages V a and V b remain the same and the element should not move from its current state. However, there may be some movement of the element, as it can become momentarily unstable. In Figure 2d, the momentary instability is eliminated when the bias voltage, V M , is returned to 20 volts. This transition from old data to new data that is substantially the same as the old data will be referred to as a "same side" transition. In Figures 2e-2h, the same sequence is performed for an element that changes state based upon the data loaded in the next frame. Figure 2e shows the element deflected to the right (or "a") side. In this case, the activated electrode has a voltage, V a , equaling 0 volts and the opposite electrode has a voltage, V b , equaling 6 volts. In Figure 2f, the new data for the next frame is loaded. Here the desire is to have the element change to the other state, so the voltage V b is set equal to 0 volts, the voltage V a equals 6 volts. In Figure 2g V M drops momentarily to 6 volts, causing the mirror to release and transition to the left ("b") side. In Figure 2h, V M is returned to 20 volts to stabilize the mirror. This transition from old data to new data that is different than the old data will be referred to as "opposite side" transition. The electrostatic potential between the mirror and the address electrode can be shown in the following table. Image Update Step Same side transition Opposite side transition Voltage Force Voltage Force Start of frame 20 Volts 400 Units 20 Volts 400 Units Load data for next frame 20 Volts 400 Units 14 Volts 196 Units Dropping bias 6 Volts 36 Units 0 Volts 0 Units Reapply bias 20 Volts 400 Units 20 Volts 400 Units Operation of these devices at high speeds requires very fine control of each element in the array. The voltages which activate and latch the elements into movement and then stabilize them must be sent to the elements via signals. The time for these signals must be tightly controlled. If the bias voltage that latches the elements is dropped for too long a period to allow the elements to transition, the elements release from their states and stray light enters the ON light path. If the bias voltage is dropped for too short a period, none of the elements will release and they will not change state to update the image. In a first embodiment of the invention, the data appears to be reversed or complementary just prior to the momentary drop in the bias voltage, V M . One example of this approach is shown in Figures 3a-h. In Figure 3a, the element is shown at the start of a new frame to the "b" side. The mirror bias, V M is at 20 volts, and the address electrodes are V b =0 and V a =6 volts. The appearance of complementary data serves to shift the equilibrium of the device away from the next state. In Figure 3b, the data for the new frame is loaded. Note that the data appears to be that for a opposite side transition as discussed in Figure 2f, but is really for a same side transition. V b changes from 0 to 6 volts, and V a changes from 6 to 0 volts. In Figure 3c, the mirror bias V M drops to 0 while the complementary data is on the address electrodes. Finally, in Figure 3d, V M is reapplied and V b and V a return to their same side voltages of 0 and 6 volts respectively. Figures 3e-3h show the voltage sequences for opposite side transitions. Figure 3e shows the now expected voltages, with the side to which the mirror is deflected at 0 volts and the opposite side at 6 volts. During the data load for the next frame, those voltages remain the same, even though an opposite side transition is the desired operation. In Figure 3g, V M drops to zero, which again causes the mirror to move to the side that has the 6 volts, which in this case is 18b. In Figure 3h, V M is reapplied and the voltages on the electrodes return to their previously expected levels. Essentially, then, the data on the address electrodes appears to be complementary when the bias is removed. The data returns to the true values (where the true values are those that were used in the prior art embodiment in Figures 2a-2h) when the bias is reapplied. The advantages of this approach are that the departure speed of an element making the opposite side transition is increased, while the departure speed of an element making a same side transition is decreased. Recall that the same side transition would ideally have no transition at all, and therefore reducing the departure speed for same side limits the instability of the elements. The effect on this complementary data method is summarized in the following table. Image Update Step Same side transition force Opposite side transition force Start of frame 400 Units 400 Units Load data for next 196 Units 400 Units Dropping bias 36 Units 0 Units Reapply bias 400 Units 400 Units In the prior art embodiment, the difference in force between the steps of loading data for the next frame and dropping the bias was 364 units for same side, and 196 units for opposite side. In the above embodiment, the difference between the same steps is 160 units for same side and 400 units for opposite side. This results in the differences in the departure speeds discussed above. The end result of these changes is an increase in the controllability of the elements in the array. Additionally, the reset operation, in which the elements are pulsed to move them to their new states is more efficient because of the boost in the electrostatic forces. The application of this complementary approach can be done in two ways. The first is what will be referred to as the complementary method, and was shown in Figures 3a-3h. In applying this method, the entire array of elements are loaded with complementary data, the bias is dropped, the true data is loaded onto the array and the bias is reapplied. A diagram of one example of a reset waveform for this embodiment is shown in Figure 5a. A second embodiment of this type of complementary addressing will be referred to as the bipolar method. In it, the true data is loaded on the array, and the polarity of the bias is reversed, then dropped, then reapplied at its original level. This second embodiment will be discussed with reference to Figures 4a-4j. A same side transition using this embodiment is shown in Figures 4a-4e. In Figure 4a the pixel is at the start of a frame, with the voltages at their expected levels. In Figure 4b, the data is loaded for the next frame. In Figure 4c, a previously unused step occurs, in which the bias on the mirror V M is reversed from 20 volts to - 14 volts. In Figure 4d, the vias is dropped to 6 volts. The reverse bias makes the data appear to be complementary just before the bias is dropped, as was done in the previous embodiment by actually using complementary data. When the bias is dropped, the same effect occurs. In Figure 4e, the bias is reapplied at 20 volts. Opposite side transitions are shown in Figures 4f-4j. Figure 4f has the voltages at their expected levels for a mirror deflected to the "a" side. In Figure 4g the data for the new frame is loaded, so the voltages V b and V a are switched. When V M is reversed the mirror "sees" the data as being opposite such that when the bias drops in Figure 4i, the mirror transitions to the new side. Finally, the bias is reapplied in Figure 4j. One example of a reset waveform for this embodiment is shown in Figure 5b. In this embodiment, the reverse of the blas serves to shift the equilibrium of the device away from the next state. It must be noted that the above discussion has revolved around a particular set of voltage values and the transitions between two states, but is not limited to that situation. The relative values of the voltages are what is important, and the idea of loaded either complementary data or reversing the bias to make the data appear complementary can be applied in other situations. For example, instead of address electrodes, the voltages could be applied to the activation circuitry of any number of micromechanical devices. The only limitation is that the micromechanical device must be addressed with data that either dictates a new state, and there can be more than one new state, or that dictates that no movement is necessary. Being able to load data that keeps the device in the same state would seem to be a disadvantage, except that when arrays of these types of devices are used, it simplifies the addressing if all devices are addressed, if they need to change state or not. Similarly, the above discussion has focused upon the use of electrical signals an electrostatic forces. The same processes could be used by micromechanical devices that are activated and trapped by any type of field, including magnetic fields. Thus, although there has been described to this point a particular embodiment for a method and structure for complementary addressing of micromechanical devices, it is not intended that such specific references be considered as limitations upon the scope of this invention.

Claims: A method for operating a micromechanical device in a first state, said micromechanical device comprising a movable element (12) supported and operable to pivot in either direction about an axis, and two electrodes (18a, 18b), each one of said electrodes located on each side of said axis such that a voltage difference between said movable element (12) and each electrode (18a, 18b), creates an electrostatic force operable to pivot said movable element (12) about said axis to either of at least two different positions, said method comprising the steps of: loading data for a next state of said device onto activation circuitry of said device, said activation circuitry applying a voltage to said electrodes (18a, 18b) determined by said data; manipulating said voltage differences between said movable member (12) and each of said electrodes (18a, 18b) such that the voltage difference between said movable element (12) and said electrode (18a, 18b) on a side indicated as the next state by said data is less than the voltage difference between said movable member and said electrode on a side not indicated as the next state by said data to shift equilibrium away from the next state; reducing the force of a trapping field on said device by reducing the magnitude of said voltage differences between said movable member (12) and said electrodes (18a, 18b); providing a signal which causes said device to transition to said next state; and reapplying said trapping field to hold said device in said next state. The method of Claim 1, wherein said manipulating step consists in loading complementary data on said activation circuitry. The method of Claim 1, wherein said manipulating step consists in reversing the polarity of said trapping field, thereby making said data appear to be complementary. The method of any of Claims 1 to 3, wherein said step of loading data for the next state comprises loading data for a next state that is the same as said first state. The method of any of Claims 1 to 3, wherein said step of loading data for the next state comprises loading data for a next state that is different than said first state.

Inventors: CARTER DUANE E, KNIPE RICHARD L, WHITE LIONEL S, CARTER, DUANE E., KNIPE, RICHARD L., WHITE, LIONEL S.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1997
Main classification: G02B26
IPC codes: G02B26