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Technology Details |
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This novel display realizes the following three technological keypoints: Making full use of shape memory alloy Using SMA as an actuator is the most important innovation of this display. Although SMA is very small and thin, it can produce a very strong force. Therefore, SMA's have been said to be a potential realization of artificial muscle. Although there have been a great variety of studies about the characteristics of SMA's, it is not popular to use SMA as an actuator, possibly because it is very difficult to control this material completely. This difficulty stems from its hysteresis: it doesn't move instantly after applying heat and needs time to shrink with cooling. This leads to a serious problem with frequency response. That is why this approach isn't very actively pursued although SMA has apparently been used as an alternative to existing actuators; in fact, there have been few studies and papers on controlling SMA. In this work, We adjusted controlling SMA with PWM pulse input according to the positional information. By this method, it is possible to actuate SMA without considering the inner model for use in mechanical design and a control system for compensating the hysteresis. Another merit to using SMA's is the ability to keep each component very simple; SMA's are thin so it is possible to arrange pin rods with high density and to modify the pin rod dimensions for tailored designs.
Matrix drive An important issue that must be addressed in large-scale pin-rod matrix displays is the complexity of the control loop. In methods to actuate a typical N ~ N pin-rod matrix such as those driven by servomotors, each pinrod must be controlled individually, requiring an amount of control information proportional to N2. Such schemes work perfectly well if the number of pin-rods is small. However, for a high-resolution display with a large value of N, the required information load could quickly hamper the control loop of the system. To speed up the control of the SMA actuators, we used a scheme that is commonly used in CMOS and LCD displays called "Matrix Drive". It is composed of an N channel FET switch positioned orthogonally at both ends of the pin array. To actuate the SMA for a single pin, each switch needs to be turned on in the corresponding horizontal and vertical channels, as a Boolean AND operation. Arbitrary height maps can be implemented with an active matrix drive algorithm, to be described in a later paper. Because the amount of information required to drive the system is proportional to 2N, which will be increasingly important as the size of the array increases.
Matrix position sensing by camera While the compactness of SMA and the flexibility of the matrix drive scheme provide a decisive advantage in supporting a dense array of pin-rods, the need for position sensors introduces a new technical challenge. Our first prototype had a pair of photo reflectors for each pin-rod, working as a linear encorder, which was readily constructed for a small number of rods. Naturally, the number of reflectors will increase if we want to add more rods, and the capacity to read the output of photo reflectors will become a more serious bottleneck for larger arrays. Therefore, we considered adopting other, more scalable sensor devices for measuring position. We adopted a kind of visual feedback sheme. We put small color markers on the bottom of each pin rod and captured image of the markers by CCD camera. Tracking each markers by image processing, we acquired height information of each pinrod. To avoid shielding problem (i.e. front markers shield back markers), the front pinrod must be shorter than the back pinrod, resulting in a characteristic stair-wise shape (see figures).
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