Overhead projector with vibration control head

Abstract: An improved overhead projector head is provided that reduces the vibration that can occur in an overhead projector head that is used to project images from an LCD panel placed on the stage of the projector. The improvements comprise adding a mounting plate between the support arm and the head bracket. The mounting plate is cantilevered from the support arm, and the head bracket is supported by spacers placed near the end of the mounting plate that is remote from the support arm. In alternative embodiments, the head bracket may be divided into two pieces separated by foam or other vibration damping material, or a counterweight may be added to the head assembly behind the support arm.

Description: FIELD OF THE INVENTION The present invention relates generally to overhead projectors, and more specifically to overhead projectors that are used to transmit images from liquid crystal display panels. In particular, the invention relates to improvements in overhead projection heads to reduce vibration and blurring of the projected image. BACKGROUND OF THE INVENTION Overhead projectors of the transmissive type include a base unit which encloses a light source and lenses for suitably focusing light from the source and a transparent stage upon which may be positioned a transparency or other film having indicia which is to be projected on a distant screen. Above the base is disposed a projection head having lenses and a mirror which function to gather light from the projector base and redirect it to the screen. The projection head is supported by a post or arm extending upwardly from the base. In typical prior art overhead projectors, as basically known from US-A-3 807 850 and US-A- 4 965 733 and as shown in Fig. 1, the primary structural member of the projection head assembly, the head bracket, is attached directly to the arm in a cantilevered fashion by screws positioned near the rear edge of the head bracket. The head bracket supports the projection lens and the mirror, as well as a head cover. Undesirable vibration in the head of an overhead projector may occur such as when the projector is used with a liquid crystal display (LCD) panel. A LCD panel is used-to allow projection of a computer-generated image by an overhead projector. With some overhead projectors, the cooling fan of the LCD panel causes an unacceptable resonant vibration of the projection head of the overhead projector. This vibration of the head, and the mirror in the head, in particular, causes unacceptable smearing or blurring of the projected image. SUMMARY OF THE INVENTION The present invention provides an overhead projector head assembly as defined in claim 1, which eliminates or reduces the occurrence of unacceptable vibration of the projection mirror and consequent blurring of the projected image. These and other advantages of the present invention will be further appreciated from the drawings and from the detailed description provided below BRIEF DESCRIPTION OF THE DRAWINGS It is to be noted that the appended drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope . Figure 1 is a perspective view of a typical transmissive overhead projector as is known in the prior art. Figure 2 is an elevation view from the side of a projection head of a typical overhead projector showing the relationship of the arm, head bracket, mirror and projection lens as are known in the prior art. Figure 3 shows an elevation view from the side of a projection head including a projection head embodiment not part of the present invention. Figure 4 is an exploded perspective view of a projection head including the improvements, not part of the present invention. Figure 5 shows a side elevation view of an alternative embodiment of a projection head embodiment not part of the present invention. Figure 6 is a front elevation view of a projection head embodiment not part of the present invention. Figure 7 is a front elevation of another projection head embodiment not part of the present invention. Figure 8 is a side elevation view of a projection head embodiment according to the present invention, employing a counterweight positioned on the head structure to move the center of gravity of the head closer to the support arm. DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 illustrates a typical transmissive type overhead projector, generally indicated as 10, which includes as major elements a base 12, an upright post 14 extending from the base 12, an adjustment mechanism 16 attached to the post 14 and a focus arm 18 extending from the adjustment mechanism 16 to support a projection head 20. The projection head 20 includes a lens or plurality of lenses 22 and a mirror 24 and is supported so that the lens 22 is positioned over a transparent stage 26. The base 12 of the projector 10 houses a light source, mirrors and lenses which direct light through the stage 26 and an imaging film such as a transparency located thereon and focus that light at the lens 22 of the projection head 20. The light passing through the projection head lens 22 is redirected by the mirror 24 to a distant vertical surface which is or serves as a screen for the projected image. A computer driven imaging display device, such as a liquid crystal display (LCD) panel, may be used to provide the image to be projected, rather than an imaging film or transparency. The LCD panel is used by positioning it on stage 26 in place of a transparency. LCD panels suitable for this application typically include cooling fans that can induce vibration into the overhead projector system. In some combinations of overhead projectors and LCD panels, it has been observed that unacceptable smearing or blurring of the projected image occurs. That problem has been identified as being caused by the existence of a resonant mode in the projection head corresponding to a vibration frequency generated by a typical LCD display fan. Vibration frequencies can also be generated by the overhead projector fan and interactions between the overhead projector fan, the LCD panel fan and other sources. Figure 2 illustrates a prior art projection head structure. This drawing is intended to represent a typical arrangement of components, but it is particularly representative of some typical projection heads as are known in the prior art. The components of the projection head are assembled on head bracket 30 which has front edge 32 and rear edge 31. Head bracket 30 is attached to support arm 18 in a cantilevered fashion near rear edge 31. Head bracket 30 typically supports projection lens 22 and projection mirror 24, as well as a head cover (not shown). When the arrangement shown in Figure 2 is used, it is suspended above the stage of the overhead projector. Light from the stage, which may have passed through a LCD display -panel, is received through projection lens 22 and reflected from mirror 24 in a forward direction toward a screen on which the image is to be displayed. It will be apparent to one skilled in the art that any vibration of mirror 24 will adversely affect the resolution and clarity of the projected image. Referring to Figure 3, a preferred embodiment of the present invention is illustrated wherein the head bracket 30 as shown in Figure 2 is connected to arm 18 via mounting plate 34 and spacer 36. In this preferred embodiment, mounting plate 34 is attached to arm 18 in a cantilevered fashion. Mounting plate 34 is generally horizontally disposed. Head bracket 30 is attached to and supported by mounting plate 34. The point of attachment between mounting plate 34 and head bracket 30 is spaced from arm 18. In preferred embodiments, mounting- plate 34 and head bracket 32 are attached at two locations, one on each lateral side of projection lens 22. A line connecting those two points of attachment may approximate a center of balance of projection head assembly including head bracket 30, projection lens 22 and mirror 24. That centerline may be approximated by centering it on the lens, or by selecting a centerline approximately midway between the front and rear edges of the head bracket. Spacer 36 may comprise a rigid material such as aluminum or other metal or plastic, or it may comprise a resilient material such as rubber, soft plastic or a foam material. Figure 4 provides an exploded view of a presently preferred embodiment of the present invention. Mounting plate 34 is attached to arm 18 by screws 42. Head bracket 30 is attached to mounting plate 34 at a location spaced from arm 18. Fastener 23, which may be a screw, rivet or equivalent, passes through holes in head bracket 30, spacer 36, and mounting plate 34. In this embodiment, there is a spacer 36 and fastener 23 on each side of the lens. A travel limiting tab 38 may be formed in mounting plate 34 for engagement with slot 44 formed in head bracket 30. Dampening pad 40 may also be placed between mounting plate 34 and head bracket 30 in the vicinity of travel limiting tab 38. Travel limiting tab 38 and dampening pad 40 cooperate to restrict and dampen any relative motion of head bracket 30 with respect to mounting plate 34, particularly to reduce the settling time of any impact-induced transient vibrations, such as when the projector is bumped by the user. Projection lens (not shown) is suspended within orifice 48 of head bracket 30, and the projection mirror (not shown) is supported by the vertical panel portions 50 of head bracket 30. Head cover 46 is also attached to head bracket 30. In presently preferred embodiments, spacer 36 is made of aluminum and dampening pad 40 consists of a small block of a shock absorbing or dampening material, which may be E-A-R ISOLOSS® "LS" high density cellular urethane foam. Figure 5 illustrates an alternative embodiment of the present invention. Figure 5 is similar in most respects to Figure 3, except that mounting plate 34 is substantially longer, extending as far forward as the front edge 32 of head bracket 30. The point of attachment between mounting plate 34 and head bracket 30 is near the front edge of both components, with spacers positioned between the mounting plate 34 and head bracket 30 to separate them from one another. Head bracket 30 is thus cantilevered back in a rearward direction from spacer 36. One or more travel limiting tabs and/or dampening pads, as described above, may be employed in this embodiment between mounting plate 34 and head bracket 30. Figures 6 and 7 illustrate further alternative embodiments of the present invention. Both of these figures show views from the front of the projector of lens 22 and mirror 24 supported by structures that isolate the mirror from the lens. Head bracket 52 supports lens 22 and is directly or indirectly attached to the projector arm (not shown). Mirror bracket 54 supports mirror 24. Mirror bracket 54 is attached to head bracket 52 through dampening material 56. This dampening material inhibits the transfer of vibrational energy from the overhead projector to mirror 24. Mirror bracket 54 and head bracket 52 may be joined together with screws, rivets or similar fasteners with dampening material 56 in the form of a washer or similar insert. Alternatively, the mirror bracket 54 and head bracket 52 may each be attached to dampening material 56 by adhesive or other means. The embodiment of Figure 7 is similar to the embodiment shown in Figure 6 except that dampening material 58 is disposed in a slanted position rather than in a horizontal position. Mirror bracket 54 and head bracket 52 are correspondingly altered as shown in the figure. Referring to Fig. 8, an alternative method for reducing vibration of an overhead projector image, particularly when using a LCD panel on the projector, is to add a rearward extension 60 to head bracket 30 and to place counterweight 62 on said rearward extension. Because counterweight 62 is located behind support arm 18, it tends to counterbalance the weight of lens 22 and the other components that are in front of support arm 18, thus moving the center of gravity of the head assembly rearward toward support arm 18. By moving the center of gravity the moments of inertia and the vibrational modes of the head assembly are altered. It will be apparent to one skilled in the art having the benefit of this description that an appropriate counterweight can be selected and positioned to eliminate or reduce resonant vibration problems in the projector head. Example Tests have been performed to compare the performance of a typical overhead projector known in the prior art with the performance of a projector employing the improvements of the present invention. In objective tests, the vibration of the projection mirror was measured using an accelerometer. Two 3M M9800 projectors were used with a M6150 LCD projection panel. Accelerometer tests were performed using a standard projection head (as shown in Fig. 1) and two split-head assemblies (as shown in Figs. 3 and 4). The results of those tests are shown in the following table: I Projector I Head I Vibration (g) I A Standard 7.50 A Split #1 1.75 A Split #2 0.75 B Standard 5.75 Split #1 12.00 B Split #2 11.00 It can be seen that the split-head design of the present invention provided a substantial decrease in the measured vibration of the projection mirror. Additional subjective tests based on observation of the vibration of the projected image have been consistent with these objective results, showing that the novel split-head design solves the problem of image vibration when a LCD panel causes unacceptable vibration when used with an overhead projector.

Claims: A projection head for attachment to a head supporting arm (18) of an overhead projector (10), comprising: a head bracket (30) adapted to support a projection lens (22) and a projection mirror (24), the lens and the mirror being positioned so as to be in front of the supporting arm; characterised by a rearwardly extending portion (60) the head bracket located so as to be behind the support arm; and a counterweight (62) attached to the rearwardly extending portion of the head bracket.

Inventors: BALDWIN DWIGHT G, DARLAND MICHAEL E, RODRIGUEZ ERNESTO M, ZHAO XIAOWEI, BALDWIN, DWIGHT G., DARLAND, MICHAEL E., RODRIGUEZ, ERNESTO M., ZHAO, XIAOWEI
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: G03B21
IPC codes: G03B21