Patent Application
20240085689
Telescopes, cameras, and other optical instruments typically employ mounts for providing stable bases for observations and photography. Such mounts allow for movement of optical instruments about two orthogonal axes to facilitate pointing and tracking. For example, so-called altitude-azimuth (Alt-Az) mounts allow for up-down (altitude) and side-to-side (azimuth) movement, whereas equatorial mounts allow for movement in celestial declination (celestial latitude) and in right ascension (celestial longitude). Some mounts are adaptable and can be switched between an Alt-Az mode and an equatorial mode.
To allow for fine positioning and control, mounts for optical instruments typically provide reduction gears on their axes. The gears enable users to rotate the instruments about their axes with high precision. Many conventional mounts employ worm gears for this purpose. A worm gear includes a worm and a worm wheel. The worm is rotatable, e.g., by a motor or manual knob, and has teeth that engage corresponding teeth of the worm wheel at right angles. The worm wheel concentrically engages a shaft that forms an axis of the mount. Rotating the worm causes the worm wheel and thus the shaft to rotate. Worm-gear arrangements can achieve high gear reduction, with one complete turn of the worm rotating the worm wheel, and thus the shaft, by as little as a fraction of a degree.
Instead of using worm gears for fine positioning and control, some mounts for optical instruments instead use harmonic gears, also known as strain wave gears. A harmonic gear typically includes three components, (1) an outer circular spline with inwardly pointing teeth, (2) a middle, flexible (flex) spline with outwardly pointing teeth that can engage with teeth of the outer spline, and (3) an inner oval that has no teeth. The middle flex spline has fewer teeth than the outer spline, and the inner oval pushes diametrically opposed teeth of the middle spine so that they engage with teeth of the outer spline at only two opposing regions. As the inner oval rotates, it causes the opposing regions of teeth engagement to follow the major axis of the oval. But owing to the fact that the middle spline has fewer teeth than the outer spline, the middle spline rotates backwards, in a direction opposite that of the inner oval and by a much smaller amount. For instance, if the outer spline has 100 teeth and the middle spline has 98 teeth, then a complete rotation of the oval causes the middle spline to rotate backwards by the angular equivalent of two teeth, i.e., 7.2 degrees. Significant gear reduction can thus be achieved. In a typical arrangement, the inner oval is driven by a motor (or hand control) and the middle flex spline is coupled to the axis of the mount to be rotated.
Unfortunately, worm gears and harmonic gears both involve deficiencies. A mount that uses worm gears typically requires a counterweight when used in equatorial mode. The counterweight attaches to a declination shaft of the mount and balances a turning moment across the right-ascension axis. Without counterweights, worm gears on equatorial mounts may slip or prematurely wear. Counterweights tend to be heavy, however, and thus can impair portability. Counterweights also increase the overall size of the mounts. By contrast, harmonic gears often can be operated without counterweights and have virtually no backlash, but they typically are much costlier than worm gears. They also tend to have much larger periodic errors. As is known, “periodic errors” are positioning errors caused by inaccuracies in mechanical components of a gearing system. What is needed, therefore, is a mount that avoids at least some of the deficiencies of worm gears and at least some of the deficiencies of harmonic gears while benefiting from the respective advantages that each of them provides.
To address the above need at least in part, an improved technique provides a hybrid mount for an optical instrument. The hybrid mount employs a worm gear for angular positioning about a declination axis and a harmonic gear for angular positioning about a right-ascension axis. Advantageously, the hybrid mount can normally be operated without a counterweight, as the harmonic gear is able to safely resist the turning moment about the right-ascension axis. The hybrid mount is less costly than mounts employing two harmonic gears. It also provides low backlash on the right-ascension axis, which is typically the more critical axis when the mount is used for astrophotography.
Certain embodiments are directed to a hybrid mount for optical instruments. The hybrid mount includes a declination axle constructed and arranged to attach to an optical instrument, a declination unit having a worm gear disposed to rotate the declination axle, and a right-ascension unit having a harmonic gear disposed to rotate the declination unit about a right ascension axis. The right-ascension unit has an optical encoder configured to produce measurements of right ascension, and the measurements provide a basis for correcting a harmonic error produced by the harmonic gear.
Other embodiments are directed to a telescope that includes an optical tube assembly, a declination axle attached to the optical tube assembly, a declination unit having a worm gear disposed to rotate the declination axle, and a right-ascension unit having a harmonic gear disposed to rotate the declination unit about a right ascension axis. The right-ascension unit has an optical encoder configured to produce measurements of right ascension, said measurements providing a basis for correcting a harmonic error produced by the harmonic gear.
Still other embodiments are directed to a method of pointing a telescope mount. The method includes driving a worm of a worm gear to rotate the telescope mount in declination, driving a harmonic gear to rotate the telescope mount in right ascension, and correcting periodic errors in right ascension using an optical encoder.
The foregoing summary is presented for illustrative purposes to assist the reader in readily grasping example features presented herein; however, this summary is not intended to set forth required elements or to limit embodiments hereof in any way. One should appreciate that the above-described features can be combined in any manner that makes technological sense, and that all such combinations are intended to be disclosed herein, regardless of whether such combinations are identified explicitly or not.
The foregoing and other features and advantages will be apparent from the following description of particular embodiments, as illustrated in the accompanying drawings, in which like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments.
Embodiments of the improved technique will now be described. One should appreciate that such embodiments are provided by way of example to illustrate certain features and principles but are not intended to be limiting.
An improved technique provides a hybrid mount for an optical instrument. The hybrid mount employs a worm gear for angular positioning about a declination axis and a harmonic gear for angular positioning about a right-ascension axis. Advantageously, the hybrid mount can normally be operated without a counterweight, as the harmonic gear is able to safely resist the turning moment about the right-ascension axis. The hybrid mount is less costly than mounts employing two harmonic gears. It also provides low backlash on the right-ascension axis, which is typically the more critical axis when the mount is used for astrophotography.
Although the harmonic gear can make the hybrid mount susceptible to periodic error, some examples reduce or eliminate periodic error by providing a high-precision optical encoder on the right-ascension axis. For example, the high-precision optical encoder precisely measures the angle of the right-ascension axis and allows corrections to be made in software. In some examples, a control signal establishes a desired angle of the right-ascension axis and a motor drives the harmonic gear under feedback control, so that the angle measured by the precision optical encoder is made to precisely match the desired angle. Periodic errors in the harmonic gear are thereby rendered irrelevant under closed-loop operation.
In some examples, the motor that drives the right-ascension axis is a stepper motor, and the stepper motor drives the right-ascension axis via the harmonic gear. For example, the stepper motor has a shaft coupled to an oval spline of the harmonic gear, and a middle flex spline of the harmonic gear provides right-ascension rotation. The shaft of the stepper motor rotates in precise angular increments in response to input signals from a drive circuit.
In some examples, the stepper motor includes at least a first coil and a second coil, with each coil having two terminals by which the respective coil is driven by the drive circuit.
In some examples, an electronic brake is provided for the right-ascension axis by a switch that connects together the terminals of one of the coils of the stepper motor. For example, short-circuiting a coil of the stepper motor via a switch has the effect of increasing the resistance to rotation of the shaft of the stepper motor in response to externally-applied torque, thereby increasing the applied torque required to rotate the optical instrument in right-ascension. One switch may be provided for a single coil of the stepper motor, or multiple switches may be provided for respective coils. The switch or switches thus act as an electronically controllable brake, which may avoid the need for any mechanical brake on the right-ascension axis.
In some examples, resistance of the brake is established or varied by short-circuiting different numbers of coils. For example, a lower level of resistance may be established by short-circuiting the terminals of a single coil, and a higher level of resistance may be established by short-circuiting the respective terminals of two coils.
In some examples, a resistor is provided between the terminals of a coil, or a respective resistor is provided between the terminals of each of multiple respective coils) to establish a desired level of braking torque, i.e., torque required of an external force to rotate the hybrid mount in right-ascension. According to some examples, multiple resistors may be provided for the coil (or for each of multiple coils), and the resistors may be selectable to vary the desired torque. In some examples, a variable-resistance potentiometer may be used in place of multiple resistors for each coil. For example, a lower-resistance (higher-torque) setting may be used for heavier optical instruments, and a higher-resistance (lower-torque) setting may be used for lighter optical instruments. In some examples, a resistor is used in place of a switch, e.g., by being connected directly to the terminals of a coil. In other examples, a resistor is used along with a switch, e.g., in series with the switch.
The RA unit 1 includes components that are constructed and arranged to rotate the DEC unit 7 in right ascension 11. In accordance with improvements hereof, the components include a harmonic gear, also known as a strain wave gear. The harmonic gear includes an outer spline fixedly attached to the RA unit 1, a middle flex spline 2 fixedly attached to the DEC unit 7, and an inner oval spline driven by an RA motor. As the RA motor turns, the harmonic gear rotates the DEC unit 7 relative to the RA unit 1 and achieves substantial gear reduction. In some examples, the RA unit 1 also houses a power supply, electronic connectors, and various control circuitry for electronically controlling the hybrid mount 100, such as a microprocessor or microcontroller, electronic memory, communications circuitry, motor-drive circuitry, and the like. In other examples, at least some of the connectors and/or control circuitry is housed within the DEC unit 7 or is distributed between the RA unit 1 and the DEC unit 7. Connectors and control circuitry may also be housed within the base 10. In addition, some embodiments may employ a separate hand controller (not shown) for facilitating user control of the hybrid mount 100.
As further shown in
Advantageously, the hybrid mount 100 may be used in many instances without a counterweight. As shown in
The hybrid mount 100 as depicted is configured in equatorial mode, in which the mount, once polar aligned, can track celestial objects by rotating primarily in right-ascension only (i.e., one revolution per day). The same hybrid mount 100 can also be configured in Alt-Az mode, however. For example, the mount may be oriented vertically on the base 10. In this mode, altitude adjustments may be achieved via the DEC unit 7 and azimuth adjustments may be achieved via the RA unit 1. Thus, embodiments are not limited to equatorial applications.
The harmonic gear 300 further includes an inner (oval) spline 320, which may be coupled to the RA motor. For example, the inner spline 320 may be connected directly to a shaft of the RA motor, such that rotating the shaft also rotates the inner spline 320. In some examples, additional gears (not shown) may be provided between the shaft of the RA motor and the inner spline 320, for achieving additional gear reduction.
The harmonic gear 300 still further includes a middle (flex) spline 2, which is also depicted in
Preferably, the RA motor 460 is a stepper motor. As is known, stepper motors provide precise control over the angle of a motor shaft in fine increments. For the same reason, the DEC motor 430 may also be a stepper motor, although the DEC motor 430 alternatively may be implemented using a DC servo motor.
As further shown in
The components still further include an optical encoder 470 for measuring right ascension angle. For example, the optical encoder 470 has a first part coupled to a body of the RA unit 1 and a second part coupled to and coaxial with the middle (flex) spline 2 of the harmonic gear 300.
In accordance with further improvements hereof, the optical encoder 470 is preferably a high-precision optical encoder, i.e., an encoder with an angular resolution of no greater than 0.1 arc-seconds and preferably no greater than 0.02 arc-seconds. As explained more fully below, the high-precision optical encoder 470 enables the hybrid mount 100 to correct for large periodic errors, which may be inherent in the harmonic gear 300. Although the encoder 440 in the DEC unit 7 may also be high-precision, this is not required and typically is not justified based on cost or performance. Thus, the optical encoder 440 is preferably a standard encoder.
In some examples, as still further shown in
As shown in
In example operation, the control loop 500 receives a control signal that indicates a desired right-ascension angle WD, and the encoder 470 generates a measured right-ascension angle WM. The desired right-ascension angle WD is a prescribed value, typically provided by the controller 410.
Summer 510 receives both WD and WM and generates a difference between the two, WE, which represents an error signal. Control laws 520 process WE to generate a control value, WC, e.g., by applying multiplication, integration, and/or differentiation for establishing desired dynamic performance and closed-loop stability. The control value WC then drives the stepper motor 460. Note that the RA motor driver 450 is assumed to be present but is omitted from
The depicted arrangement reduces to zero or near zero any periodic error introduced by the harmonic gear 300. Because feedback ensures that WM precisely matches WD, any errors introduced by the harmonic gear 300 have negligible effect on the output RA angle. The main source of error in control loop 500 is rather the encoder 470, but the encoder 470 is precise by design.
Rather than relying on a mechanical brake, we have recognized that the stepper motor 460 itself may be used as a type of brake. More specifically, a typical stepper motor includes at least two coils for controlling its shaft rotation. Normally, motor drive circuits energize the coils to induce a desired degree and direction of rotation. Stepper motors can also act as generators, however, when their shafts are subjected to externally-applied rotation. In this generator mode, the torque required to turn the shaft increases markedly when at least one of the coils is short-circuited. Thus, by short-circuiting one or more coils of the stepper motor 460, the shaft of the stepper motor 460 is made to resist rotation and thus acts as a type of brake. When such resistance is combined with gear multiplication (caused by back-driving the reducing harmonic gear), a large resisting torque can be created, such that significant torque is required to rotate the hybrid mount 100 in right ascension. Such resisting torque is preferably larger than the torque induced by the optical instrument 8 held by the hybrid mount 100 without a counterweight, such that the hybrid mount 100 does not slump down under the weight of the optical instrument 8.
To realize an electronic brake, the above-described RA brake signal 480 controls a pair of Form-C (single pole, double throw) switches 630. The switches 630 may be realized as relays, solid-state switches, discrete transistor switches, or switches of any other type. Each of the switches 630 (e.g., 630a or 630b) has a pole that connects to a first terminal T1 of a respective coil, a first contact that receives a drive signal (D2 or D4), and a second contact that connects (e.g., via a wire or circuitboard trace) to a second terminal T2 of the respective coil. When the switches 630 are in the Down position (as shown), all of the drive signals D1-D4 reach the coils 630 and operation of the stepper motor 460 is normal. But when the switches 630 are in the Up position, the drive signals can no longer form complete circuits and the coils 630 are short-circuited. As indicated above, short-circuiting the coils 630 turns the stepper motor 460 into a type of brake. Thus, electronic braking may be realized by controlling switches 630 using brake signal 480.
Although two switches 630a and 630b are shown, some embodiments may use only a single switch. A single switch may be preferred, for example, in cases in which short-circuiting a single coil 630 is sufficient for establishing the desired braking torque. Also, greater than two switches 630 may be used, as desired, in stepper motors that have greater than two coils. In some examples, a switch 630 is provided on each coil of the stepper motor 460, and the switches may be independently controlled, such that more switches may be used when higher braking torque is desired and fewer switches may be used when lower braking torque is desired.
Preferably, the switches 630 have a normally-closed position, which is the Up position in which the coils 630 of the respective switches are short-circuited. In this manner, the electronic brake engages automatically when power is turned off, protecting the optical instrument from damage. It is noted that no external source of power is needed for operating the electronic brake.
In some examples, resistors 660 may be provided as variable-resistance elements, such as potentiometers, which allow resistance to be varied (e.g., via a knob or set screw). In other examples, banks of resistors may be provided, in place of individual resistors, in which one of multiple resistors may be selected at a time for achieving a desired braking torque. Any number of switches 630 and resistors 660 may be provided, e.g., for only one coil or for each of multiple coils.
Although not shown, still another implementation of an electronic brake may be realized within the RA motor driver 450 itself. For example, the RA motor driver 450 may be configured automatically to short circuit D1 to D2 and to short-circuit D3 to D4 when power is turned off or in response to a brake command 480. Such an arrangement would avoid the need for any additional parts.
At 710, the controller 410 drives a worm 5 of a worm gear 6 to rotate the optical instrument 8 in declination. At 720, the controller 410 drives a harmonic gear 300 to rotate the optical instrument 8 in right ascension. At 730, the controller 410 corrects periodic errors in right ascension, e.g., those introduced by the harmonic gear 300, using a high-precision optical encoder coupled to the right ascension axle. At 740, the controller 410 applies an electronic brake to the right-ascension axle by creating a low-impedance path (e.g., a short-circuit or resistor) across at least one coil of the stepper motor 460.
An improved technique has been described which provides a hybrid mount for an optical instrument. The hybrid mount employs a worm gear for angular positioning about a declination axis and a harmonic gear for angular positioning about a right-ascension axis. Advantageously, the hybrid mount can normally be operated without a counterweight, as the harmonic gear is safely able to resist the turning moment about the right-ascension axis. The hybrid mount is less costly than mounts employing two harmonic gears. It also provides low backlash on the right-ascension axis, which is typically the more critical axis when the mount is used for astrophotography.
Having described certain embodiments, numerous alternative embodiments or variations can be made. For example, although the hybrid mount 100 is shown as having a particular design (similar to a German equatorial mount), embodiments are not limited to German equatorial mounts or their variants. Rather, embodiments may be used with other types of equatorial mounts or with Alt-Az mounts.
Also, although the electronic brake has been described in connection with a hybrid mount that includes a worm gear for declination and a harmonic gear for right-ascension, the electronic brake may also be used in mounts that use harmonic gears on both axes. In such embodiments, an electronic brake like any of the ones described above may also be used on the declination axis.
Further, although features have been shown and described with reference to particular embodiments hereof, such features may be included and hereby are included in any of the disclosed embodiments and their variants. Thus, it is understood that features disclosed in connection with any embodiment are included in any other embodiment.
Further still, the improvement or portions thereof may be embodied as a computer program product including one or more non-transient, computer-readable storage media, such as a magnetic disk, magnetic tape, compact disk, DVD, optical disk, flash drive, solid state drive, SD (Secure Digital) chip or device, Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA), and/or the like (shown by way of example as medium 750 in
As used throughout this document, the words “comprising,” “including,” “containing,” and “having” are intended to set forth certain items, steps, elements, or aspects of something in an open-ended fashion. Also, as used herein and unless a specific statement is made to the contrary, the word “set” means one or more of something. This is the case regardless of whether the phrase “set of” is followed by a singular or plural object and regardless of whether it is conjugated with a singular or plural verb. Also, a “set of” elements can describe fewer than all elements present. Thus, there may be additional elements of the same kind that are not part of the set. Further, ordinal expressions, such as “first,” “second,” “third,” and so on, may be used as adjectives herein for identification purposes. Unless specifically indicated, these ordinal expressions are not intended to imply any ordering or sequence. Thus, for example, a “second” event may take place before or after a “first event,” or even if no first event ever occurs. In addition, an identification herein of a particular element, feature, or act as being a “first” such element, feature, or act should not be construed as requiring that there must also be a “second” or other such element, feature or act. Rather, the “first” item may be the only one. Also, and unless specifically stated to the contrary, “based on” is intended to be nonexclusive. Thus, “based on” should be interpreted as meaning “based at least in part on” unless specifically indicated otherwise. Although certain embodiments are disclosed herein, it is understood that these are provided by way of example only and should not be construed as limiting.
Those skilled in the art will therefore understand that various changes in form and detail may be made to the embodiments disclosed herein without departing from the scope of the disclosure.
This application claims the benefit of U.S. Provisional Application No. 63/406,032, filed Sep. 13, 2022, the contents and teachings of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63406032 | Sep 2022 | US |
