Patent Publication Number: US-2022221139-A1

Title: Absolute position sensing system for stepper motor mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 63/168,913 filed Mar. 31, 2021 by Pavel Juřík, et al. entitled, “Absolute Position Sensing System for Stepper Motor Mechanism”, which is incorporated by reference herein as if reproduced in its entirety. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     The disclosure generally relates to automated luminaires, and more specifically to a method for sensing an absolute position of stepper motors controlling functions of an automated luminaire. 
     BACKGROUND 
     Luminaires with automated and remotely controllable functionality (referred to as automated luminaires) are well known in the entertainment and architectural lighting markets. Such products are commonly used in theatres, television studios, concerts, theme parks, night clubs, and other venues. A typical automated luminaire provides control from a remote location of the pan and tilt functions of the luminaire allowing an operator to control the direction the luminaire is pointing and thus the position of the light beam on the stage or in the studio. Often, this position control is done via control of the luminaire&#39;s position in two orthogonal rotational axes usually referred to as pan and tilt. Many automated luminaires additionally or alternatively provide control from the remote location of other parameters such as intensity, focus, zoom, beam size, beam shape, and/or beam pattern of light beam(s) emitted from the luminaire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in conjunction with the accompanying drawings in which like reference numerals indicate like features. 
         FIG. 1  presents a schematic view of a luminaire system according to the disclosure; 
         FIG. 2  presents a block diagram of a control system according to the disclosure; 
         FIG. 3  presents a flow chart of a process according to the disclosure for establishing an absolute position of a load of an automated luminaire by determining an absolute position of a stepper motor associated with the load; 
         FIG. 4  presents an exploded view of a first absolute position sensing system according to the disclosure; 
         FIG. 5  presents a second exploded view of the first absolute position sensing system of  FIG. 4 ; 
         FIG. 6  presents an exploded view of a pan movement system including a second absolute position sensing system according to the disclosure; 
         FIG. 7  presents an isometric view of a pan movement system and tilt movement system of an automated luminaire, including absolute position sensing systems according to the disclosure; 
         FIG. 8  presents a schematic view of a rotary indexing system for motor shaft rotation counting according to the disclosure; and 
         FIG. 9  presents a view of focus and zoom movement systems of the automated luminaire according to the disclosure. 
     
    
    
     SUMMARY 
     In a first embodiment, a luminaire includes a luminaire mechanism, a stepper motor, an absolute multi-turn rotational position sensing system, and a control system. The stepper motor has a motor shaft that is mechanically coupled to the luminaire mechanism. The stepper motor is configured to move the luminaire mechanism from a first position to a second position. The absolute multi-turn rotational position sensing system senses an absolute multi-turn position of the motor shaft and includes a first absolute rotational sensor and a second absolute rotational sensor. The first absolute rotational sensor detects an absolute rotational position of a cam indexer directly coupled to the motor shaft. The second absolute rotational sensor detects an absolute rotational position of an indexer wheel that is mechanically coupled to the cam indexer. The indexer wheel and cam indexer are configured to rotate the indexer wheel by a predetermined amount in response to one full rotation of the cam indexer. The control system is electrically coupled to a data link, the stepper motor, the first absolute rotational sensor, and the second absolute rotational sensor. The control system receives a first signal from the first absolute rotational sensor, where the first signal includes first information relating to the absolute rotational position of the cam indexer. The control system also receives a second signal from the second absolute rotational sensor, where the second signal includes second information relating to the absolute rotational position of the indexer wheel. The control system determines an absolute position of the luminaire mechanism based on the first information and the second information. The control system receives a luminaire mechanism command via the data link, where the luminaire mechanism command specifying a commanded position for the luminaire mechanism, and the control system causes the stepper motor to rotate to move the luminaire mechanism to the commanded position based on the absolute position of the luminaire mechanism. 
     In a second embodiment, a method of controlling a position of a luminaire mechanism of a luminaire includes detecting an absolute rotational position of a cam indexer that is directly coupled to a motor shaft of a stepper motor, where the motor shaft is mechanically coupled to a luminaire mechanism and moves the luminaire mechanism from a first position to a second position. The method also includes detecting an absolute rotational position of an indexer wheel that is mechanically coupled to the cam indexer, where the indexer wheel and the cam indexer are configured to rotate the indexer wheel by a predetermined amount in response to one full rotation of the cam indexer. The method further includes determining an absolute position of the luminaire mechanism based on the absolute rotational position of the cam indexer and the absolute rotational position of the indexer wheel. The method also includes receiving a luminaire mechanism command via a data link, where the luminaire mechanism command specifies a commanded position for the luminaire mechanism and causing the stepper motor to rotate to move the luminaire mechanism to the commanded position based on the absolute position of the luminaire mechanism. 
     In a third embodiment, an absolute multi-turn rotational position sensing system includes a first absolute rotational sensor and a second absolute rotational sensor. The absolute multi-turn rotational position sensing system senses an absolute position of a motor shaft of a stepper motor and a number of full rotations of the motor shaft. The first absolute rotational sensor detects an absolute rotational position of a cam indexer that is directly coupled to the motor shaft. The second absolute rotational sensor detects an absolute rotational position of an indexer wheel that is mechanically coupled to the motor shaft, where the indexer wheel is configured to rotate by a predetermined amount in response to one full rotation of the cam indexer. 
     DETAILED DESCRIPTION 
     Preferred embodiments are illustrated in the figures, like numerals being used to refer to like and corresponding parts of the various drawings. 
     A luminaire includes a luminaire mechanism, a stepper motor, an absolute multi-turn rotational position sensing system, and a control system. The stepper motor moves the luminaire mechanism from a first position to a second position. The absolute multi-turn rotational position sensing system includes absolute rotational sensors that detect absolute positions of (i) a cam indexer on the motor shaft and (ii) an indexer wheel coupled to the cam indexer. The indexer wheel rotates by a predetermined amount in response to one full rotation of the cam indexer. The control system determines an absolute position of the luminaire mechanism based on information from the absolute rotational sensors relating to the positions of the cam indexer and the indexer wheel. The control system receives a commanded position for the luminaire mechanism and causes the stepper motor to move the luminaire mechanism to the commanded position based on the absolute position of the luminaire mechanism. 
       FIG. 1  presents a schematic view of a luminaire system  10  according to the disclosure. The luminaire system  10  includes a plurality of luminaires  12  according to the disclosure. The luminaires  12  each contains on-board a light source, one or more of color changing systems, light modulation devices, and pan and/or tilt systems to control an orientation of a head of the luminaire  12 . Mechanical drive systems to control parameters of the luminaire  12  include motors or other suitable actuators coupled to a control system, as described in more detail with reference to  FIG. 2 , which is configured to control the motors or other actuators. 
     The luminaire  12  includes a luminaire head  12   a  mounted in a yoke  12   b . The yoke  12   b  rotates around a pan axis of rotation (vertical in the plane of the page in  FIG. 1 ). The luminaire head  12   a  rotates within the yoke  12   b  around a tilt axis of rotation (perpendicular to the page in  FIG. 1 ). 
     In addition to being connected to mains power either directly or through a power distribution system, the control system of each luminaire  12  is connected in series or in parallel by a data link  14  to one or more control desks  15 . Upon actuation by an operator, the control desk  15  sends control signals (such as commands) via the data link  14 , where the control signals are received by the control system of one or more of the luminaires  12 . The control systems of the one or more of the luminaires  12  that receive the control signals may respond by changing one or more of the parameters of the receiving luminaires  12 . The control signals are sent by the control desk  15  to the luminaires  12  using DMX-512, Art-Net, ACN (Architecture for Control Networks), Streaming ACN, or other suitable communication protocol. 
     The luminaire head  12   a  comprises an optical system comprising one or more luminaire mechanisms, each of which includes one or more optical devices such as gobo wheels, effects wheels, and color mixing (or other color changing) systems, as well as prism, iris, shutter, and lens movement systems. The term luminaire mechanisms further includes a pan mechanism configured to rotate the yoke  12   b  relative to a fixed portion of the luminaire  12  and a tilt mechanism configured to rotate the luminaire head  12   a  relative to the luminaire yoke  12   b . Some or all of the luminaire mechanisms include stepper motors or other rotating actuators to cause movement of their associated optical device(s). 
     The stepper motors and/or rotating actuators of the luminaire  12  are electrically coupled to and under the control of the control system of the luminaire  12 . Such luminaire mechanisms and their motors and/or actuators under the control of the control system of the luminaire  12  may be referred to as load movement systems. The control system of the luminaire  12  receives signals from the control desk  15  or other source via the data link  14 . At least some of the signals include luminaire mechanism commands that comprise information identifying a luminaire mechanism and specifying a commanded position for the luminaire mechanism. In response to receipt of such command signals, the control system of the luminaire  12  moves the identified luminaire mechanism to the commanded position, using an absolute position sensing system according to the disclosure. In some embodiments, the data link  14  has a plurality of logical channels and the luminaire mechanism is identified by the identifier(s) of the logical channel(s) over which the luminaire mechanism command is received. 
     The luminaires  12  may include stepper motors to control the movement of the yoke, head, and/or internal optical systems. A stepper motor has a fixed number of full steps in each 360° full revolution, in some cases a stepper motor has 200 steps each of 1.8°, but other motors may have other numbers of full steps. The stepper motor can be moved through one or more of these full steps in either direction by a motion control system. 
     Absolute and incremental position control of a motor and its load may be understood through the example of an hour hand (a load) of a clock attached to the stepper motor. An incremental motion control system (IMCS) is able to move the clock hand by 30 degrees (or one hour on the clock face), but the IMCS does not know whether it has moved the clock hour hand from, for example, 1:00 to 2:00 or from 7:00 to 8:00. For a large or heavy clock hand, the motor axle might be coupled to the clock hand by a 2:1 belt drive system, where two full rotations of the motor axle are required to rotate the clock hand through 360 degrees (or one full rotation). For such a system, when the IMCS turns the motor through one full rotation, it knows that it has moved the clock hand through 180 degrees but it will not know whether it has moved the clock hand from, for example, 2:00 to 8:00 or from 9:00 to 3:00. 
     If a light sensor were placed in the clock face at the 12:00 position to signal the IMCS when the clock hand was blocking light to the sensor, the IMCS would be referred to as an indexed IMCS. The indexed IMCS can calibrate itself by rotating the clock hand until the signal is received from the sensor and, from that “index position,” the indexed IMCS can reliably move the clock hand to 3:00 by rotating the motor by 180 degrees (rotating the clock hand by 90 degrees) in the clockwise direction from the index position and then to back to 1:00 by rotating the motor 120 degrees (rotating the clock hand by 90 degrees) in the counterclockwise direction from its 3:00 position. 
     Unlike a clock hand, which is able to rotate continuously about the clock face, some loads controlled by an indexed IMCS may have limits to the extent of the load&#39;s motion (e.g., a rotating load that only rotates through 270 degrees). The index position of such a load may be established by moving the load in a first direction into a limit switch, sensor (as above), a physical end stop, or other technique to reach a known physical position of the load. In some such indexed IMCSs, an opposite extent of motion (an opposite known position) is established by moving the load in an opposite direction into a second limit switch, sensor (as above), or a physical end stop. 
     Either a stepper motor or a servo motor may be used in an IMCS (or indexed IMCS). A stepper motor is inherently an indexed device, because it moves in steps of known size (known angle of rotation) in a known direction under the control of the IMCS. A servo motor is a free-running motor whose direction and rotational velocity is controlled by its MCS, which requires positional feedback on the motor or load to predictably control the position of the load. Where the feedback is provided by a position encoder that senses only increments of angular rotation, the servo MCS may be considered an incremental MCS. 
     In contrast, an absolute MCS (AMCS) has information that is available at startup relating the current physical position of the motor and/or load. Such an encoder may include a patterned object (e.g., a disc for a rotary encoder or a tape scale for a linear encoder) attached to the load and a sensor capable of reading the pattern currently under the sensor and unambiguously determining therefrom the current actual physical position of the load. Such patterned objects typically have fine-grained details to permit the sensor to read the current position to a high degree of accuracy. Either a stepper motor or a servo motor may be used in an AMCS. 
     In some stepper motor motion control systems, a stepper motor may be positioned between its full steps by using a technique called microstepping. The motion control system may cause the stepper motor to move to a static position (microstep) between two adjacent full steps by rapidly alternating power applied to the motor between the signal that would cause the motor to move to a first of the adjacent full steps and the signal that would cause the motor to move to a second of the adjacent full steps, allowing very precise positioning of the motor and luminaire mechanism. The duty cycle between the alternating steps controls a current microstep position of the motor between the two full steps. Unlike the incremental full steps of the stepper motor, the position of a microstep between the adjacent full steps is under absolute control of the motion control system and is determined by the duty cycle between the alternating steps, thus the motion control system reliably knows which microstep the motor is on between the adjacent full steps. 
     When used in an automated luminaire  12 , the indexing process of an indexed IMCS described above may have disadvantages for the user: (i) the process may take a significant amount of time to perform, in some cases tens of seconds; (ii) the luminaire mechanism may be noisy while performing the process; and (iii), in the case of pan and tilt, the process may cause a significant amount of movement of the head as it rotates to its known fixed position(s). Similarly, an AMCS may have disadvantages: (i) the patterned object may have to be replaced if it is damaged by, for example, transport or rough handling of the luminaire  12 , or (ii) the luminaire  12  may have to be partially disassembled for cleaning if the pattern becomes obscured by a buildup of dirt, oil, or other substances that may occur when the automated luminaire is used in harsh environments. 
     An absolute position sensing system according to the disclosure provides a method for determining an absolute position of a stepper motor and its associated load in an automated luminaire at startup without such disadvantages. In particular, an absolute position sensing system according to the disclosure requires little or no movement or time to establish the position of the stepper motor and its associated load. 
       FIG. 2  presents a block diagram of a control system (or controller)  200  according to the disclosure. The control system  200  is suitable for use to control the stepper motor movement systems of an automated luminaire according to the disclosure. The control system  200  is also suitable for controlling the light source, color changing devices, light modulation devices, pan and/or tilt systems, load movement systems, and other control functions of the luminaires  12 . 
     The control system  200  includes a processor  202  electrically coupled to a memory  204 . The processor  202  is implemented by hardware and software. The processor  202  may be implemented as one or more Central Processing Unit (CPU) chips, cores (e.g., as a multi-core processor), field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), and digital signal processors (DSPs). 
     The processor  202  is further electrically coupled to and in communication with a communication interface  206 . The communication interface  206  is coupled to, and configured to communicate via, the data link  14 . The processor  202  is also coupled via a control interface  208  to one or more sensors, motors, actuators, controls, and/or other devices. The processor  202  is configured to receive control signals from the data link  14  via the communication interface  206  and, in response, to control systems and mechanisms of the luminaire  12  via the control interface  208 . 
     The processor  202  is further electrically coupled to and in communication with motors  210 , optional motor brakes  212 , motor rotation counter position sensors  214 , and motor shaft position sensors  216 . The processor  202  is configured to receive control signals from the data link  14  via the communication interface  206  and, in response, to control the motors  210  and the optional motor brakes  212 , based on signals from the motor rotation counter position sensors  214  and the motor shaft position sensors  216 , to control physical positions of systems and mechanisms (or loads) of the luminaire  12 . In some embodiments, a value read from one of the motor rotation counter position sensors  214  is an absolute rotational position (in degrees or radians) of an indexer wheel (as described below) and may be converted into a number of complete rotations of a motor based on a number of teeth on the indexer wheel. In some embodiments, a value read from one of the motor shaft position sensors  216  is an absolute rotational position of the motor shaft (in degrees or radians) and may be converted into a step position of the motor based on a number of steps in one full rotation of the motor shaft. 
     The control system  200  is suitable for implementing processes, module control, optical device control, pan and tilt movement, parameter control, motor control, position sensor control, brake control, and other functionality as disclosed herein, which may be implemented as instructions stored in the memory  204  and executed by the processor  202 . The memory  204  comprises one or more disks and/or solid-state drives and may be used to store instructions and data that are read and written during program execution. The memory  204  may be volatile and/or non-volatile and may be read-only memory (ROM), random access memory (RAM), ternary content-addressable memory (TCAM), and/or static random-access memory (SRAM). 
       FIG. 3  presents a flow chart of a process  300  according to the disclosure for establishing an absolute position of a load of an automated luminaire by determining an absolute position of a stepper motor associated with the load. A stepper motor controlling, for example, pan movement of the luminaire  12  may have a geared or pulley driven connection to a load and thus the motor shaft may rotate multiple times while the load moves through its full range of motion. In some embodiments, a controller of the stepper motor has the capability of microstepping multiple smaller steps in between the full steps. These microsteps do not need additional sensing as they are controlled by a signal from the controller and, thus, the current microstep position is known to the controller. 
     The absolute position sensing system according to the disclosure takes into account these two (or three) parameters in order to determine a current absolute position of the load. The parameters are: an absolute rotational position of the stepper motor shaft, an absolute rotational position of a device indicating the rotation number that the motor shaft is on, and (optionally) a current microstep position. In steps of the process  300 , after the luminaire  12  is powered up in step  302 , the processor  202  reads the parameter values. The motor shaft position sensor  216  and the motor rotation counter position sensor  214  are physical sensors mechanically attached to the motor system, and their current values are read in steps  304  and  306  respectively. In optional step  308 , where a control system provides microstepping control of the stepper motor, motor microstep information is read from the memory  204  or inferred from a high-resolution absolute rotational position of the cam indexer. The combination of the parameters&#39; values (in some embodiments along with stored values such as gear ratios or pulley ratios) describes an absolute position of the stepper motor and enables the processor  202  to determine the current absolute position of the load, in step  310 . The absolute position sensing system according to the disclosure is configured to make this determination at power up from an initial, static value of the absolute rotational position of the stepper motor shaft and an initial, static value of the motor rotation counter position, e.g., without requiring movement of the load in an indexing process as described above for an indexed IMCS or other movement of the load. 
     In an example, in a mechanism that moves a load through 540 degrees of rotation, using an 8:1 drive mechanism, the stepper motor will rotate 12 full rotations in moving the load through its full 540 degrees of rotation. Assume the motion control system creates 100 microsteps between pairs of full steps. In such a system one full motor rotation rotates the load through 540/8=67.5 degrees; one degree of motor shaft rotation rotates the load through 67.5/360=0.1875 degrees; and one microstep rotates the load through 0.1875/100=0.001875 degrees. Further assume (for simplicity of the example) that the motor rotation counter position value, motor shaft position value, and microstep value are all zero at one end of travel of the load. Under such assumptions, if the processor  202  reads a value of 13 degrees from the motor shaft position sensor  216 , a value of 1 from the motor rotation counter position sensor  214 , and has a stored microstep value of 33 (three values describing an absolute position of the stepper motor), the processor  202  determines that the load is at an absolute position of (1*67.5)+(13*0.1875)+(33*0.001875)=70 degrees from the end of travel position. 
       FIG. 4  presents an exploded view of a first absolute position sensing system  400  according to the disclosure. The first absolute position sensing system  400  comprises a rotary indexing system  406  and a sensor assembly  410 . The rotary indexing system  406  comprises a cam indexer  407  and an indexer wheel  408 . In other embodiments, other suitable rotary indexing systems may be used. The first absolute position sensing system  400  senses an absolute multi-turn rotational position of a stepper motor and is suitable for use in a tilt system (or other stepper-driven system) of the luminaire  12 . A stepper motor  402  has a motor shaft  412  which protrudes from opposite sides of the stepper motor  402 . In other embodiments, the motor shaft  412  may extend from only a single side of the stepper motor  402 . One end of the motor shaft  412  is coupled to a drive belt  404  or to another suitable driving mechanism such as gears or a linear actuator. In other embodiments, for applications other than pan or tilt, the motor shaft  412  may be coupled to a linear actuator or other drive mechanism. 
     The same end of the motor shaft  412  is further directly coupled to the cam indexer  407 . The cam indexer  407  comprises a gear having a single tooth. As the cam indexer  407  rotates through one complete rotation of the motor shaft  412 , it moves the indexer wheel  408  through one tooth of rotation. The indexer wheel  408  is a gear mechanically coupled to the motor shaft  412  via the cam indexer  407  and configured to rotate by a predetermined amount in response to one full rotation of the motor shaft  412 . As such, the rotational position of the indexer wheel  408  provides an indication of an integral number of full rotations of the motor shaft  412 . 
     Absolute rotational positions of both the cam indexer  407  and the indexer wheel  408  are measured using sensors in the sensor assembly  410 . These sensors may be magnetic, optical, mechanical, inductive, resistive or any other type of absolute position rotational sensor. The sensors may have a greater or lesser degree of resolution. For example, a sensor may read cam indexer position in steps or fractions of a step, or indexer wheel position in degrees or in fractions of a degree. 
     In the embodiment shown in  FIG. 4 , the sensors are magnetic. A first magnet is fixed to the center of cam indexer  407  and a second magnet to the center of indexer wheel  408 . These magnets align with first and second magnetic sensors (not shown) of sensor assembly  410 , which are configured to read the absolute positions of their associated magnets. 
     In some embodiments, one or both of the first and second magnets are two-pole diametric magnets with their magnetic poles oriented orthogonally to the axis of rotation of the cam indexer  407  or the indexer wheel  408 . In other embodiments, one or both of the first and second magnets may be bar magnets mounted across an axis of rotation of the cam indexer  407  or the indexer wheel  408 , with the magnetic pole of the bar magnet oriented orthogonal to its axis of rotation. In both such embodiments, the first and second magnetic sensors of the sensor assembly  410  may comprise magnetic rotary position sensors that produce an output signal representing an absolute rotational position of a two-pole magnet. 
     In some applications, the total movement of the yoke  12   b  in the pan direction is 540 degrees from a first end of travel to a second end of travel. In some such applications, gear ratios and pulley ratios require the motor shaft  412  to turn through 12 full rotations to move the yoke  12   b  through the full 540 degrees of rotation. Thus, the cam indexer  407  will make 12 full rotations and move the indexer wheel  408  by 12 teeth of rotation as the yoke  12   b  moves from its position at one end of travel to its position at the other end of travel. In such applications, the indexer wheel  408  is designed with at least 13 teeth, so that its absolute position of angular rotation at one end of travel is distinct from its absolute position of angular rotation at the other end of travel. In general, where the motor shaft  412  and the cam indexer  407  make n full rotations, the indexer wheel  408  is designed with at least n+1 teeth. 
       FIG. 5  presents a second exploded view of the first absolute position sensing system  400  of  FIG. 4 . In this figure, rotated from the view in  FIG. 4 , it may be seen that the sensor assembly  410  includes a circuit board  518  that includes absolute rotational sensors  514  and  516 . The absolute rotational sensor  516  detects an absolute rotational position of the cam indexer  407  (and thus the motor shaft  412 ) and the absolute rotational sensor  514  detects an absolute rotational position of the indexer wheel  408  (and is thus configured as a rotation counter position sensor of the number of complete rotations of the motor shaft  412 ). Although the absolute rotational sensors  514  and  516  are here shown mounted to a circuit board  518 , the invention is not so limited and sensors  514  and  516  may be mounted to the structure of the luminaire or of the motor or another suitable mounting structure. 
     In some cases, a luminaire according to the disclosure is powered up for the first time after manufacture with one or more of the luminaire mechanisms in known mechanism positions. Where those one or more mechanisms have absolute position sensing systems according to the disclosure, the readings on the absolute rotational sensors  514  and  516  for each such mechanism may be recorded by the control system  200  as the rotation number of the motor shaft and the absolute rotational position of the motor shaft (respectively) corresponding to the known mechanism position. 
     If the luminaire according to the disclosure is powered up for the first time after manufacture with one or more of the luminaire mechanisms in unknown mechanism positions, then an indexing process as described above may be used to move the mechanisms into known positions. Then, the readings on the absolute rotational sensors  514  and  516  for each such mechanism may be recorded by the control system  200  as the rotation number of the motor shaft and the absolute rotational position of the motor shaft (respectively) corresponding to the known mechanism position. 
       FIG. 6  presents an exploded view of a pan movement system  650  including a second absolute position sensing system  600  according to the disclosure. The second absolute position sensing system  600  comprises a rotary indexing system  606  and a sensor assembly  610 . The rotary indexing system  606  comprises a cam indexer  607  and an indexer wheel  608 . The second absolute position sensing system  600  senses an absolute multi-turn rotational position of a stepper motor and is suitable for use in a pan system (or other stepper-driven system) of the luminaire  12 . A stepper motor  602  includes a motor shaft  612  which has first and second ends protruding from opposite sides of the stepper motor  602 . The first end of the motor shaft  612  is coupled to a drive belt  604  (or to another driving mechanism such as gears). In other embodiments, for applications other than pan or tilt, the motor shaft  612  may be coupled to a linear actuator or other drive mechanism. The drive belt  604  drives a pan gear  620  which is fixedly coupled to a head coupling  624  which is rotatably coupled to a chassis  622 . The second, opposing end of the motor shaft  612  is coupled to the cam indexer  607  and thereby to the indexer wheel  608 . Absolute rotational positions of both the cam indexer  607  and the indexer wheel  608  are measured using sensors in the sensor assembly  610 , which includes a circuit board  618 . 
       FIG. 7  presents an isometric view of a pan movement system  650  and tilt movement system  750  of the luminaire  12 , including absolute position sensing systems according to the disclosure. As shown in  FIG. 6 , the stepper motor  602  drives the pan movement of the yoke  12   b  (and thereby the luminaire head  12   a ) of the luminaire  12  through head coupling  624  and utilizes the second absolute position sensing system  600  to determine absolute rotational positions of both the motor shaft  612  (via the cam indexer  607 ) and the indexer wheel  608  (neither is shown in  FIG. 7 ). Similarly, a tilt stepper motor  702  drives the tilt movement of the luminaire head  12   a  of the luminaire  12  through a drive belt  704  (or another driving mechanism such as gears) and a coupling  724  and utilizes an absolute position sensing system  700 , which includes a circuit board  718 , to determine absolute rotational positions of both a shaft of the tilt stepper motor  702  and an associated indexer wheel (not shown) within the absolute position sensing system  700 . The absolute position sensing system  700  is coupled to the same end of the shaft of the tilt stepper motor  702  as the tilt driving gear, while the second absolute position sensing system  600  is coupled to the opposite end of the motor shaft  612  of the stepper motor  602  than the pan driving gear. In practice the absolute position sensing systems may be connected to either end of the motor shaft or may instead be connected through gears or drive belts to an ancillary pulley or shaft. 
       FIG. 8  presents a schematic view of a rotary indexing system  806  for motor shaft rotation counting according to the disclosure. The rotary indexing system  806  is suitable for use in the absolute position sensing systems of  FIGS. 4-7 . The rotary indexing system  806  includes a cam indexer  807  and an indexer wheel  808 .  FIG. 8  shows three sequential positions of the cam indexer  807  ( 807   a ,  807   b , and  807   c ) and the indexer wheel  808  ( 808   a ,  808   b , and  808   c ) as the cam indexer  807  rotates while coupled to a motor shaft. The single tooth on the cam indexer  807  in position  807   a  is moving towards a currently active tooth (shown colored black) on the indexer wheel  808  in position  808   a . The tooth of the cam indexer  807  in position  807   b  has contacted the currently active tooth on the indexer wheel  808  in position  808   b  and is rotating the indexer wheel  808 . The tooth of the cam indexer  807  in position  807   c  is exiting contact with the currently active tooth on the indexer wheel  808  in position  808   c  and thus the indexer wheel  808   c  stops rotating, having turned through one tooth of rotation. An absolute rotational sensor (e.g.,  516 ) senses an absolute rotational position of the cam indexer  807  by sensing a rotational position ( 817   a ,  817   b , and  817   c ) of a magnet  817  that is fixedly coupled to the cam indexer  807 . Similarly, an absolute rotational sensor (e.g.,  514 ) senses an absolute rotational position of the indexer wheel  808  by sensing rotational position ( 815   a ,  815   b , and  815   c ) of a magnet  815  that is fixedly coupled to the indexer wheel  808 . 
     An indexer wheel of the disclosure moves incrementally from one position to the next during a portion of the rotation of the motor shaft (in these embodiments, by the action of the single tooth of the cam indexer) and remains static during the remainder of one full rotation of the motor shaft. The indexer wheel does not rotate continuously along with the motor shaft, but rather indexes (or incrementally rotates) to a new position only as the motor shaft rotates through a portion of its rotation. In the example shown in  FIG. 8 , it may be seen that the indexer wheel  808 , having 16 teeth, moves 22.5 degrees for each full rotation of the cam indexer  807  (motor shaft). Thus, the indexer wheel  808  moves only during approximately 22.5 degrees of rotation of the cam indexer  807 , i.e., while the single tooth of the cam indexer  807  is in contact with the currently active tooth of the indexer wheel  808 . The indexer wheel  808  remains static during the remaining 337.5 degrees of rotation of the motor shaft and the cam indexer  807 . While the embodiment of the present disclosure comprises a single-tooth cam indexer and a multi-tooth indexer wheel, in other embodiments other suitable mechanisms may be used that provide incremental rotation of an indexer wheel once per full rotation of a motor shaft. 
     Use of the disclosed absolute rotational position sensing system is not limited to the pan and tilt systems of an automated luminaire and may be used for any of its motorized movement and optical functions.  FIG. 9  presents a view of a focus movement system  920  and zoom movement system  960  of the luminaire  12 . The focus movement system  920  includes an absolute position sensing system  900  according to the disclosure, a stepper motor  902 , and a belt system  904 . The stepper motor  902  drives movement of a lens  964  through the belt system  904  to focus a light beam projected by the luminaire  12 , and the absolute position sensing system  900  determines absolute rotational positions of both a cam indexer (not shown) and an indexer wheel (not shown). Similarly, a stepper motor  952  drives movement of a lens carriage  962  through a belt system  954  to change a beam angle of a light beam projected by the luminaire  12 , and the absolute position sensing system  950  determines absolute rotational positions of both a cam indexer  956  and an indexer wheel (not shown). 
     The pan movement system  650  of  FIGS. 6 and 7 , the tilt movement system  750  of  FIG. 7 , the focus movement system  920 , and the zoom movement system  960  of  FIG. 9 , under the control of the control system  200 , disclose embodiments of load movement systems according to the disclosure. In a load movement system according to the disclosure, an absolute multi-turn rotational position sensing system for sensing an absolute position of a motor shaft of the load movement system provides information allowing the control system  200  to determine an absolute position of the load. 
     While only some embodiments of the disclosure have been described herein, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the disclosure herein. While the disclosure has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the disclosure.