Abstract:
A position encoder provides one or more trigger outputs based on position signals developed within the encoder, in addition to traditional position output signals used by other system components such as a motion controller. The trigger outputs may be used directly by a triggered device, bypassing the motion controller and obviating any separate trigger generation electronics. The trigger output(s) can be fully synchronous with the encoder&#39;s position output signal(s) with essentially no latency or jitter, increasing accuracy and providing improved system performance. The trigger functionality can be incorporated in a variety of encoder types (e.g., absolute and incremental) and technologies (optical, magnetic, inductive etc.), and used in conjunction with different position output signal formats (e.g., quadrature, serial).

Description:
BACKGROUND 
     The present invention is related to the field of position encoders and their use in motion control applications. 
     There are many motion control applications in which an event is triggered once or multiple times at pre-determined precise positional states of a system. These events may include the activation of a device such as a laser, camera, radar, sonar, x-ray, etc. The motion of a stage relative to the device is controlled by a controller, and a position encoder is used to detect relative position and provide position feedback information to the controller. These systems may employ trigger generation electronics to create a digital signal that triggers the event(s) when the stage has particular position(s). The trigger generation electronics operate in response to signals from the controller, which in turn are generated based on the position feedback information from the encoder. The trigger generation electronics may be packaged separately from the controller, or in some cases together with the controller. 
     With the current technology, the controller deciphers position data from the encoder and outputs corresponding position data to the trigger generation electronics, which uses the position data to generate trigger(s) at desired position(s). There can be significant delay (or latency) between the detection of a position by the encoder and the generating of a trigger signal by the trigger generation electronics. This delay can contribute to inaccuracy in operation, because the stage continues to move during the response time of the controller and trigger generation electronics and thus the resulting event (e.g., firing of a laser) does not occur precisely at the desired position. Additionally, trigger outputs may not be accurately spaced due to “jitter” (variability in the response time relative to motion speed), so that the resulting events are irregularly spaced. 
     SUMMARY 
     It is desired to achieve greater accuracy in motion control applications which employ events generated at specific relative positions. 
     To address this goal, a position encoder is disclosed which can provide one or more trigger output signals based on position signals developed within the encoder. The trigger output signals are separate from the position output signals provided to a controller that controls motion in the system. The trigger output signals may be used directly by a triggered device, bypassing the controller and obviating any trigger generation electronics. The trigger output signals can be fully synchronous with the encoder&#39;s position output signal with essentially no latency or jitter, increasing accuracy and providing improved system performance. In addition, cost savings can be realized because separate trigger generation electronics are not be needed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention. 
         FIGS. 1 and 2  are block diagrams of systems employing motion control; 
         FIG. 3  is a block diagram of a position encoder; and 
         FIGS. 4 and 5  are examples of trigger signals generated by a position encoder. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example system employing motion control of a stage  10  along with position-based triggering of a device  12 . The motion of the motor-driven stage  10  relative to the device  12  is controlled by a controller  14  generating motor command signals  15 , and a position encoder  16  is used to detect relative position and provide position feedback information in the form of encoder position signals  17  to the controller  14  for use in the motion control function. Trigger generation electronics  18  creates a binary trigger signal  19  that triggers the event(s) when the stage  10  is at particular position(s). The trigger generation electronics  18  operates in response to controller position signals  21  from the controller  14 , which in turn are generated based on the encoder position signals  17  from the encoder  16 . 
     The device  12  may include a source of pulsed energy and the triggered event is the generating of a pulse of energy from the device  12 . Examples of such pulsed sources include a laser, a radar or sonar, and an x-ray generator. The device  12  may also include, either alternatively or in addition to a source, a receiver of input energy and the triggered event is receiving a predetermined unit of input energy. Examples of such receivers include a camera and a radar/sonar receiver. In the case of a camera, the event can be operating a shutter of the camera to capture an image. 
     As generally known in the art, there are two widely used types of encoder position output signals. One type is referred to as “quadrature” output or, more colloquially “A quad B”, and the other is a serial output. These are briefly described to provide context for the remainder of the present description. 
     Quadrature output typically employs two binary signal channels (A and B) whose outputs are nominally offset by ¼ of a cycle, with a state transition occurring alternately on the channels whenever the encoder  16  moves over a certain position increment. An example is provided below. The controller  14  employs a counter to track position of the stage. The counter may be reset when the stage  10  is brought to a reference position, and then it is incremented and decremented in response to the state transitions of the A and B signals, thereby tracking incremental position changes and maintaining a representation of the absolute position in the form of the count value. The direction of the stage  10  is determined by monitoring the relative phasing of the A and B channels while the stage  10  is in motion. 
     A serial output encoder generates a serial output word which represents the current position. Typically the serial output word is generated in response to a request input signal from a separate controller, e.g. controller  16 , which the controller asserts to the encoder when the controller needs to know the position of the stage  10 . 
     For either type of encoder, in the configuration of  FIG. 1  the controller  14  converts the encoder position signals  17  (whether quadrature or serial) into the controller position signals  21  which are provided to the trigger generation electronics  18 . In many cases the position information in the controller position signals  21  has a different resolution than the resolution conveyed by the encoder position signal  17 . The trigger generation electronics  18  compares the position information in the controller position signals  21  to predetermined position values which correspond to trigger points for the device  12 , which may be either programmed or hard-wired position values. When the position information matches a given predetermined position value, the corresponding trigger signal is asserted. As noted above, due to the delay through the controller  16  and trigger generation electronics  18 , motion of the stage  10  can cause an offset and jitter between the trigger signal  19  and the actual stage position, resulting in inaccurate system operation. 
       FIG. 2  shows an example system also employing motion control along with position-based triggering of a device, but exhibiting less offset and jitter between the triggering of the device and the desired trigger position than in the configuration of  FIG. 1 . The motion of the motor-driven stage  10  relative to the device  12  is again controlled by a controller  14  generating motor command signals  15  based on position feedback information in encoder position signals  17  from an encoder  20 . The encoder  20  also creates the trigger signal  19  which is provided to the device  12  to trigger the desired event(s) when the stage  10  is at particular position(s). The trigger signal  19  passes directly from the encoder  20  to the device  12 , bypassing the controller  14  and any separate trigger generating electronics and their attendant delay and jitter. 
     An encoder system generally includes a read head, a scale which is affixed to an element that moves relative to the read head, and electronics to provide a user interface. In the systems of  FIGS. 1 and 2 , for example, the scale may be affixed to the stage  10  while the item labeled “encoder” ( 16  or  20 ) represents the read head which has a fixed position relative to the device  12 . There are many types of encoder position sensing technologies including optical, magnetic, and inductive. There are absolute encoders, which have a unique pattern at every location on the scale, and incremental encoders, which have repeating patterns and typically a reference point pattern to be used as a home position at power up. 
       FIG. 3  shows a block diagram of a detector (or sensor) and electronics of the encoder  20 . The detector  22  is a transducer that generates electrical output position signals in response to patterns of energy it detects. For example, the detector  22  may be a photo-detector, magnetic detector, inductive detector, etc. which is in close proximity to a scale which is capable of modulating the light or magnetic field provided to the detector  22  according to changes in the position of the scale as it moves relative to the detector  22 . The signals from the detector  22  are conditioned with analog conditioning electronics  24  and then converted to digital signals by analog-to-digital conversion circuitry (A/D)  26 . Digital processing circuitry  28  converts the digitized detector signals from the A/D  26  to position signals and provides a digital interface for the user. The processing circuitry  28  may be embodied in any of several forms, for example as a field-programmable gate array (FPGA) or a digital signal processor (DSP). 
     In  FIG. 3 , the processing circuitry  28  is shown as including a position generator  30 , a trigger generator  32 , a quadrature state generator  34  and a parallel to serial converter  36 . In the case of using an FPGA or DSP, these components may advantageously be realized as modules or sections of firmware code. In alternative embodiments one or more of these components may be realized by hardware logic that does not employ instruction-set processing. 
     The position generator  30  may contain a number of subsections such as raw signal correction, signal conditioning features, home (also called an index or reference point) calculation, and position interpolation. The output of the position generator  30  is a generic parallel position word PARALLEL POS that is available to other independent firmware code sections within the FPGA or DSP, such as the components  32 - 36  as shown. The position information in the parallel position word is passed to other system elements as one or more encoder outputs.  FIG. 3  shows three options for output types. One or more trigger signals  19  are generated by the trigger generator  32 . Quadrature output signals A, B and INDEX (reference position) (collectively identified by ref  17 - 1 ) are generated by the quadrature generator  34 . Serial output signals DATA and CLOCK (collectively 17-2) are generated by the parallel to serial converter  36 . The three components  32 - 36  all use the same parallel position word from the position generator  30  as an input, and each converts the position information to a respective user-friendly output. The quadrature output signals  17 - 1  and serial output signals  17 - 2  basically convey the same information, so in most applications only one or the other, but not both, will be used. The trigger signal(s)  19  can be used advantageously in conjunction with either the quadrature or serial outputs. 
     With an incremental encoder, the parallel position word represents the position relative to where the encoder powered up or relative to an index or reference location on the scale. With an absolute encoder, the parallel position word represents the absolute position of the encoder on the scale without any requirement for initially passing through an index location, because the scale has a unique pattern at every location. The format of the parallel position word may be the same in each case. In addition, the format of the parallel position word may be same regardless of the particular position sensing technology (optical, magnetic, or inductive), and also regardless of the exact configuration of the analog conditioning electronics  24  and A/D circuitry  26 . For this reason, the trigger output may be a feature of virtually any type of encoder. 
     Additionally, although the trigger signal  19  can be implemented with discrete electronics in the encoder, this may add cost and complexity and may limit the trigger output to fixed predetermined positions rather than programmable positions. When an FPGA or DSP is used, the trigger output functionality may be added to the encoder with no extra associated cost and can be fully programmable, which can be advantageous or even required for certain applications. 
       FIG. 4  provides an illustration of a pattern of a programmable trigger signal. A pattern of this type may be achieved by a user loading trigger settings into the encoder  20  through a set-up process using a built-in communication port, such as a USB port, Ethernet port, etc., or using a separate set-up tool. As illustrated, typical settings may include the following, all of which are measured in encoder counts such as described above:
         Distance START from reference point to start the trigger output   Width W of the trigger pulse   Distance SEPARATION between trigger pulses   Distance END from reference point to end the trigger output       

     Once the settings are loaded, the trigger pattern occurs automatically as the stage  10  moves relative to the encoder  16 . The reference point (shown as REF) may refer to the index position of an incremental encoder or some predetermined point on the scale for an absolute encoder. 
     The basic example illustrated in  FIG. 4  may cover a large variety of applications. However, the trigger pattern can be as exotic as may be needed in a particular application. For example, the width W and the SEPARATION could be replaced by a look up table that can define much more complex trigger patterns. 
       FIG. 5  shows an example of the relationship between quadrature output signals A, B and a trigger signal TRIGGER generated by the encoder  20  of  FIG. 3 . Because the trigger generator  32  is built into the encoder  20 , the trigger pattern is fully synchronous with the quadrature state transitions and exhibits substantially no delay or jitter between the encoder position output and the trigger output. In the case of a serial output, the controller  14  must request a position sample and obtain it from the encoder, which as described above can be problematic for generating accurate triggers. With the trigger generator  32  built into the encoder  20 , the trigger signal may be output independent of the serial signals. In other words, the trigger signals occurs at the correct positions without any need for the controller  14  to first obtain the position information via the serial signals. 
     While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.