Abstract:
An improved servo system for galvanometers, scanners and similar devices which uses digital processing to increase the dynamic range and provide greater effective resolution. The system provides digital control of a reference point that directly influences the excitation, which in turn directly influences the gain of the circuit. A high and low resolution switching path is provided to optimize dynamic range. Wide angle torque compensation improves uniformity of response at large angular deflections from zero. Improved thermal protection allows safe operation near system thermal limits. A graphical user interface allows adjustments and changes response on the fly for tuning due to input conditions or user control.

Description:
[0001]    This application relates and claims priority for all purposes to pending U.S. provisional application serial No. 60/234,405, filed Sep. 21, 2000. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention relates to an improved servo system for an optical scanning device, and more specifically, to a servo system for optical scanning systems that incorporates digital servo control with an on-board processor and a graphical user interface (GUI) setup tool.  
         BACKGROUND OF THE INVENTION  
         [0003]    A servo system is a control system that causes some output variable(s) to track some input variable(s). Traditional servo systems have employed analog servo circuits and processing techniques. These systems often use a “PID” (Proportional Integrator Derivative) control algorithm where an error signal contributes to subsequent control signals.  
           [0004]    As digital sensors and processing techniques have become more common and cost-effective, many analog servo circuits have been replaced by digital implementations. Digital servo circuitry and digital processing provides significant advantages to the analog scheme. Namely, the digital version provides greater flexibility and stability as compared to the analog system. Digital circuitry also has many advantages such as smaller size and less heat dissipation.  
           [0005]    Scanner and galvanometer servo systems have existed for many years in a variety of applications. These systems were slow to adapt to a digital topology because of the ruggedness and proven reliability of the existing designs. In addition, the typical scanner operation requires the very high dynamic range achieved with existing analog servo designs. Finally, the use of analog position feedback devices in these systems has contributed to the continuing use of analog servo systems.  
           [0006]    There have been many attempts to effectively incorporate digital circuitry into servo systems and improve the digital processing techniques. However, many of these implementations still do not provide sufficient resolution during the entire servo process. Some examples are referenced below.  
           [0007]    In Baker et al&#39;s U.S. Pat. No. 4,398,241, a computing means controls the servo and processes the feedback signals. The position and velocity signals are processed in the computer and issued to the servo through the described circuitry.  
           [0008]    A servo feedback system is described in Barker et al&#39;s U.S. Pat. No. 4,282,468, where the feedback signals are generated in a resolver digitizer circuit. The resolver digitizer circuit generates pulses corresponding to the feedback. This digital error signal is fed to a digital to analog converter and the error signal is then applied to the servo. The advantages that accompany digital processing techniques produce a more accurate response.  
           [0009]    A digital servo positioning system incorporating a lead/lag integrator is disclosed in Evans&#39; U.S. Pat. No. 4,893,068. The integrator in the position control system is placed in parallel with the feedback loop and allows the integrator to switch between velocity error during pathtracking to position error during holding and moving states.  
           [0010]    What is needed is a servo system that provides an increased dynamic range and greater effective resolution.  
         SUMMARY OF THE INVENTION  
         [0011]    Accordingly, an object of the present invention is to provide a servo system that allows digital processing techniques to work with appropriate analog command and position feedback signals to provide a greater effective resolution and dynamic range of optical scanner operation.  
           [0012]    A further object of the invention is the digitizing of an analog error signal in addition to separately digitized command and position feedback signals. In particular, the present invention employs a dual resolution scheme. A high-resolution path is available for low error conditions, while a low-resolution system can be used for higher error conditions. This provides greater effective resolution and dynamic range.  
           [0013]    A further object of the invention is to permit the full dynamic range of the servo to be consistently applied to any part of the field of view. In order to accomplish this, the gain of the sensor and the gain and offset of the sensor processing circuitry are varied under digital control. In addition, compensation for diminished torque constant at the extremes of scanner position is provided.  
           [0014]    An additional object is real-time, online tuning of the servo system operating algorithm and parameters described above. A graphical user interface (GUI) with simulated analog presentations and inputs is used to provide a simple operator interface tool to measure and alter system parameters through an on-board digital processor.  
           [0015]    Another object is to store multiple “tunes” or sets of operating parameters within the on-board processor, and thereby permit switching between tunes in real-time in response to internal states and/or external commands. A tune consists of a servo algorithm as well as the variables that describe it. Changing a tune can consist of changing variables for use with the present algorithm or substituting a completely new algorithm with the same or new variables.  
           [0016]    For example, the system could be commanded to have a high bandwidth tune for the flyback period of a raster waveform, and a quieter, low bandwidth tune for the forward scan period. As another example, for controlling laser marking applications, one tune can be used for marking, giving smooth, well-controlled motion, and another tune used for jumps between marks, giving the fastest speed and settling times. When marking large, filled shapes or characters, one tune can be used for marking the outline with smooth, well-controlled motion and another tune used to fill the interior at higher speed. As another example, the system can change its tune in response to detection of over-position or clipping of the output amplifier.  
           [0017]    Yet another object of the invention is to provide a real-time scanner capacity to monitor rotor magnet thermal margins by calculating peak coil temperatures from scanner current and thermal parameters, and combining this information with baseline temperature readings from a sensor located in the scanner. The on-board sensor and thermal model of current allows continual operation of the device close to the thermal limits of the scanner by accounting for both ambient temperature and coil current heating.  
           [0018]    Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein only a preferred embodiment of the invention is described, simply by way of illustration of a best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The present invention will be readily understood by the following detailed description in conjunction with the attached drawings, and in which:  
         [0020]    [0020]FIG. 1A is a prior art block diagram analog servo circuit element with digital error signal, using an analog input and A/D converter, and a D/A converter in the feedback circuit.  
         [0021]    [0021]FIG. 1B is a prior art block diagram of an all digital servo circuit element.  
         [0022]    [0022]FIG. 2 is a simplified schematic of a preferred embodiment of the invention showing High/Low resolution digital error switching in a servo circuit.  
         [0023]    [0023]FIG. 3 is a graph of raster scan of position vs. error signal.  
         [0024]    [0024]FIG. 4A is a schematic of a prior art position detector circuit.  
         [0025]    [0025]FIG. 4B is an idealized representation of the position detector circuit of FIG. 4A.  
         [0026]    [0026]FIG. 5A is a functional schematic of a preferred embodiment servo circuit with adjustable reference voltage for adjusting sensor gain, differential amplifier and gain section, and adjustable offset voltage for adjusting circuit gain.  
         [0027]    [0027]FIG. 5B is a first example embodiment of the differential amplifier section of the circuit of FIG. 5A.  
         [0028]    [0028]FIG. 5C is a second example embodiment of the differential amplifier section of the circuit of FIG. 5A.  
         [0029]    [0029]FIG. 5D is a third example embodiment of the differential amplifier section of the circuit of FIG. 5A.  
         [0030]    [0030]FIG. 6 is a wide angle compensation embodiment of a servo circuit with selectable algorithms in the digital section.  
         [0031]    [0031]FIG. 7A is a physical sideview depiction of scanner components with a thermistor.  
         [0032]    [0032]FIG. 7B is an electrical equivalent of a scanner components thermal model of the embodiment of FIG. 7A.  
         [0033]    [0033]FIG. 8 is a schematic diagram of elements of the invention in a preferred embodiment of the digital control servo system for an optical scanner device.  
         [0034]    [0034]FIG. 9 is a screen shot of expert level tuning with a graphic user interface.  
         [0035]    [0035]FIG. 10 is a screen shot of applications level tuning with the graphic user interface of FIG. 9.  
         [0036]    [0036]FIG. 11 is a screen shot of the notch filter tuning window of the graphic user interface of FIG. 9.  
         [0037]    [0037]FIG. 12 is screen shot of the tunes matrix of the graphic user interface of FIG. 9, with arrows showing the path between tunes.  
         [0038]    [0038]FIGS. 13A and B are a diagram and table showing the  5  bit word construction and symbol meaning for tune selection using the graphic user interface of FIG. 9.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0039]    The invention is susceptible of many embodiments, including the following preferred embodiments. The description includes reference to prior art practices for context. FIGS. 1A and 1B depict prior art servo feedback system elements. There are basically two forms of existing servo feedback architectures, an analog system in the form of a resolver and a digital feedback system in the form of an encoder. FIG. 1A is an analog position feedback servo system element, where the analog command and position feedback signals are separately digitized by A/D converters  10  and  40 . The error is then the arithmetic sum of these two digitized values as represented by summation symbol  30 , the function being performed by the system processor. FIG. 1B illustrates the digital feedback servo system element where a digital command and a digital position feedback signal are summed to produce a resultant digitized error signal.  
         [0040]    Referring now to FIG. 2, a preferred embodiment of the invention is illustrated as a functional schematic. The servo system uses a switch selectable low-resolution/high-resolution circuit where the arithmetic sum of the separately digitized analog command and position feedback signals is available as a digital error signal, as is the digitized sum of the analog signals. The analog summing circuit element  130  includes a fixed or variable gain K to provide a higher resolution error signal which is digitized at A/D converter  140  to produce a digital high resolution error signal  1 . A low resolution error signal  2  is generated by converting analog command signal  100  at A/D converter  160 , analog position feedback signal  110  at A/D converter  170 , and summing the result as represented by summation symbol  180 . Switch  150  is automated to alter the selection of error signal output from high resolution signal  1  to low resolution error signal  2  when large errors cause A/D converter  140  to saturate. This is condition is defined as when the output signal value of converter  140  reaches either its positive or negative limit. The error signal selection reverts back to high resolution signal  1  when the condition abates, under control of the servo system processor and software. The digital functions of the circuit, such as the summation symbol  180  and switch  150 , are executed in an embedded system processor and computer code.  
         [0041]    To illustrate the use of this hi/low resolution error signal capability, a typical raster scan waveform is shown in FIG. 3, with position signal S P  and error signal S E  shown for a cycle of scanner motion. The error signal is small while the servo is operating in a low velocity condition, also called the forward scan state, from T 0  to T 1 . During the reverse, or flyback state, from T 1  to T 2 , the error signal increases with the higher velocity. The switching between high and low resolution error signals referenced in FIG. 2 is triggered when the error signal from the gain circuit  130  exceeds the limitations of the A/D converter  140 . Switch  150  then switches to the lower resolution error signal  2 . As explained above, this occurs only when A/D  140  is saturated, which is a function of variables such as frequency and amplitude of waveform and how the circuit is tuned.  
         [0042]    In prior art FIG. 4A, a simplified schematic of a position detector in a scanner is shown. This type of position detector is ubiquitous in the industry and is described, for example, in Rohr&#39;s U.S. Pat. No. 4,864,295 and in Dowd&#39;s U.S. Pat. No. 5,537,109. An excitation voltage E is connected to an oscillator. The scanner moving element has plates that modulate currents which are used to detect the position of the scanner. The scanner position detector can be idealized as two current sources as shown in FIG. 4B. The Pos+ and Pos− currents are a function of the excitation voltage E and scanner angular position θ. The excitation voltage level directly influences the size of the Pos+ and Pos− signals such that, for a given angular position, increasing the excitation voltage also increases the Pos+ and Pos− currents and the difference between them.  
         [0043]    The cited patents also describe the Sum/Difference Detector architecture. The circuitry in the servo system serves not only to calculate the difference between Pos+ and Pos− but the sum as well. It compares that sum to some reference voltage Ref, and adjusts the excitation voltage E to keep the sum and Ref equal.  
         [0044]    One of the unique features of the present invention is the manipulation of this reference level Ref. A preferred embodiment places Ref under digital control via a D/A converter, allowing digital control of the excitation voltage which thereby provides digital control of the gain of the sensor. In a preferred embodiment, this gain can be varied by a factor of  10  or more, enabling a fuller dynamic range than the prior art systems.  
         [0045]    Referring now to the preferred embodiment servo circuit of FIG. 5A, each current source  200 ,  210 , is connected to a respective transconductance amplifier  220 ,  230 , which outputs a respective voltage signal  221 ,  231 . Scanner position is represented by the difference between these two voltages, as determined by the differential amplifier  250 , whose gain K is adjustable. An offset signal voltage under digital control of the system processor and computer code, and applied via a D/A converter  251 , adjusts the zero point of the system via summing node  252 .  
         [0046]    Differential amplifier  250  and offset summing node  252  may be implemented in a variety of ways. For example, FIG. 5B shows a preferred embodiment example in which variable resistors  240  and  260  are ganged or varied simultaneously to the same value for adjusting gain K. FIG. 5C shows another preferred embodiment example in which resistor  241  is variable for adjusting gain K. FIG. 5D shows another preferred embodiment example in which resistor  242  is variable for adjusting gain K. In each of FIGS. 5B, 5C, and  5 D, the variable resistor(s) may be digitally controlled variable resistors or a set of fixed resistors with digitally controlled switches. Other schemes are commonplace in other servo applications. For example, embodiments may be implemented using an Analog Devices™ AD8400 (no claim is made to the trademark or product designator) digitally controlled variable resistor.  
         [0047]    Referring back to FIG. 5A, the positive and negative position voltage signals are also summed via a resistor network  280  and the resulting signal is compared to a digitally controlled reference signal Ref, by operational amplifier  290 . The operational amplifier adjusts the position detector excitation voltage E, to keep the sum equal to the reference voltage.  
         [0048]    In summary, there is digital control via the system processor and computer code of the reference signal Ref, that controls the excitation voltage E, which in turn controls the gain of the sensor. There is also a control of the gain of the electronics associated with the sensor.  
         [0049]    Referring now to FIG. 6, another aspect of the invention is compensating for changes in torque constant due to scanner angular position. When designing a servo system for a typical commutated rotating DC motor, it is reasonable to assume that the relationship between coil current and generated torque is constant regardless of rotor position. However, this is not the case for limited-angle non-commutating galvos, where torque per unit current falls slightly at large angles. Wide angle compensation boosts output control effort, which is a voltage, at large angles to correct for this loss of torque, as illustrated in FIG. 6. The result of PID and command feedforward servo control  300  would usually be sent directly to D/A converter  310  which drives an amplifier  320  attached to the galvo motor. In the present invention, a boost factor  330  is digitally calculated at compensation calculator  328  from the digital output of A/D  326  from the analog position feedback signal, and fed to multiplier  340 . By compensating for torque constant variation with angle, system response is more nearly uniform at all angles of operation.  
         [0050]    A further variation of the preferred embodiments is illustrated in FIGS. 7A and 7B, which shows a graphical side view of a scanner and an electrical analogue of the thermal model. Scanners have a critical temperature threshold. At a maximum temperature of around 100-120° C., depending on material, magnets will demagnetize, rendering the system useless. Therefore, it is desirable to have a system, which can monitor scanner temperature and shut down the servo if necessary to protect the scanner. Ideally, the instantaneous temperature of the surface of the magnet would be the most helpful measurement. For practical purposes the coil temperature provides the most accurate reflection of the magnet temperature in moving magnet scanners.  
         [0051]    As shown in FIG. 7A, moving magnet scanners constitute a magnet  70 , a pair of coils  60 , and a stator  50 . According to the present invention, a thermocouple or thermistor  80  is tightly coupled to the stator to track stator temperature.  
         [0052]    Employing an RMS current limit is known in the art, which roughly translates coil current (I) into peak temperature. The prior art systems use the formula (I 2 •R coil ) to calculate the instantaneous heat input to the system. This is combined with an integrator whose time constant corresponds to the thermal time constants of the scanner, and gives a rough indicator of the thermal rise of the coil system above the baseline temperature.  
         [0053]    However, the prior art systems do not know the baseline temperature of the system. Thus, the prior art methodologies either assume a higher temperature than actually exists and rob the system of available thermal margin or assume a lower ambient temperature than actually exists and run the risk of overheating and destroying the system.  
         [0054]    According to the preferred embodiment of the present invention, the temperature of the stator is measured by a thermistor and used as the baseline temperature. The RMS model then calculates coil temperature excursions above this baseline, yielding a much more reliable picture of actual coil temperature. In the electrical analogue of the thermal model, shown in FIG. 7B, heat corresponds to current, temperature to voltage, and coil and stator thermal resistance to the resistor.  
         [0055]    In a moving coil scanner, the present invention would again be used, this time to protect the coil itself from overtemperature damage.  
         [0056]    [0056]FIG. 8 is a schematic representation of many of the elements of the invention in a present implementation of a digital control servo system for a galvanometer optical scanner device. In particular, the lo-res/hi-resolution error switch  400  as described above and the wide angle compensation  410  as described above. As illustrated in FIG. 8, a present implementation supports a second order (also known as “error proportional”, “speed”, or “type 0”) PID (Proportional  420 -Integrator  430 -Derivative  440 ) with Command Feedforward  450  algorithm and a third order (also known as “position proportional”, “accuracy”, or “type 1”) PID algorithm as well as a notch filter  460 . Other algorithms are possible. For example, the derivative block  440  shown in FIG. 8 takes digitized position feedback as the input for both second order and third order tunes; an alternative would be to use error as the input for second order tunes.  
         [0057]    Referring now to FIGS.  9 - 13 , the graphical user interface (GUI), of which several screens are illustrated, is designed to give users skilled in the art an experience which is not unlike that of an analog servo system. The GUI, which runs on a personal computer or other general purpose computer and keyboard which interfaces with an embedded control processor in the servo device, is a set up device for programming the operation of the scanner. Continuously variable mouse-controlled sliders act much like the potentiometers of an analog system and perform many of the same functions. Boxes may be checked to enable various optional features such as a notch filter and wide angle compensation. The GUI can assume multiple configurations depending on the intended user and the required tune algorithm. For example, FIG. 9 shows an “expert” configuration with a large set of controls and controls for a second order tune algorithm. Once the servo device is programmed, the GUI is no longer necessary.  
         [0058]    [0058]FIG. 10 shows a more restricted configuration where certain options have been eliminated for simplicity. The configuration shown here is for a third order tune algorithm. Note that the Error Proportional slider of FIG. 9 has been replaced by the Position Proportional slider and that the command feedforward adjustments have been dimmed because they are not available for the third order tune algorithm. Selection of the algorithm is made by the user via the Second Order and Third Order buttons in the upper left region of each screen shot. Adjustment to a tuning parameter is transmitted automatically to the servo hardware for real-time, “on-line” tuning.  
         [0059]    Similarly, another feature of the servo is an integrated notch filter, shown in FIG. 11. The filter includes a user interface that allows design of the appropriate filter visually in the frequency domain. Adjustments are transmitted automatically to the hardware.  
         [0060]    The response behavior of any servo system is determined, in part, by its tuning. For a PID-type tuning, a tune is the state of a number of variables, including P, I, D, gain, offset and filter coefficients, to name a few. There are times when it is desirable to be able to change from one tune to another while the system is in use. For example, in laser marking, a pair of galvanometers direct a high energy laser beam to mark an article. There are at least two operations with different requirements: low-noise, well-controlled velocity while marking, and high-speed, fast-settling jumping to the next letter. In addition, for filled letters, an intermediate speed tune may be desirable for the fill operation.  
         [0061]    Another feature of the servo is illustrated by the matrix in FIG. 12 and addressing scheme in FIGS. 13A and 13B, showing a means of associating specific tunes with a set of internal and external system states. For each axis, a tune matrix is a set of tunes and their 5-bit selection rules for testing the 5-bit “address” formed by the axis OverPosition Status Flag and four logic level sync input pins. Selection rules must be mutually exclusive: two tunes for the X axis with selection rules 0XXXX and XXXXX would not be allowed because, for instance, 01111 (Not OverPosition, all sync inputs high) would fit both rules. Incomplete rule sets are permissible, e.g. selection rules XXX00, XXX01, and XXX10 for three Y-axis tunes would not include 01111 and the servo would not change tunes in response to this address. This matrix of tunes may be stored in the servo&#39;s non-volatile flash memory.  
         [0062]    When two tunes have markedly different characteristics, an abrupt switch from one to another may cause system instability. Transitioning through a number of intermediate tune states may be required. The off-diagonal elements in the tune matrix indicate how “tune ladders” are to be built between tunes to ensure smooth operation. “Steps” is the number of intermediate tunes used to move from one tune to another. “Dwell” is the number of servo cycles to spend on each step. These numbers may be entered directly. As the arrows in FIG. 12 indicate, the matrix is traversed clockwise when moving from one tune to another. Thus, moving from “low noise” to “high speed” involves the Tune Ladder Block at the intersection of row  2  and column  3  in FIG. 12; the X axis of will employ 5 steps, the Y axis will employ 9 steps, and X &amp; Y will each employ 8 steps when recovering from an overposition state to the “high speed” tune.  
         [0063]    The invention is susceptible of other and various embodiments, all within the scope of the claims that follow. For example, the servo described here can be used alternatively to refocus the lens of the optical scanning device or system.  
         [0064]    As another example, again similar to FIG. 2, there is a digital servo system for an optical scanner incorporating a digital processor and computer code and a signal processing circuit with inputs for an analog position command signal and an analog position feedback signal, and an error signal output. The circuit includes a first circuit element for producing an analog position error signal from the position command signal and the position feedback signal. A second circuit element produces from the analog position error signal a first digitized error signal known as a high resolution error signal. A third circuit element produces a digitized analog position command signal. A fourth circuit element produces a digitized analog position feedback signal. The digital processor and computer code generates a second digitized error signal known as a low resolution error signal as a difference signal between the digitized command signal and the digitized feedback signal. The digital processor and computer code provide automatic switching of the error signal output from the high resolution error signal to the low resolution signal during periods when the second circuit element is at its maximum positive or negative output limit.  
         [0065]    There may be generated by the processor and computer code a digitally operative scanner angle torque compensation control effort multiplier voltage, derived from the digitized position feedback signal input.  
         [0066]    The processor and computer code may be connected to a graphical user interface. The graphical user interface may provide simulated analog presentations of servo algorithms and operating parameters, and permit simulated analog control inputs in real time via hand or finger motion input devices like a mouse or a joy stick or other devices accepting physical, analog actuation, for manipulating a multiplicity of servo system operating parameters.  
         [0067]    As an additional example, related to the embodiment of FIG. 5, there may be a digital servo system for an optical scanner, where the servo system includes an analog position command signal input, a digital error signal output, an analog position feedback signal processing circuit with positive and negative angular position sensors connected to an analog differential amplifier, and an analog position error signal with gain. The analog position error signal is derived from the analog command and the differential amplifier by the feedback signal processing circuit.  
         [0068]    There is a digital processor and computer code, and a reference voltage controlled by the processor and computer code and connected to the feedback signal processing circuit for controlling the circuit gain. There is a high resolution error signal consisting of a digitized form of the analog position error signal, and a low resolution error signal consisting of a digital differential of a digitized form of the command signal input and a digitized form of the analog position error signal.  
         [0069]    As in some other embodiments, the digital processor and computer code provides automatic switching of the error signal output from the high resolution error signal to the low resolution signal during periods when the second circuit element is at its maximum positive or negative output limit.  
         [0070]    There may be a digital control output from the processor and computer code connected to the feedback signal processing circuit for adjusting the gain of the differential amplifier. The gain of the differential amplifier may be adjustable through two equal resistor sets in the feedback signal processing circuit, one each of the resistor sets being connected between a respective position sensor and the differential amplifier, with the resistor sets ganged mechanically or digitally for concurrent selection of equal subsets of resistance value by operation of the processor and computer code.  
         [0071]    There may be an offset voltage controlled by the processor and computer code and connected to the feedback signal processing circuit for adjusting the zero point of the servo system. There may be a digitally constructed scanner angle torque compensation control effort multiplier voltage derived from the feedback position signal by the processor and computer code.  
         [0072]    Again, there may be in this example a graphical user interface connected to the processor and computer code. The graphical user interface may provide a simulated analog presentation or multiple forms of presentations, of servo algorithms and operating parameters, and permit simulated analog control inputs in real time via hand or finger motion input devices for a multiplicity of servo system operating parameters. The graphical user interface may be a personal computer or a general purpose computer, keyboard and a hand or finger analog motion cursor control device like a mouse or joy stick.  
         [0073]    Further, the processor, computer code and graphical user interface may provide for simulated analog control inputs for programming of sequentially executable user constructed tunes or algorithms, where the tunes are recordable in the processor and computer code for automated sequencing during subsequent scanner operations.  
         [0074]    As still another example, there is a process for calculating rotor magnet operating temperature in a galvanometer, including the of the steps of sensing the stator temperature of a galvanometer with a thermal sensor proximate the stator, sensing coil current in said galvanometer, calculating temperature rise in the system from the effects of coil current, and adding the calculated temperature rise to ambient stator temperature.  
         [0075]    For implementing the process, there may be in an optical scanner a thermal sensor proximate the stator, a coil current sensor, a thermal model in the processor associating temperature rise in the system with coil current level, and a program in the processor for calculating temperature rise assumed from affect of coil current and adding to it the actual temperature from said thermal sensor so as to indicate the system operating temperature.  
         [0076]    The present invention has been particularly shown and described with respect to certain preferred embodiments of features in the written description, figures, and claims. However, it should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and details may be made without departing from the spirit and scope of the invention. Additional objects and advantages of the present invention may be further realized and attained by means of the instrumentalities and combinations all within the scope of the claims. The drawings and description are to be regarded as illustrative in nature, and not as restrictive.