Abstract:
A pen-lift system is provided which can achieve high speed plotting while maintaining line definition of drafting quality. The system includes a pen-lift mechanism for raising and lowering the pen, a pressure control system in order to provide a constant pen force for different kinds of pens, a velocity controller for varying the vertical velocity of the pen in response to changes in platen height and relative lateral pen position, and a position controller for sensing the present pen height relative to the platen and for actuating the pen-lift mechanism to achieve the desired pen-lift heights for different plotting situations.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to X-Y plotters, and in particular to an adaptive servo-controlled pen-lift system for raising and lowering a pen relative to a platen in such a plotter. 
     Generally, the pen-lift system in a high quality plotter becomes as important as the lateral positioning system if plots of drafting quality are to be obtained. For example, even if the lateral positioning system moves the pen accurately in a straight line, a drafting quality line will not result if there is uncontrolled pen bounce or if there is an incorrect pen force. 
     In the past, pen-lift systems have typically used a spring system to hold the pen to the recording medium and platen with a prescribed force, that force depending on the weight of the pen and pen carriage, the effective force constant of the spring system, and the pen-lift height. Attempts have been made to control pen bounce through spring damping systems of various kinds. In most implementations, the complications inherent in such mechanically intensive systems have led to two-state pen-height system, i.e., pen-up or pen-down. 
     It is important to note that such two state systems are manifestly inefficient in the often encountered plotting situation, e.g., lettering or drafting dashed lines, in which many pen lifts are required. Despite the fact that most pen strokes in these situations are in close proximity, the pen travels to its full height between successive separate strokes. Hence, a substantial amount of time is spent raising and lowering the pen rather than in plotting. 
     To date, no available plotting system has been developed which utilizes a feedback control system adaptive to platen height irregularities to control pen bounce and to increase the plotting speed by controlling pen-lift height. 
     SUMMARY OF THE INVENTION 
     In accordance with the illustrated preferred embodiment of the present invention, a pen-lift system achieves high speed plotting while simultaneously maintaining line definition of drafting quality. The system includes a pen-lift mechanism for raising and lowering the pen, a pressure control system for providing a constant pen force, a velocity controller for varying the vertical velocity of the pen in response to changes in platen height and relative lateral pen position, and a position controller for sensing the present pen height relative to the platen and for actuating the pen-lift mechanism to achieve the desired pen-lift heights for different plotting situations. 
     The pen-lift mechanism is a voice coil actuator for providing a constant pen force which is linearly dependent on coil current and which is independent of pen position. The pressure control system effectuates this constant pen force via a microprocessor controlled force adjustment potentiometer to supply suitable magnitudes of current to the voice coil actuator. Eight different levels of pen force are selectable by the user with this system by utilizing a 3-bit force selector A/D converter. 
     The pen position controller and velocity controller form an interactive system under microprocessor control for raising and lowering the pen more quickly than is possible in systems encountered in the prior art. Variations in platen height from one lateral position to another are reasonably small for small lateral distances. Hence, if the pen is to plot a series of vectors in which the end point of one is close to the beginning point of another, then it is necessary to lift the pen a small distance in order to clear the writing medium before the pen is moved from the end point of one vector to its next location, the beginning point of the succeeding vector. This process is achieved by storing the height of the pen in contact with the writing medium just prior to pen-lift. In this way, the pen may be lifted during successive operations based on its last vertical position rather than relative to some artificial reference. 
     Furthermore, knowing the last height of the pen allows appropriate velocity control. Only when the pen is within a certain distance of the platen is a low vertical pen velocity required in order to avoid damage to the pen on impact. Hence, for high pen lifts which are necessary in moving the pen a substantial distance, the uppermost portion of the pen drop may occur at a relatively high velocity, followed by a terminal portion at a sufficiently low velocity to avoid pen damage. Control of the pen drop velocity in this manner provides a substantial increase in plotting speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the general relationship of the major components of the preferred embodiment of the present invention. 
     FIG. 2 shows the details of a voice coil system used to effect the pen-lift. 
     FIGS. 3A and 3B show a schematic of the pen-lift control system. 
     FIG. 4 is a diagram showing the mechanical configuration of the pen-lift system. 
     FIG. 5 illustrates the function of the optical position sensor. 
     FIGS. 6A, 6B, and 6C illustrate pen position as a function of time for different pen commands. 
     FIG. 6D is a state diagram of the pen-lift system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a pen-lift control system 11 of the preferred embodiment of the present invention. The system 11 includes a control system 13 and a pen-lift mechanism 15, both of which may be included, for example, in a plotter apparatus. Operating under computer (microprocessor) control, the control system 13 controls the pen-lift mechanism 15 and, hence, the pen-lift operations of the plotter. Control system 13 includes a pen position controller 17, a pen velocity controller 19, and a pen pressure controller 21. Taken together, the lift mechanism 15 and the control system 13 form an adaptive servo system. 
     FIG. 2 shows the pen-lift mechanism 15, including a voice-coil actuator (linear pen actuator) 23 having a magnet 24, a voice-coil cup or housing 25, a pole cup assembly 27, and a voice coil bobbin 29 with windings 31 for moving a pen-lift arm (with or without a pen) vertically downward onto, or up and away from, a medium such as paper on a platen. 
     Referring to FIG. 3, position controller 17 (not particularly shown) includes an optical position sensor 33, and a position sensor signal amplifier 35. The position sensor 33 includes a light-emitting diode (LED) 37 with a current source 39, a phototransistor 41, and an opaque shutter or flag 43 positioned for movement with actuator 23 affixed to pen-lift arm 45 (shown in FIG. 4) carried by pen carriage 47. Flag 43 is disposed a selected distance from the LED 37 for movement relative to the LED 37 in accordance with the action of actuator 23 and, hence, with movement of lift arm 45. The LED 37 and the phototransistor 41 are fixed with respect to the pen-lift housing 49 (shown in FIG. 3) such that the optical path between them, and hence the infrared beam from LED 37 to phototransistor 41, can be partially occluded in proportion to the position of flag 43 attached to the pen actuator 23. The optical position sensor 33 senses the voice coil actuator position relative to the pen-lift carriage frame by sensing the amount of light reaching phototransistor 43 due to occlusion by flag 43. 
     As shown in FIG. 5, an end portion of the flag 43 may be slanted at, for example, a thirty degree angle causing it to block varying portions of the beam from LED 37 as the pen arm 45 moves up and down in accordance with the action of actuator 23. Although not shown, the optical position sensor 33 is temperature compensated to render it substantially insensitive to temperature changes that would ordinarily affect the optical output of LED 37, and the gain of phototransistor 33. 
     As shown in FIG. 3, the output of position sensor 33 is amplified by amplifier 35, then inverted by an invertor 51 before being stored by a sample and hold circuit 53. Under microprocessor control, this circuit 53 stores a voltage representing the pen position (i.e., the vertical or Z-axis position of a pen 44 in pen arm 45, just prior to the last (immediately previous) pen-lift. This stored pen position signal represents the position of pen arm 45 relative to pen-lift carriage 47 when pen tip 44 was last down and in contact with the writing medium. Exactly how this sampled position signal is used is described as follows. 
     In a pen-up or position control mode of operation, switches B0 through B7 are opened by the microprocessor. The all switches open condition causes the pen 44 to be lifted (raised) to its full up position until the output of the optical position sensor 33 becomes substantially zero. (A sensor offset potential signal amplifier 35 shown in FIG. 3 is adjusted so that this full up position is approximately 0.025 mm below the upper mechanical limit of travel.) In a velocity-control mode of operation, the pen 44 is lowered at a controlled rate (velocity) onto the platen 34. In this mode, switches B3, B4, B5, B6, and B7 are closed while switches B0, B1, and B2 are opened. The closing of switch B3 by the microprocessor disables the pen-lift position control loop operation by setting the gain in the position control loop to zero. In the velocity-control mode, switches B5, B6, and B7 apply a downward input force signal, and switch B4 applies a velocity feedback signal to the input of voice coil drive amplifier 57 (current controlled current source). Approximately ten milliseconds after entering this mode, switches B5 and B7 are opened by the microprocessor. This results in a high initial velocity for the first ten milliseconds, and a slower constant velocity (e.g., approximately 4 mm/sec) until the pen 44 contacts the platen 34, as shown in FIG. 6A. Thus, after a long move (i.e., a move greater than 100 ms in elapsed time), the pen 44 is dropped at a high initial velocity then at a slower constant velocity. As shown in FIG. 6B, during short lateral moves, the pen 44 is lifted a smaller distance above the current platen 34 height and is then dropped after the move to the same platen 34 height. Further, as shown in FIG. 6C, if a pen-up move takes longer than 100 ms, the pen 44 is raised to its upmost or full position and the move is treated as a long move for purposes of subsequent pen movement. Also, it should be noted that although a short travel time condition (e.g., a long pen combined with a high spot in the writing surface produces a short travel time of about 20 ms) may result, because this condition is not known prior to the move, a full delay period (e.g., 45 ms) must elapse before start of writing and commencement of lateral pen motion. 
     With respect to the level of writing force selected to match the type of pen 44 used (fiber tip, ball point or drafting) eight levels of writing force may be set by microprocessor control of switches B5, B6, and B7. These switches control the output current of a 3-bit force selection A/D converter 55 (shown in FIG. 3) which is applied to the input of the current-controlled current source output amplifier 57. This results in application of a constant pen 44 force since the voice-coil force is independent of pen position and is linearily dependent on coil current. Other input signals to the output amplifier 57 are applied from a force bias adjustment circuit 59 and from a power-off pen-up spring compensator adjustment 61. Spring compensator adjustment 61 feeds back a signal that is proportional to the actuator 23 position and minimizes the effect of the power-off pen-up spring 63 upon the commanded pen force from the microprocessor. This adjustment and the force bias adjustment are required since tolerances of springs are generally not closely controlled. Once the commanded force is applied (i.e., the force pursuant to the command signal) an additional 5 ms is allowed for pen settling before lateral pen motion is started. 
     At the time the microprocessor sets the force by application of selected signals via switches B5, B6, and B7, it also closes switch B3 which disables the position mode and switch B0 which sets the platen height sample-and-hold circuit 53 to the sample state enabling this circuit&#39;s output to now follow and track the pen 44 position as the pen 44 moves in the lowered position to trace a vector. 
     When the end of a vector is reached and a pen-up command signal is recognized by the microprocessor, the following state changes take place. First, the platen-height sample-and-hold circuit 53 is placed in the hold state by the microprocessor opening switch B0. This causes a voltage level to be stored representing the present platen position of the pen 44. The microprocessor also opens force input switches B5, B6, and B7 at this time. Next, the position mode is enabled by the opening of switch B3 and the closing of switches B1 and B2. This causes the pen 44 to lift about 0.64 mm above its last pen-down position. This lift requires only about a 3 ms delay before lateral pen-up movement can begin. In the event a pen-up time, caused by a long movement or no further commands, lasts longer than 100 ms, the pen position control status is modified by the opening of switches B1 and B2. This modification results in the pen 44 being lifted to the full up position and to the state associated therewith, namely, a subsequent initial high velocity pen drop followed by a predetermined lowered velocity pen drop. Where the pen up time is less than 100 ms, the pen 44 drops at a predetermined lower velocity, but this time the approximate distance that the pen 44 must travel is known and a combined drop time and settling time of only 20 ms is found to be satisfactory. This shows that plotting involving short pen-up moves saves about 25 ms per pen lift cycle. A lateral pen-up move of about 6 cm takes about 100 ms. It is assumed that the platen irregularities are smooth enough that the required pen height will not change significantly in less than that distance. Of course, other values of time and distance for negligible irregularities may be assumed. 
     A safety shutdown circuit (not shown) may be included in the pen lift mechanism to prevent damage to the voice coil by excessive power dissipation that could be caused by either a malfunction or misadjustment of the mechanism. 
     From the foregoing, therefore, it is shown that, whereas time spent lifting and lowering the pen of a plotter is time not spent plotting, the system disclosed herein reduces the time spent lifting and lowering the pen. This is especially beneficial when the plotting task is pen-lift intensive, as when alphanumeric characters and symbols are written. The actual pen-lift rates selected (in response to pen-up and pen-down command signals from the microprocessor) are approximately 36 cycles per second for the short lift cycle and approximately 17 cycles per second for the high lift cycle. 
     Returning now to FIG. 3, the interaction of the various circuit elements including pen position controller 17, pen velocity controller 19, and pen pressure controller 21, is described in greater detail below. Pen position controller 17 (shown in block form in FIG. 1 and shown as a position control loop in FIG. 3) includes input node 65 (a summing node for receiving pen position command signals from the microprocessor controlled switches B1 and B2), position summing amplifier 67, position compensator circuit 69, current controlled current source 57, feedback or sensor circuit 33, and position sensor signal amplifier 35. Position command signals from the microprocessor, and feedback signals from amplifier 35 are summed at node 65 and are applied to position control loop summing amplifier 67 (which is also a gain control stage) for enabling the position control loop. The output signal from summing amplifier 67 is given phase lead by a compensator circuit 69, the output then being applied, via node 71, to current source 57. The output of current source 57 is then applied via conductors 73, 75 to voice coil actuator 23. Thus, the effective output of position controller or control loop 17 is a control current applied to voice coil actuator 23 in response to signals applied from the microprocessor and from feedback amplifier 35. The voice coil actuator 23 has a predetermined linear force to current ratio (for example, 100 grams per ampere). Upon passage of current through the windings 31 (shown in FIG. 2) of the voice coil bobbin 29, the voice coil bobbin 29 slides up and down over the magnetic pole piece 24, moving with it flag 43, pen-lift arm 45 (shown in FIG. 4), and any pen 44 that may be positioned in the arm. 
     The following Table I, when read in conjunction with FIGS. 6A, 6B, 6C, and 6D serves to explain the various states that the pen-lift control system 11 is made to assume. 
     
                                           TABLE I__________________________________________________________________________MICROPROCESSOR CONTROLLED BIT STATE(Each 1 bit represents a closed switch condition.)(F0, F1, and F2 are binary pen-down force parameters selectableby a user.)                     B3                     B0               B4    POSITION               SAMPLE               DAMP- LOOP    B2             HOLDB7   B6   B5   ING   CONT.   SHORT  B1      CONT.FORCE     FORCE          FORCE               1 = ON                     1 = OPEN                             LIFT   SHORT LIFT                                            1 = TRACK                                                   COM-STATECONT.     CONT.          CONT.               0 = OFF                     0 = CLOSED                             DISTANCE                                    REFERENCE                                            0 = HOLD                                                   MENTS__________________________________________________________________________A    0    0    0    0     0       0      0       0      Full-Up                                                   StationaryB    0    0    0    0     0       0      0       0      Full-Up                                                   Mobile C1  1    1    1    1     1       0      0       0      Long Drop                                                   1st 10 msec C2  0    1    0    1     1       0      0       0      Long Drop                                                   Last 30 ms                                                   (next 30 ms)D    F2.sup.     F1.sup.          F0.sup.               1     1       0      0       1      Down                                                   StationaryE    F2.sup.     F1.sup.          F0.sup.               1     1       0      0       1      Down MobileF    0    0    0    0     0       1      1       0      Short Lift                                                   StationaryG    0    0    0    0     0       1      1       0      Short Lift                                                   MobileH    0    1    0    1     1       0      0       0      Short__________________________________________________________________________                                                   Drop 
    
     In the above Table and in FIGS. 6A, 6B, 6C and 6D, state B represents long pen-up moves, states C1, C2, and D (shown in FIG. 6A) represents long pen-drops of unknown height, state E represents any length pen-down moves, and states F, G, H, and D (shown in FIG. 6B) represent short pen-up moves. In FIGS. 6A and 6B, the slope shown for states C2 and H may be set by a damping adjustment 79. State D allows for settling time of the pen 44 after impact and writing force selection before lateral movement begins. Elapsed time in state H is shown to be a minimum because the approved distance for pen 44 drop to the platen 34 is known (and small, 0.64 mm). Time in state C2 must be set to a selected maximum figure since the distance from pen 44 to platen 34 is not known. In the state diagram shown in FIG. 6D, N represents the number of milliseconds that the pen 44 remains in a given state. This number is contained in a counter of the microprocessor, which counter is reset upon the entering of each state. As described herein, the microprocessor, upon execution of the program listed in Appendix A hereto, enables various control functions shown in blocks 17, 19, 21 (of FIG. 1) by performing various calculations, and opening and closing the various switches B0-B7 shown in FIG. 3 in accordance with the bit settings shown in Table I above. 
     In response to signals from the microprocessor, pen velocity controller 19 controls the rate at which the pen 44 is dropped to the platen 34, as shown in states C and H. Pen velocity controller 19 includes control switch 77, adjustment potentiometer 79, capacitor-resistor network 81, and buffer amplifier 83. With switch 77 closed, pen velocity controller 19 receives an output signal from position sensor signal amplifier 35 and differentiates it through the capacitor-resistor network 81 producing a signal proportional to velocity of the pen 44. In producing such a signal, the output from the RC network 81 is then applied to buffer amplifier 83, then to damping adjustment potentiometer 79. 
     Also, as shown in FIG. 3, pressure controller 21 includes A/D converter 55, input node 71, spring adjustment potentiometer 61 and force adjustment potentiometer 59. In order to lower the pen 44 and have it make contact with the platen 34 with an appropriate writing force or pressure, a current of suitable magnitude is applied to voice coil actuator 23 from current source 59 in response to one or more input signals applied via input node 71 (actually, a summing node for force command signals from the processor). From a possibility of eight force levels, specifiable by the microprocessor in controlling switches B5, B6, and B7 of 3-bit force selector A/D converter 55, any one of these eight possible forces may be preselected by the user to provide the writing forces suitable for the different possible pen 44 tips. The closing of switch B3 disables the position control loop and allows the pen 44 to be lowered and to contact the platen with a writing force corresponding to the force level (number) selected at converter 55. The rate at which the pen 44 is lowered prior to contacting the platen 34 may be selected by closing switch B4 in velocity controller or damping circuit 19. The closing of switch B4 causes a force proportional to the velocity of the pen 44 to act counter to the direction of motion of the pen 44 thereby setting a certain constant velocity for the pen 44 as it moves downward toward the platen 34. This constant downward velocity may be changed by adjusting potentiometer 79. The constant downward velocity is, however, affected by the pen-up action of spring 63 when the latter is distended. Spring 63 is provided to hold and maintain the pen 44 in an upward position relative to the platen 34 when power is off. To counteract the effect of the spring 63 when the pen 44 is being lowered toward the platen 34, a power-off spring compensation adjustment circuit or potentiometer 61 is provided which applies a current to node 71 (in proper phase relationship with other input signals to the node) substantially eliminating the effects of the spring 63. Thus, as shown in FIG. 3, signals to node 71 include input signals applied via converter 55, feedback signals from amplifier 35 via velocity controller or damping circuit 19, and input signals from power-off spring compensation adjustment circuit 61. 
     Spring 63 exerts a force F equal to Ax+B, where x represents the height or length of the spring 63, A represents the spring constant of spring 63, and B represents a parameter proportional to the initial (substantially non-distended) tension of the spring 63. The output from circuit 61 compensates for the effect off the spring constant A. However, compensation is needed for the effect of parameter B. Accordingly, a force bias adjustment circuit or potentiometer 59 is provided which applies a selected constant current to node 71 to compensate for the initial tension of the spring 63, and to provide a selected downward bias (offset) force (e.g., ten grams) to the pen 44. 
     Potentiometer 85 changes the gain of the current controlled current source 57. This allows normalization of the control system 13 necessary due to variations in the force constant (i.e., grams) amplifier of pen-lift actuator 23 from one unit to another. 
     A voltage reference circuit 87 provides a substantially stable reference, for example, a -6.92 volts and a +6.92 volts used by the position control circuit in determining short lift heights and by the LED 37 in providing light of substantially uniform intensity. The stable reference voltage from circuit 87 is also used in conjunction with resistors 89 and 91 as shown in FIG. 3, with resistor 91 arranged parallel to LED 37, to substantially eliminate temperature drift and variations in intensity of the LED 37. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6##