Multiloop positioning control system

Claim 2

A position control system for driving a motor-driven machine to a programmed final position, the combination comprising: a servoamplifier connected to drive said motor in response to a position error signal; a position transducer connected to sense a positional of a machine and generate an analog position error signal having a magnitude varying as a function of the distance between said position of said machine and a position transducer reference point; a command signal generator connected to the position transducer and adapted to reset the position transducer reference point to the programmed final position; a digital error signal generator connected to sense a position of said machine and generate a digital position error signal when the machine is not in the programmed final position; and a loop control switch adapted to connect the digital position error signal to drive the servoamplifier until the machine comes to within a predetermined distance from the programmed final positioned, at which point it connects the analog position error signal to drive the servoamplifier.

Claim 3

The position control system of claim 1 wherein, the digital error signal generator has a position data control connected to receive the analog position error signal and is adapted to generate an incremental feedback signal when the analog position error signal surpasses a predetermined magnitude.

Claim 4

The position control system of claim 2 wherein the digital error signal generator has: a position storage connected to receive the incremental feedback signals from the position data control and is adapted to compile these incremental feedback signals and generate a digital absolute feedback signal; a command storage adapted to receive the programmed final position data and generate a digital command signal; and a digital position comparator connected to receive the digital command signal and digital absolute feedback signal, and adapted to generate the digital position error signal when there is more than a predetermined arithmetic difference between the digital command and digital absolute feedback signals.

Claim 5

The position control system of claim 2 wherein; there is a position storage connected to receive the incremental feedback signals from the position data control with is adapted to compile these incremental feedback signals, and the command signal generator is connected to the position storage to sense when an incremental feedback signal is compiled and reset the position transducer reference point one unit distance.

Claim 6

The magitude of this feedback signal varies to maintai the speedof the motor 2 constant at a predetermined value. The speed control 6 exercises control over the magnitude of this feedback signal to slow theemotor down when a slow down command signal is received at its input terminal 9. The multiloop position control system is designed to start the machine moving in the proper direction at the predetermined maximum speed, to decelerate it in steps as the programmed final position is approached, and to bring it smootyly to a stop at the programmed final position. THus is accomplished by first injecitnt a constant DC error signal into a second input terminal 10 on the servoamplifier 4, the polarity of which determines the desired direction of machine movement; generating slow down command signals to the input terminal 9 of the speed control 6 when the machine is typically 1 inch and 0.1 INCHES FROM THE PROGRAMMED FINAL POSITION: AND FINALLY, INITIATE A GRADUAL AND continuous DECREASE OF THE DC error signal when the machine is between 0.005 and 0.02 inches its progmvmmtx final position. The final position data needed to generate an error signal for tHe servoamplifier 4 is fed into a data input terminal 11. The data originates fromeither a punched tape reader or a manual data input (not shown) and is processed by a decoding block (not shown to be received in a command storage circuit 12. This final position data is stored in seven decimal digits from 0.0001 to 100 in a natural binary coded decimal format. The command storage 12 is scanned sequentially from the lest significnt decimal figit of the programmed dimension to the most signiffican decimal digit. The output of the command storage 12 is scanned serially, to minimize the duplication of equipment by allowing the use of time sharing of components when the control system forms a pat of a larger multiaxis control system. To obtain the desired error signal at the input terminal 10 of the servoamplifier 4, the final position data stored in the command storage 12 is periodically scanned and compared with digital absolute feedback signal that indicates the present position of the machine 1. To generate this digital absolute feedback signal the present position of the machine 1 must first be accurately determined and converted into a digital form. This is accomplished in the preferred embodiment by mechainically attaching a position transducer 13 to the machine 1. The position transducer 13 generates an accurate analog error signal for a range of machining displacement of at leas t 0.02 inches. Because the position transducer 13 is chosen to indicate accurately small displacements, it cannot be used along to measure large machine displacements. However, alone to measure large machine by a unique method of repositioning, or resetting, the position transducer reference point, a single position transducer 13 is used. The position transducer reference point, when used herein, refers tp a machine position which will result in no analog error signal output from the position transducer 13. Applying this reset method, the position transducer reference point is reset when the machine moves at least 0.01 inches, and the new null, or position transducer reference point is advanced 0.01 inches in the direction of machine motion. Resetting of the position transducer 13 is accomplished by means of a closed loop system having a position data control 14, a position 15 and a command signal generator 16. As the machine moves and the transducer reference point is resetk the magnitude of the analog error signal appearing at the output terminal 17 swints repeatedly over a voltage range During ideal operating conditions this voltage cycle repaeats once for each unit distance of machine displacement, or in the preferred embodiment, approximately each 0.01 inches. The analog error signal is fed to the input of the position data control 14 which is designed to generat a DC level at its output when the analog error eignal reaches or surpasses a predetermined voltage level. The output of the position dta control 14 is a voltage level which is maintained as long as the analog error signal surpasses the predetermined level. The polarity of the voltage level is determined by the direction the machine 1 is moving. The voltage level produced by the position data control 14 is fed to the position storage 15. The position storage 15 also receives a clock signal from an external source (not shown) which makes an entry into the position storage 15 when the position data control output indicates the predetermined error level has been reached.Each such entry constitutes an incremental feedback signal, which inuuu------v--uuuuuuu2222;;,,,u22eevel. RTING CONDITIOND THISTANCE RPRTING CONDITIOND THISTANCE RPRESENTED BY THIS INCREMENTAL FEEDBACK SIGNAL IS ALSO SUBJECT TO UNINTENDED VARIATIONS SUCH AS THAT CAUSED BY NOISE. Aunique fearure of this invention, to be described below is the ability to compensate for these unintended and undesirable variations in the amount of machine displacement occurring between incremental feedBack signals. As a consequence, the eincremental feedback sigmal herein differs from thse prior systems in its retention of the information necessary to correct error caused by factors external to the control system The position storage 15 compiles the 0.01 inch entries made by the incremental feedback signal into a digital absolute feedback signal which represents the total machine displacement from a zero reference point. The decimal digits 0.01 through 100 of this up-to-date machine position dimension are stored in a series of shift registers ia natural binary coded decimal. The additions made to the 0.01 decimal digit in the position storage 15 by the incremental feedback signal are indicated by a voltage change at an output 18 which is connected directly to the input 19 on the command signal generator 16. Aswill be discussed below, the command signal generator 16 is appropriately designed to work in combination with the position transducer 13. When a signal is received at the input terminal 19, a command signal is generated by the command signal generator 16 which causes the transducer reference point to reset exactly 0.01 inches in the direction of machine motion. By feeding back information derived from entries made to the posit.ntmvgt 15 to reset the position transducer 13, theanalog error signal is caused to shift in voltage level with the result that the DC output level of the position data control 14 drops back to zero. P The closed loop thus formed provides two functions First, as discurssed previously, it serves to peridically reset the position trandeucer reference point to deep the position transducer 13 within its operating rang. Secondly, it provides a means of retianing the information necessary to generate incremetnal feedback signals that will correct automatically for e rors caused by noise or miscounts. P The self correting feature of theclosed loop operates as follows. The erroneous additional entry to the position storage 15 is sensed at the input terminal 19 of the command signal generator 16 similar to a correct entry. Conseuqently, the command signalgenerator 16 repositone the transducer reference point an additional b 0.01 Linches in the direction of machine travel. This causes the analog error signal level at the output of the position transducer 13 to increase accordingly with the result that the machine 1 must travel an additional b 0.01 Linches before the magnitude of the analog error signal will reach the level it had attained prior to the injection of the erroneous entry. In other words, the machine 1 mjst travel an additional unit distance of b 0.o1 L INCHES BEFORE the position DATA CONTROL % SENSES THE NECESSARY LEVEL TO MAKE ANOTHER ENTRY INTO THE POSITION STORAGE /. The closed loop causes the system to skip an dentry,

Claim 7

The magitude of this feedback signal varies to maintai the speedof the motor 2 constant at a predetermined value. The speed control 6 exercises control over the magnitude of this feedback signal to slow theemotor down when a slow down command signal is received at its input terminal 9. The multiloop position control system is designed to start the machine moving in the proper direction at the predetermined maximum speed, to decelerate it in steps as the programmed final position is approached, and to bring it smootyly to a stop at the programmed final position. THus is accomplished by first injecitnt a constant DC error signal into a second input terminal 10 on the servoamplifier 4, the polarity of which determines the desired direction of machine movement; generating slow down command signals to the input terminal 9 of the speed control 6 when the machine is typically 1 inch and 0.1 INCHES FROM THE PROGRAMMED FINAL POSITION: AND FINALLY, INITIATE A GRADUAL AND continuous DECREASE OF THE DC error signal when the machine is between 0.005 and 0.02 inches its progmvmmtx final position. The final position data needed to generate an error signal for tHe servoamplifier 4 is fed into a data input terminal 11. The data originates fromeither a punched tape reader or a manual data input (not shown) and is processed by a decoding block (not shown to be received in a command storage circuit 12. This final position data is stored in seven decimal digits from 0.0001 to 100 in a natural binary coded decimal format. The command storage 12 is scanned sequentially from the lest significnt decimal figit of the programmed dimension to the most signiffican decimal digit. The output of the command storage 12 is scanned serially, to minimize the duplication of equipment by allowing the use of time sharing of components when the control system forms a pat of a larger multiaxis control system. To obtain the desired error signal at the input terminal 10 of the servoamplifier 4, the final position data stored in the command storage 12 is periodically scanned and compared with digital absolute feedback signal that indicates the present position of the machine 1. To generate this digital absolute feedback signal the present position of the machine 1 must first be accurately determined and converted into a digital form. This is accomplished in the preferred embodiment by mechainically attaching a position transducer 13 to the machine 1. The position transducer 13 generates an accurate analog error signal for a range of machining displacement of at leas t 0.02 inches. Because the position transducer 13 is chosen to indicate accurately small displacements, it cannot be used along to measure large machine displacements. However, alone to measure large machine by a unique method of repositioning, or resetting, the position transducer reference point, a single position transducer 13 is used. The position transducer reference point, when used herein, refers tp a machine position which will result in no analog error signal output from the position transducer 13. Applying this reset method, the position transducer reference point is reset when the machine moves at least 0.01 inches, and the new null, or position transducer reference point is advanced 0.01 inches in the direction of machine motion. Resetting of the position transducer 13 is accomplished by means of a closed loop system having a position data control 14, a position 15 and a command signal generator 16. As the machine moves and the transducer reference point is resetk the magnitude of the analog error signal appearing at the output terminal 17 swints repeatedly over a voltage range During ideal operating conditions this voltage cycle repaeats once for each unit distance of machine displacement, or in the preferred embodiment, approximately each 0.01 inches. The analog error signal is fed to the input of the position data control 14 which is designed to generat a DC level at its output when the analog error eignal reaches or surpasses a predetermined voltage level. The output of the position dta control 14 is a voltage level which is maintained as long as the analog error signal surpasses the predetermined level. The polarity of the voltage level is determined by the direction the machine 1 is moving. The voltage level produced by the position data control 14 is fed to the position storage 15. The position storage 15 also receives a clock signal from an external source (not shown) which makes an entry into the position storage 15 when the position data control output indicates the predetermined error level has been reached.Each such entry constitutes an incremental feedback signal, which inuuu------v--uuuuuuu2222;;,,,u22eevel. RTING CONDITIOND THISTANCE RPRTING CONDITIOND THISTANCE RPRESENTED BY THIS INCREMENTAL FEEDBACK SIGNAL IS ALSO SUBJECT TO UNINTENDED VARIATIONS SUCH AS THAT CAUSED BY NOISE. Aunique fearure of this invention, to be described below is the ability to compensate for these unintended and undesirable variations in the amount of machine displacement occurring between incremental feedBack signals. As a consequence, the eincremental feedback sigmal herein differs from thse prior systems in its retention of the information necessary to correct error caused by factors external to the control system The position storage 15 compiles the 0.01 inch entries made by the incremental feedback signal into a digital absolute feedback signal which represents the total machine displacement from a zero reference point. The decimal digits 0.01 through 100 of this up-to-date machine position dimension are stored in a series of shift registers ia natural binary coded decimal. The additions made to the 0.01 decimal digit in the position storage 15 by the incremental feedback signal are indicated by a voltage change at an output 18 which is connected directly to the input 19 on the command signal generator 16. Aswill be discussed below, the command signal generator 16 is appropriately designed to work in combination with the position transducer 13. When a signal is received at the input terminal 19, a command signal is generated by the command signal generator 16 which causes the transducer reference point to reset exactly 0.01 inches in the direction of machine motion. By feeding back information derived from entries made to the posit.ntmvgt 15 to reset the position transducer 13, theanalog error signal is caused to shift in voltage level with the result that the DC output level of the position data control 14 drops back to zero. P The closed loop thus formed provides two functions First, as discurssed previously, it serves to peridically reset the position trandeucer reference point to deep the position transducer 13 within its operating rang. Secondly, it provides a means of retianing the information necessary to generate incremetnal feedback signals that will correct automatically for e rors caused by noise or miscounts. P The self correting feature of theclosed loop operates as follows. The erroneous additional entry to the position storage 15 is sensed at the input terminal 19 of the command signal generator 16 similar to a correct entry. Conseuqently, the command signalgenerator 16 repositone the transducer reference point an additional b 0.01 Linches in the direction of machine travel. This causes the analog error signal level at the output of the position transducer 13 to increase accordingly with the result that the machine 1 must travel an additional b 0.01 Linches before the magnitude of the analog error signal will reach the level it had attained prior to the injection of the erroneous entry. In other words, the machine 1 mjst travel an additional unit distance of b 0.o1 L INCHES BEFORE the position DATA CONTROL % SENSES THE NECESSARY LEVEL TO MAKE ANOTHER ENTRY INTO THE POSITION STORAGE /. The closed loop causes the system to skip an dentry,

Claim 8

The position control system of claim 3 wherein said digital position comparator stops generating a digital error signal when said machine is a predetermined number of unit distances from said programmed final position and it generates a predetermined number of incremental feedback signals to said position storage.

Claim 9

A position control system for driving a motor-driven machine to a programmed final position, the combination comprising: a servoamplifIer adapted to drive the motor when a position error signal is received at its input; a position transducer having a transducer reference point, connected to sense the distance between this reference point and the machine position, and adapted to generate an analog position error signal, the magnitude of which is determined by this distance; a position data control connected to receive the analog error signal, adapted to sense its magnitude, and generate an incremental feedback signal when the magnitude of said analog position error signal surpasses a predetermined level; a position storage connected to receive the incremental feedback signal, compile a digit representing a unit distance of machine displacement for each such incremental feedback signal received and generate an absolute digital feedback signal; a command signal generator connected to the position storage and position transducer and adapted to reset the transducer reference point one unit distance for each digit compiled in the position storage; a digital position comparator connected to receive the digital absolute feedback signal and adapted to generate a digital position error signal when the machine position is greater than a predetermined distance from the programmed final position; and a loop control switch connected to receive the digital position error signal and the analog position error signal and adapted to connect the digital position error signal to the servoamplifier, and to connect the analog position error signal to the servoamplifier when the machine position is within the predetermined distance from the programmed final position.

Claim 10

A position control system for driving a motor-driven machine to a programmed final position, the combination comprising: a servoamplifIer adapted to drive the motor when a position error signal is received at its input; a position transducer having a transducer reference point, connected to sense the distance between this reference point and the machine position, and adapted to generate an analog position error signal, the magnitude of which is determined by this distance; a position data control connected to receive the analog error signal, adapted to sense its magnitude, and generate an incremental feedback signal when the magnitude of said analog position error signal surpasses a predetermined level; a position storage connected to receive the incremental feedback signal, compile a digit representing a unit distance of machine displacement for each such incremental feedback signal received and generate an absolute digital feedback signal; a command signal generator connected to the position storage and position transducer and adapted to reset the transducer reference point one unit distance for each digit compiled in the position storage; a digital position comparator connected to receive the digital absolute feedback signal and adapted to generate a digital position error signal when the machine position is greater than a predetermined distance from the programmed final position; and a loop control switch connected to receive the digital position error signal and the analog position error signal and adapted to connect the digital position error signal to the servoamplifier, and to connect the analog position error signal to the servoamplifier when the machine position is within the predetermined distance from the programmed final position.

Claim 11

The position control system of claim 6 wherein the digital position comparator is also adapted to generate a predetermined number of digits to the position storage when the machine position is within the predetermined distance from the programmed final position.

Claim 12

The position control system of claim 6 wherein the position transducer is a resolver mechanically connected to sense machine displacement, and a position error detector connected to the resolver output and adapted to generate the analog position error signal.

Claim 13

The position control system of claim 8 wherein the command signal generator is connected to generate electrical command signals to the resolver that determine the position of the position transducer reference point. BACKGROUND OF THE INVENTION The basic objective of a numerical positioning control system is to move a portion of a machine as rapidly as possible from one point to another. This is usually accomplished by the use of aclosed loop servosystem. In such systems an error signal having a polarity indicating the direction of desired travel and a magnitude indicating the distance of desired travel is injected into a servoamplifier which drives an electric or hydraulic motor on the machine.A transducer is mechanically linked to sense the motion of the machine and feed back a signal to the input of the servoamplifier which will null the error signal when the machine has reached theprogrammed final position. There are two basic types of transducers used in positioning control systems; an incremental feedback transducer, and an absolute feedback transducer. In the first type, the feedback signal is interpreted merely as a numerical value corresponding to an amount of machine displacement. While theesecond generates a feedback signal indicating the actual location of the machine from a fixed reference position. Additionally, the feedback signals produced by position transducers may be either an anayyyyylggggxstomary in these position control systems to use a plurality of position transducers, each having progressively narrower operating ranges to provide greater accuracy as the programmed final position is approached An electronic switching circuit such as that shown in U.S. Pat. No. 3,240,951, to Lynn H. Matthias and Odo J. Struger issued March 15, 1966 and entitled STATIC ITCH FOR MULTISPEED ERROR DETECTOR CONTROL STEM is used to shift control progressively from the coarse to the fine position transducers as theemachine approaches the programmed final position. To increase the accuracy of such position control systems, therefore, additional position transducers must be provided along with the additional switching circuitry. A second approach is the use of a position transducer that generates an incremental feedback signal. These systems are usually digital, and when the machine moves a predetermined amount, the position transducer accurately records this by generating an electric pulse or digit. These pulses are either subtracted directly from the digitally stored final position command signal or are accumulated to form a digital absolute feedback signal which is compared periodically to the digital command signal to produce an error signal representing the distance to be traveled. in either of these cases, incremental or absolute programmed, there are a number of problems encountered when an incremental feedback signal is used. Because control systems operate in an industrial environment, noise signals are often induced ito their circuitry. These signals may be interpreted as pulses from the position transducer which ultimately result in an inaccruate error signal. Additionally, the accuracy or resolution of the system is limited to the finite distance the machine must travel to initiate a pulse from the position trasducer. Both of these problems can be minimized by decreasing the incremental distance needed to register the pulse. Such a solution, howevrm is not without cose because the capacity of storage circuits must be increased to accommodate smaller signal increments, and the speed with which the counts from the position transducer are registered must be substantially increased. Also, decreasing the incremental distance needed to produce a pulse from the position transducer does not solve the noise problem, but only decreases the error introduced when the noise pulses regitter. SUMMARY OF THE INVENTION This invention relates to a position control system which combines the advantageous features of the incremental feedback method with the advantageous features of the analog absolute feedback method by combining an analog feedback loop and a digital feed back loop into a single system. More particularly, the invention resides in a servoamplifier connected to drive the machine motor; a posityon transdecer which produces an analog error signal related in magnitude and polarity to a transducer reference point, a digital error signal generator connected to receive the analog error signal and generate a digital error signal; and a loop control switch which cothe digital error signal to the servoamplifier input until the machine comes to within a predetermined distance from its programmed final position, at which time the analog error signal from the position transducer is connected to control the servoamplifier input until theprogrammed final position is reached. The invention also relates to a control system in which the digital error signal generator has a position data control circuit connected to receive the analog error signal and control the entry of a digit into a position storage circuit when their error signal reaches or surpasses a predetermined level, and the system has a command signal generator connected to electrically or mechanically move or reset the podition transducer reference point an incremetnal amount when a digit is registered in the position storage. A general objective of the invention is to provide a hybrid position controlsystem which operates with digital feed back for large movements of the machine by registering and rememering the number of times the position transducer reference point is repositioned, and switching the system to an analog feedback system when the machin nearly reaches its programmed final position. Another objEctive of the invention is to use an analog position transducer which is accurate for small increments of machine displacement and to provide circuitry which periodically repositions the reference point from which the transducer measures distance to keep its signal output well within theaccurate operting range of the transducer. Still another objective of the invention is to provide an incremental feedback system in which false counts caused by factors such as noise can be neglected by utilizing the additional information contained in the analog error signal which is not available in digital incremental feedback systems. Amore specific objective is to provide a system in which a position transducer can be used to generate a feedback signal in a digital feedback system, and to generate an analog error signal in an analog feedback system. Other objects and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawings, forming apart hereof, in which there is shown, by way of illustation and not of limitation, a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the preferred embodiment of the multiloop position control system; FIG. 2 is a schematic electrical diagram of the loop control switch shown in FIG. 1; and fig. 3 is a schematic block diagram of the position transducer shown in FIG. 1. DESCRIPTION OF THE PREFEMBODIMENT The block diagram of FIG. 1 shows a machine 1 mechanically linked to a motor 2 which will drive the machine 1 from a zero reference position to a programmed final position The machine 1 is typically a machine tool, such as a drilling or milling machine, in which rapid and accurate positioning from point to point is desired. The motor 2 is a DC electric motor, however, any prime mover having a controllable speed is satisfactory. The field coils (not shown) of the DC motor 2 are supplied a constant DC voltage while the motor armature receives current from the output terminal 3 of a servoamplifier 4. The polarity and magnitude of the output signal from the servoamplifier 4 determines the direction and speed

Claim 14

The difference between these two digital signals produces either a positvie or negative digital error signal that drives the motor 2 in a direction which causes this digital errir signal to decrease in magnitude. The combined position data control 14, position storage 15, command storage 12, and digital position comparator 26, form a digital error signal generator. When the difference between the digital command signal and the digital absolute feedback signal reaches b 0.02 Linches the digital position comparator 26 is locked to cease generating a digital error signal. Simultaneously, the pg,21 digital position comparator 26 causes additional entires to be mde to the b 0.01 decimal of the position storage 15 through the line 40 until the digital comparator output is zero. As when erroneous entries are made to the position storage 15, these entries cuse the command signal generator 16 to reset the position transducer reference point an additional b 0.02 L inches. This causes the analog error signal from the position trandducer 13 to increase in magnitude an additional amount. because thereeis no longer a digital error signal being applied to the loop control of the servo amplifier 4 and motor 2. The operational amplifier 30 of the loop control 27 remains saturated because of the additional b 0.02 L inches entry made during conversion. When the machine 1 approaches to within approximately b 0.01 L inches of its programmed final position, this output drops out of saturation and the analog error signal continuously and smoothly slows the machine, bringing it to a stop at the programmed final position. The point where control is switched to the analog feedback loop is easily adjusted to any value by locking the digital position comparator 26 at a different point. Additionally, the point at which the operational amplifier 30 comes out of saturation is adjustavle to accommodate the particular machine and speeed used. P The invention dicdred eerein rdve manp rf tee prrkdem encruntered kp prirr pritirning crntrrd meterd. Frr darge maceine mrvement 22 k 5.51 D INCEE/ accuracp i pg,22 inured kp uing digitad feedkack tr tee ervrampdidier 7. Errors caused by induced nrise signals are minimized by an additirnal feedback loop between tee pritirn trrage 15 and positirn transducer 13 that compensates for the error. Furthermore, the digital increments handled by the digital feedback system are quite large, thus minimizing the amount and complexity of the storage and logic circuits. Instead of demanding increased accuracy from the digital feedback system, vontorl is switched to an analog feedback system control is switched to an analog feed system when thE programmed final position is approached. This final apprach is made by means of a highly accurate analog feedback signal generated by a position transducer 13 that is specifically designed to operate accurately over a small range of machine dqsplacement. This system also allows a significat cost reduction by its unique use of a single pritione transducer 13 in both the digital and analog feedback loops and for both large and small machine movements. P The aforesaid description of the invention, and the manner of making and using it, is i such full, clear, concise and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make an duse the same, and sets forth the est mode contemplated by the inventors of carrying out their invention. P In contrast to the foregoing description, the followi ng pg,23 claims particularly point out and distnctly claim thesub ect matter which theapplicants regard as their invention. pg,24 cm We claim cm 1. A position control system for driving a motor driven machine to a programmed final position, thecom ination comprising PP a servoamplifier connected to drive said motor in response to a position error sigaml PP a position transducer connected to sense a positional of a machine and generate an analog position error signal having a magnitude varying as a function of the distance betwe e said position of said machine and a position transducer reference point PP a command signal generator connected to the position transduver and adapted to reset the position transducer referencepoi nt to the programmed final position PP a digital error signal generator connected to sense a position of said machine and generatie a digital position error signal lwhen the machine is not in the programmed final position and PP a loop control switch adapted to connect the digital position error signal l to drive the servoamplifier until the machine comes to within a predetermined distance from the programmed final positioned, at which point it connects the analog position error signal to drive the servo amplifier. pg,25 cm 2. The position control system of claim 1 wherein, the digital error signal generator has a position data control connected to receive the analog position error signal and is adapted to generate an incremental feedback signal when the analog position error signal surpasses apredetermimed magnitude. cm 3. The position control system of claim 2 wherein the digital eirir signal generator has PP a position storage connected to receive the incremental feedback signals from the position data control and is adapted to compole these incremental feedback signals and generate a digitil absolute feedback signal PP acommand storage adapted to receive the programmed final position data and generate a digital command signal and PP a digital position comparator connected to receive the digital command signal land digital absolute feedback signal, anadapted to generate the digital position error signal when there is more than a predetermined arithmetic difference between the digital command and digital absolute feedback signals. pg,26 cm 4. The position control system of claim 2 wherein there is a position storage connected to receive the incremental feedback signals from the position data control with is adapted to compile these incremental feedback signals, and the command signal generator is fonnected to the position storage to sense when an incremental feedback signal is compiled and reset the position transducer reference point one unit distance. cm 5. The position control system of claim 3 wherein said digital position comparator stops generating a digital error signal when said machine is a predetermined number of unit distances from said programmed final position and itgenerates a predetermined number of incremental feedback signals to said position storage. pg,27 cm 6. A position control system for driving a motor driven machine to a programmed final posItion, the combination comprising p1 a servoamplifier adapted to drive the motor when a position error signal is received at its input p1 a position transducer having a transducer reference point, connected to sense the distance between this reference point and the machine position, and adapted to generate an analog position error signal, the magnitude of which is determined by this distance p1 a position data control connected to receive the analog position error signal, adapted to sense its magnitude, and generate an incremental feedback signal when the magnitude of said analog position error signal surpasses a predetermined level p1 a position storage connected to receive the incremental feedback signal, compile a digit representing a unit distance of machine displacement for each such incremental feedback signal received and generate an absolute digital feedback signal p1 a command signal generator connected to the position storage and position transducer and adapted to reset the transducer reference point one unit distance for each digit compiled in the position storage p1 a digital position comparator connected to recevie the pg,28 digital absolute feedback signal and adapted to generate a digital position