CONTROLLED ACCELERATION/DECELERATION CIRCUIT FOR OPTICAL TRACERS

Abstract: In an optical line tracer sudden changes of velocity can cause instability and vibration. Means are provided to limit acceleration or deceleration to a specific rate of change. Speed change signals are processed through a ramp circuit which converts the step function signal to a ramp function before application.

Description: This invention relates to a drive system for a coordinate drive optical pattern tracer, said drive system being of the kind defined in the preamble of claim 1. Optical pattern tracers are well known in the machine tool field and are commonly used to control the cutting of various machine tools, particularly gas and some cutting machines, which may be caused to cut specific shapes from material based upon a predetermined optical pattern. One class of such pattern tracers is an optical pattern tracer which views a pattern such as a dark silhouette or line on a white surface and causes the tracer and its associated machine tool to follow the convolution of the pattern. One of the preferred forms of optical tracers, scans a circular area repetitively across the edge of the pattern and produces an electrical signal representative of the change of reflectance as the scanning action crosses the edge of the pattern. This electrical signal is then used together with reference signals and other information to produce coordinate drive information to control X and Y drive motor which cause the machine to move with constant tangential velocity around the pattern. Typical of tracers of this class are those described in U.S. Patent No. 3,704,372, issued November 28, 1972 to Robert E. Parker, et al., U.S. Patent No. 3,727,120, issued April 10, 1973 to George Jewell, et al., U.S. Patent No. 3,860,862, issued January 14, 1975 to William Dell et al., and U.S. Patent No. 3 883 735, issued May 13, 1975 to Francis P. Murphy et al. EP-A-155 395 discloses a pattern tracer slowdown circuit which utilizes adjustable durations for the normal scanner signal and advance scanner signal, but of the same value, and so the degree of overlap indicates a pattern direction deviation to permit the slowdown signal to be produced. In many such tracers the tracing velocity is changed not only at the beginning and end of a trace, but also during certain convolutions, for example, on certain corners and at times when other procedures are required. This slowdown may be initiated either by a special mark on the pattern or by the mere existence of a rapid change of direction or, in the simplest form, such a slowdown may be initiated by the operator by operating operation of a suitable switch, push bottom, etc. In all of these situations, the change in velocity tends to be a step function, abruptly changing from one velocity to a higher or lower velocity. It will be obvious that such step changes in the velocity may cause the machine to vibrate or shake, due to the demand for maximum acceleration or deceleration. Such vibrations will cause imperfections in the cut or tracing action. A drive system of the kind defined in the preamble of claim 1 is included in an optical line tracer of the circular scanning, non-steering type, as taught in CA-A-1 177 142, in which two scan systems are used for coincidence with one being the normal scanner for producing the desired signals to enable the pattern tracer to follow the pattern and the other being an advance signal scan which indicates deviation of the pattern from a relatively straight line. To obtain a gradual change of velocity the velocity change signal will be applied to a ramp function generating means which will generate a ramp function that replaces the step function. Since the ramp fraction generating means is placed in the speed control circuit the machine is prevented from responding to normal tracing demands. An object of the invention is to provide a drive system of the kind defined in the preamble of claim 1, said drive system being designed to avoid such vibrations mentioned above without preventing the tracer from responding to normal tracing demands. This object is achieved by applying the elements of the characterizing part of claim 1 to a drive system of the kind defined in the preamble of claim 1. In accordance with the present invention, changes in velocity indicated by a stepped slowdown circuit or by startup or stopping of the machine inititate a specific gradual rate of change circuit output, i.e., a ramp signal. The ramp circuit is not in the speed control circuit, since this would prevent the machine from responding to normal tracing demands, but instead precedes the sample and hold circuits in the electronic circuitry. An embodiment of the invention will be described by way of example and with reference to the accompanying drawings in which Figure 1 is a block diagram of a drive system for a coordinate drive optical pattern tracer, Figure 2 is a schematic circuit diagram of the ramp circuit of Figure 1; and Figure 3 is a representation of the wave form at certain points in the circuit. Considering first Figure 1, a nonsteered head tracing system includes a tracing head 1 which provides a sine and cosine signal on conductors 2 and 3. These sine and cosine signals are controlled by the ramp circuit 4 and applied with selectable amplitude to the sample and hold circuits 5 and 6. The photocell output from the tracing head is applied to the signal processor 7 and a sampling signal representative of the time of occurrence of the intersection of the trace and the pattern is applied to the sample and hold circuits from the signal processor. The sample and hold circuits produce from these signals, suitable coordinate drive signals X, Y which appear at terminals 8 and 9. The system also includes a slowdown circuit 10, which may be manually activated or activated by a signal from the signal processor 7. The output from the slowdown circuit is applied to the ramp circuit 4. In operation, the output from sample and hold circuit 5, for example, is determined by the value of the sinusoid applied to it from the ramp circuit, at the time determined by the time of occurrence of the sampling signal from the signal processor. It will be evident therefore, that the output at terminal 8 is a function of the amplitude of the sine wave applied to the sample and hold circuit. Upon the occurrence of a slowdown, either indicated by a signal received from the signal processor or by a manually introduced signal provided by the operator, the output from the slowdown circuit applied to the ramp circuit changes in a step function. The ramp circuit, however, is arranged so that this step function generates a ramp of predetermined slope and amplitude, which controls the output amplitude of the sine waves passing through it. Thus, the sine wave applied to the sample and hold circuit 5, rather than abruptly changing amplitude, when a slowdown signal is received, changes in a controlled manner from a first value to a second value. The output from the sample and hold circuits, which controls the coordinate drive system, therefore varies in a similar manner. Turning now to Figure 2, a detailed circuit diagram of the ramp circuit 4 is shown. The input from the slowdown circuit is applied to terminal 11, which operates a relay 12 having a plurality of contacts, including a single-pole double-throw portion which connects the moving of the contact, either between resistors 13 and 14 or 14 and 15 of the potentiometer, comprising resistors 13, 14 and 15, connected from a suitable volt source of potential to ground. The output from the moving contact is applied to the noninverting terminal of operational amplifier 16 and the output of the operational amplifier 16 is coupled back to the inverting input of the operational amplifier, in a unity gain noninverting configuration. Alternatively an inverting configuration may be used. Suitable potentials are applied to terminals 7 and 8 of the operational amplifier. A selectable capacity is coupled between terminal 8 of the operational amplifier and a suitable potential. A plurality of switches 17, 18, 19 and 20 permit selection of the desired capacity between terminal 8 and the potential. When connected in this manner, the operational amplifier output goes from a first voltage to a second voltage at a linear rate determined by the value of the capacitor connected to terminal 8, when the input to the plus terminal of the operational amplifier is switched from a first value to a second value. With the values shown, the ramp produced may have a duration in the order of one half second. Other forms of ramp generators may be used for this purpose. This ramp function is applied to a pair of analog multipliers 21 and 22. Also connected to the analog multipliers are the conductors 2 and 3 which apply to them the sine and cosine waves from the tracing head. The output of these analog multipliers is, in the case of multiplier 21, the sine voltage times the ramp voltage applied from operational amplifier 16 divided by 10. Similarly, the output from analog multiplier 22 is the cosine voltage times the ramp function applied from operational amplifier 16 divided by 10. The output from these multipliers is then applied to the sample and hold circuits, as shown in Figure 1. Operation : A clearer understanding of the operation of the system described, may be had from a consideration of Figure 3, which is a series of wave forms. Wave form A in Figure 3 represents the potential applied to terminal 11 in Figure 2. As will be seen, the potential abruptly changes from a first value to a second value, indicating a slowdown. Wave form B, presents the output from operational amplifier 16. As will be seen, the abrupt changes in wave form A have been converted to slope changes or ramps. The time d in wave form B is a function of the value of the capacity connected to terminal 8, that is, the number of switches 17, 18, 19 and 20 closed. The time d may be in the order of .5 seconds. The larger the capacity the more gradual the ramp and the longer the period d. Wave form C, represents the output from the analog multipliers and as will be seen, the amplitude of the sine wave gradually changes from a first amplitude to a second lesser amplitude over a period of time, equal to period d. It will be evident that with this arrangement, there is no abrupt change in velocity of the system and the machine is therefore not subject to impulses or abrupt changes in attempting to attain a change in velocity in zero time. While the invention has been described specifically in relation to a circuit using a sample and hold circuitry, it will be evident that a similar arrangement can be applied to tracers incorporating sine and cosine voltage generators such as those used in steered head systems. In tracing heads where the amplitude of the sine and cosine produced are dependent on a reference voltage the output from the operational amplifier would be directly applied to the tracing head as a reference voltage. It will also be evident that while the invention has been illustrated with reference to a tracing system which produces both a sine and cosine output, it will have equal application to those situations where a single sinusoid is produced and from it a pair of sinusoids are produced right angles to each other. It should be understood that the slowdown circuit 10, of Figure 1, which produces a step function, which is applied to terminal 11, may be a simple switch controlled by the operator or may be a complex circuit which derives a signal from the tracing head, either by means of command marks on the pattern or by means of advanced detection which indicates when there is an abrupt change or direction of the tracing head.

Claims: A drive system for a coordinate drive optical pattern tracer, comprising a pair of sample and hold circuits (5,6) for producing coordinate drive signals (X,Y), a tracing head (1) for producing first and second signals which are applied to said sample and hold circuits, said first signals being coordinate sinusoidal voltages and said second signals being timing signals indicating the time of occurrence of the intersection of the trace and the pattern, velocity change input means (10) for producing velocity change signals in response to some predetermined stimulus, and ramp function generating means (4) for gradually varying the velocity of said drive system, said velocity change input means (10) being connected to said ramp function generating means (4) for applying said velocity change signals to said ramp function generating means (4), characterized in that said ramp function generating means (4) are connected between said tracing head (1) and said sample and hold circuits (5,6) so as to be able to vary the amplitude of said sinusoidal voltages in accordance with the ramp function upon the receipt of a velocity change signal. A drive system as claimed in claim 1, wherein said velocity change input means comprises a speed control relay (10). A drive system as claimed in claim 2, wherein the output of said speed control relay (10) is applied to said ramp function generating means (4). A drive system as claimed in any one of the preceding claims, wherein said sinusoidal voltages and said ramp function are applied to a pair of analog multipliers (21, 22), and the outputs of said multipliers are applied to said sample and hold circuits (5,6).

Inventors: ENN, VALI
Applicants: WESTINGHOUSE CANADA INC.
Application year: 1988
Main classification: B23Q35
IPC codes: B23Q35, G05B19