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CONTROL-SIM Version 2.0 Users Manual Copywrite 1992, Roger W. Thompson, All rights reserved RTW Software P.O. Box 710292 Houston, TX 77271 CONTROL-SIM Users Manual Page 2 REGISTRATION Please feel free to evaluate CONTROL-SIM and distribute copies of CONTROL-SIM as long as all files are unaltered and distributed together. If you continue to use CONTROL-SIM you are obligated to register this program by sending a U. S. check or money order for $35 ($32 + $3 handling) to: Roger W. Thompson RTW Software P.O. Box 710292 Houston, TX 77271 Texas residents should include 8.25% sales tax. Upon registration, you will receive the latest version of CONTROL-SIM, without the shareware screen. In addition, QuickBASIC source files for the main program and controller subroutines, plus library files, will be included to allow controller development. You will also be notified of future updates. The registered version of CONTROL-SIM will be licensed to you for single computer system use and it may not be distributed in any manner. Please send any comments or suggestions you may have with regard to CONTROL-SIM. If feasible, they may be incorporated in future updates. DISCLAIMER Although CONTROL-SIM has been tested extensively and is believed to produce accurate results if used correctly, no guarantee is provided. RTW Software disclaims all warranties relating to this software, whether express or implied, as to the merchantability or fitness of this program for any purpose. RTW Software also assumes no liability for any damages, either direct or inconsequential, which may result from use of this program. CONTROL-SIM Users Manual Page 3 1.0 INTRODUCTION This program simulates operation of a simple plant control loop. It is intended to be used by engineers and students involved in practical digital control system design and application. Significant uses include: Tuning a controller Specification (modeling) of a plant response Evaluation of PID and other types of controllers Simulation of a typical bounded controller output and input Evaluation of effects of quantization and sample rate Time response of LaPlace transforms Conversion from LaPlace transforms to state equations Development of control algorithms (registered users only) The program is designed to be easy to use allowing the user to concentrate on control system performance characteristics rather than program commands and variables or extensive math. Full featured menu's and an initial (default) example permit the beginner to experiment quickly, while the more advanced user can create, evaluate, and save plant configuration files for future reference. a. CONTROL-SIM System Requirements: * IBM PC, XT, AT or compatible * EGA OR VGA color display * IBM/Epson compatible printer * MS-DOS 2.1 or higher b. Installation For backup, make a working copy of the original distribution disc. CONTROL-SIM can be run from either a floppy disc or from the hard disc. If hard disc operation is desired, create a new separate directory and copy all CONTROL-SIM files to the new directory. For example, at C:\ prompt enter: MKDIR ctrl COPY A:\*.* C:\ctrl The original disc and the new directory should contain the following files: README Initial instruct PROCESS.PLT Sample config file CS.EXE Program TEMP.PLT Sample config file MANUAL.DOC Documentation SERVO.PLT Sample config file DX1.PRX Utility file EXAMPL1.PLT Sample config file CTRLBLK.EGX Picture file EXAMPL2.PLT Sample config file DAT1.PRN Sample data file COMPCKT.PLT Sample config file FZVALVE.PLT Sample config file FZSERVO.PLT Sample config file PIPDEL1.PLT Sample config file PIPDEL2.PLT Sample config file CONTROL-SIM Users Manual Page 4 2.0 BASIC OPERATION Begin CONTROL-SIM operation by entering at C:\ prompt: CD \CTRL CS After introductory screens are displayed, the Main Menu will appear with the following selections available: 1. DISPLAY DATA 2. DISPLAY BLOCK DIAGRAM DATA 3. PLOT DATA 4. MODIFY PLANT MODEL 5. MODIFY CONTROLLER 6. PLAYBACK DATA 7. CONFIGURATION FILES 8. EXIT The first 3 selections will display time response data as follows: DISPLAY DATA - Meter and Bar Graph DISPLAY BLOCK DIAGRAM DATA - Meter PLOT DATA - Chart Recorder traces In all three, the user can enter setpoints, activate a load change, change loop mode (open or closed), print or record data, and observe elapsed time from setpoint initiation. The DISPLAY DATA/DISPLAY BLOCK DIAGRAM DATA selections also provide an IAE (Integrated Absolute Error) indication to aid in tuning. The PLOT display allows user selection of up to 4 chart speeds depending on sample rate. The MODIFY PLANT MODEL selection is concerned with plant configuration, consisting of the plant model (LaPlace transform), delay time (or system lag time), sample rate, and equation display status. The MODIFY CONTROLLER selection is concerned with controller selection, control constants, quantization level, and range of controller output. The PLAYBACK DATA selection permits the user to view data text files recorded previously. CONFIGURATION FILES provides the means to save and retrieve plant/controller configuration files and to designate an alternate directory. CONTROL-SIM Users Manual Page 5 3.0 SYSTEM DESCRIPTION (Reference DISPLAY BLOCK DIAGRAM DATA) The simulated system includes the CONTROLLER and the PLANT. The CONTROLLER includes data acquisition and control output functions, controller function, loop mode function, and display of setpoint, measurement, and control values. The PLANT consists of the plant transfer function, transport delay, and load change function. The CONTROLLER simulates a typical hardware digital control system where conditioned analog data (voltage) from a PLANT sensor is sampled and converted to a digital value. The resolution of the digital value, or measurement, depends on the quantization level of the analog-to-digital converter. Typically, 12 bits is used, resulting in 4096 resolvable steps. The measured value is then compared with the user input setpoint value within the controller function. The difference between setpoint value and measured value, or error, is then used as input to the controller algorithm. The controller algorithm produces an output signal which is then converted from digital to analog and sent to the PLANT actuator as a voltage signal. Again, the resolution of the digital-to-analog converter is dependant on the quantization level, typically 12 bits. If the output is allowed to swing both positive and negative about zero, it is said to be bipolar and the resolution applies to the complete range. For example, 12 bits, or 4096 steps, would actually be 2048 steps above zero and 2048 below zero. The PLANT is simulated by a transfer function and a delay. The transfer function is assumed to represent all the elements of the plant loop from actuator input to sensor output. The transport delay is the total deadtime of the plant loop which is the sum of the time delays attributable to each element, including transport time between elements. The transfer function can be as simple as a single time constant or as complex as a multiple of time constants up to 4th order. Typical plant hardware elements are actuators, control valves, servo valves, electrical power controls, motors, heaters, sensors, and signal conditioners. The user can specify the plant transfer function as a LaPlace transform. As an example, the LaPlace transform of a simple integration function is: 1 .5 G(s) = -------- = -------- (2s + 1) (s + .5) where .5 = the reciprocal of the time constant, 2 seconds this is equivalent to the following RC network: R = 2 megohm o-----^\/^\/----------------o | ___ INPUT ___ C = 1 microfarad OUTPUT | | o----------------------------o CONTROL-SIM Users Manual Page 6 CONTROL-SIM converts the LaPlace transform into state equations (also called state space or state variables) for ease of computation. A further conversion is necessary from the continuous form of state equations to discrete form state equations, to account for the effects of sampled data. The resulting difference equations, which are updated each scan cycle, are simple multiply and add/subtract operations which the computer performs handily. 4.0 DETAILED OPERATION 4.1 DISPLAY DATA Selection Displays Process - the 3 basic control system variables are displayed in both bar graph and digital form. The (S)etpoint value, or reference value, is the desired value set by the user. The (C)ontrol value (also called manipulated variable) is the value output by the Controller. The (M)easure value (also called controlled variable) is the Plant output value. This is the variable which the control system is attempting to control. Elapsed Time - is displayed in seconds. The time is reset to 0 when a setpoint or load change is set, to reflect the response time of the plant. Mode indicator - shows the loop status (see Mode change below). Integrated Absolute Error (IAE) - is displayed to give a measure of relative performance of the same upset conditions (setpoint or load change) with different tuning constants or other controller/plant settings. It is reset to 0 when a setpoint or load change is activated. IAE is a measurement of the area existing between the setpoint value and the measured value response. User Controls Setpoints - Enter setpoint value (0 - 100) and depress the S key to activate the new setpoint. All system inputs and outputs are represented as 0 to 100% full scale. An audible beep is sounded to signify acceptance of the new value. In event an incorrect number is entered, the X key can be used to erase the entry, prior to activating the setpoint. If a numerical key is held depressed, the new value will become 100. AUTOSET is a feature that, when activated by the A key, causes the last 2 setpoints entered to be reset. Each time the S key is depressed, the setpoints alternate. This simplifies repeated execution as when evaluating the effects of tuning changes. Load transient - the L key causes the Control value (input to Plant transfer function) to change by 50% full scale for 1 sample. The load change alternates polarity each time L is depressed. This allows evaluation of the control response to a plant upset. CONTROL-SIM Users Manual Page 7 Often times a system tuned for best response to setpoint change will not fare as well during a load transient as a system tuned for best load response. The converse is also true. Mode change - the M key changes the loop mode from Closed Loop to Open Loop or vice versa. The default Closed Loop mode causes the measurement of plant output to feedback to the Controller algorithm, thereby allowing corrections to made in the next Controller output. The Open Loop mode bypasses the Controller algorithm and connects the setpoint value directly to the Controller output. Open Loop allows the Plant response to be evaluated without any controlling influence. For example, if the simple integration transfer function illustrated previously is implemented in Open Loop, the resulting time response will be indicated as 63.2% (approx. 1 - 1/e) of the setpoint change at 2 seconds. Print/Record - the P key is used to start or stop data recording. Initially, when P is depressed, the user is requested to select File or Printer. If File is selected, the user is then requested to enter the file name (without extension), after which all data is logged to the disc file. The extension .PRN is automatically appended. If the file already exists, the user is given opportunity to change the name or reuse the same name. Files are limited to approx. 900 records, or samples, to conserve disc space and to aid in data review. If longer runs are desired, reselect and enter another file name (e.g. DAT1, DAT2, etc). If Printer is selected, data printing begins immediately. In both cases, a header is recorded followed by elapsed time, setpoint, control, and measure values for each sample point. Logging continues until the P key is once again depressed or the T key is used to terminate the display. Terminate - the T key is used to terminate the display (or plot) operations and return to the initial selection menu. All variables are retained and if one of the display operations is again selected, all values will resume from the point of termination. 4.2 DISPLAY BLOCK DIAGRAM DATA Selection Same as DISPLAY DATA except that data is displayed in meter form only and related to the CONTROL-SIM Block Diagram, as test points. See 3.0 System Description for block diagram discussion. 4.3 PLOT DATA Selection Chart Display Data is presented in a strip chart type display, except that the "pens" move and the "paper" remains still. When the right edge of the screen is reached, the "pens", or vertical bar, continue from the left edge, erasing old traces and writing new traces as CONTROL-SIM Users Manual Page 8 it proceeds. Grid markers are also written to provide appropriate time scaling. User Controls User controls are the same as DISPLAY DATA with the addition of: Chart Speed - can be changed to better depict the area of interest for a given sample rate. Speed is increased by depressing the "+" key and decreased with the "-" key. Four possible settings are available. For each setting, the elapsed time between grid marks (vertical dots) is displayed in seconds. Depending on sample time selected, some speed positions may not be effective, resulting in grid marks too dense, or too spread out, to be useful. If the Space bar is depressed, all updates are suspended until the Space bar is pressed again. 4.4 MODIFY PLANT MODEL selection After selection, a window is opened containing a display of the current Plant Transfer Function in factored form. In addition to the transfer function, the current transport delay (deadtime) is indicated as Td and the current sample period is indicated as Ts. The user is asked "Is the plant correctly defined?" and a "yes" response results in computation of state equations and return to the main menu. If "no", or any key other than "y", is entered, the PLANT MODEL MODIFICATION menu is presented. Modify Transfer Function When selected, the format of the transfer function is displayed as follows: b0(s + b1)(s + b2)(s + b3) Gp(s) = --------------------------------- 1(s + a1)(s + a2)(s + a3)(s + a4) and the user is asked to enter the nominator term, b0. Term b0 must be a non-zero constant. After each term is entered, the next in sequence is requested, proceeding through the nominator terms, b0 to b3, and then the denominator terms, a1 to a4. The b(i) terms are the "zeros" and the a(i) terms are the "poles" of the plant transfer function, which can be fourth order. Entering "0" for a term results is the operator "s" only, as in the case of a "pole" at zero value. The operator "s" is equivalent to jw (j omega = j2(pi)f). Depressing the Enter key without a constant, terminates entry of the b(i) terms, jumping to the a(i) terms. Depressing Enter again without constants terminates all entry and the previous display of the actual transfer function is presented again for review. The program checks for a(i) values that match b(i) values and if present, both terms are negated, since they cancel each other. CONTROL-SIM Users Manual Page 9 At this point, if the user is satisfied with the result, he can answer "yes" to compute state equations, or he may enter "no" to re-enter the PLANT MODEL MODIFICATION menu. Modify Delay Time When selected, a value for delay time in seconds can be entered. Note, that the program only allows delay time that is a multiple of sample time. In the event a delay less than sample time is entered, the resulting delay will be 0. When satisfied with value, depress "E" to exit to previous menu. Modify Sample Time When selected, a value for sample period in seconds can be entered. The sample period should be less than 1/2 the plant time constant to insure data integrity and maintain control. Note, that although sample time is really a Controller parameter, it is included here because it affects the discrete state equation computation. When satisfied with value, depress "E" to exit to previous menu. Display Equations This selection allows the user to display the resulting plant continuous and discrete state equations, in matrix form. In both cases, state variables, input variables, and output variables are shown. Depress "C" to change the display status (ON/OFF) and "E" to exit. Exit Exit from PLANT MODEL MODIFICATION returns to the current transfer function display and subsequent computation of state equations, if the user is satisfied with its definition. 4.5 MODIFY CONTROLLER The menu of changes to the Controller includes the following selections: Select Controller Three controller choices are currently available: PID, FUZZY, & PIP. (a) The PID (Proportional-Integral-Derivative) Controller is the default controller. It is a general purpose, widely used technique that allows the user to adjust control system performance to accommodate various plant characteristics. The transfer function of the PID controller is: Gc(s) = Kp + Ki/s + Kds The 3 constants are user specified and affect proportional gain, reset rate (inverse integral time), and derivative time CONTROL-SIM Users Manual Page 10 respectively. If Ki and Kd are set to zero, the controller output is proportional to the error between setpoint and measurement. The amount of output can be changed with the proportional gain (Kp) factor. This factor is also called Proportional Band which is 100/Kp. The proportional band is the band over which the control output is allowed to swing. With Kp = .1, the proportional band is 10% of the 100% control output range. A disadvantage of proportional control is that an offset results between the setpoint and the measured value. To overcome offset error, Integral, or Automatic Reset, action is used to slowly shift the controller output as long as an error exists. The Ki factor is the number of repeats per second, or the number of repeats of proportional action required to equal the integral action. The inverse of Ki is the reset time constant, which is the amount of time it takes the integral control to produce the same action as proportional control. Since the reset action can build to a large number in certain situations, resulting in excessive overshoots, the reset windup action is limited to the range of the control output. This feature is also called anti-reset-windup. The fast action of the proportional control and the slow action of integral control are adequate for many applications, but the reset action may result in too much overshoot. By adding derivative control, also called rate action, to the controller, it is possible to decrease the effect of sudden change in the measurement value by anticipating the effect on controller output. The Derivative time constant (Kd) is the time in seconds for the proportional controller to produce a signal equal to the rate signal. (b) The FUZZY Controller is a rule based algorithm which responds to classes of current error and past error. The current error between setpoint and measurement is classified as positive big, positive small, zero, negative small, or negative big. With this fuzzy relationship established for both current and past errors, a rule will fire resulting in a controller output signal which is similarly classified. If the change in setpoint is larger than the big classification, the class definitions are scaled up by an order of magnitude. When the errors become less than the big classification, the class definitions are scaled back down. This controller allows the user to determine constants from subjective evaluation of the plant response, including non-linear action. The user can define the class sizes in the Modify Control Constants menu selection. Kp is used to specify positive & negative big, Ki is used to specify positive & negative small, and Kd is used to specify zero. (c) The PIP Controller is a predictive PI controller. In this design the derivative portion of the PID controller is replaced with another integration function. The derivative function has no CONTROL-SIM Users Manual Page 11 effect on long delays. This controller is especially suited for plants with long deadtimes with respect to the plant's dominant time constant. The predictor portion integrates the difference in control outputs between present and delayed values. The significance of the predictor portion is determined by the users estimate of L, the total deadtime. The resulting response is considerably faster than the PI controller alone. For this controller, the user has 3 adjustments available. The Kp and Ki constants are similar to the PID controller, but the Kd constant should be used to specify deadtime in seconds. After controller selection is complete, enter E to return to the MODIFY CONTROLLER menu. Modify Control Constants This selection allows the user to enter values for the 3 controller constants. After depressing the P, I, or D key, the value for the Kp, Ki, or Kd (or equivalent) constant may be entered. Depressing E returns to the Main menu. Modify Quantization Level This selection permits the user to change the resolution of the simulated analog-to-digital and digital-to-analog converters. The default selection is 12 bits which is fairly standard in hardware. This results in an full scale continuous input signal being quantized into 4096 steps (or approx. .0244 %FS per step). As less bits are used, control becomes less smooth. A maximum of 128 bits is permitted. Depressing B allows entry of Number of bits with resulting step size indicated. Depressing E returns to the Main menu. Change Output Range The default controller output signal is UNIPOLAR, ranging from 0 to 100 %FS. While this range is suitable for many plant devices such as valves which move from closed to open, some devices such as servo motors benefit from a BIPOLAR control signal, which can move from -100 to 100 %FS. Note that when bipolar output is specified though, that the selected resolution (say 12 bits) applies across the complete range (200 %FS) resulting in a step size of .0488 %FS. Depress C to change the range selection and E to return to the Main menu. 4.6 PLAYBACK If data has been recorded under the display menu selections, it can be reviewed with the PLAYBACK selection. All files having the .PRN extension are displayed with size, date, and time shown. The highlight bar can be moved up and down to select the desired file. The data is displayed in column format identical to printer output, with Elapsed Time, Setpoint, Control, and Measure values presented. The arrow up and down keys, PgUp and PgDn keys, plus Home and End CONTROL-SIM Users Manual Page 12 keys can be used to scroll to the desired section of the file. Note that the recorded, or printed, data includes more decimal places than the display screens indicate, to provide better detail of the control action. The header at the beginning of the file contains the name of the configuration file (.PLT) in use when the data was recorded as well as the mode (open or closed loop) and the date. The Esc key is used to terminate the data file playback and return to the Main menu or select another data file for review. 4.7 CONFIGURATION FILES The CONFIGURATION FILES selection results in choices of RETRIEVE, SAVE, DIRECTORY, or EXIT. Retrieve When selected, Retrieve presents a display of all configuration files (.PLT), including size, date, and time. The highlight bar can be moved up and down to select the desired previously saved configuration. A retrieved configuration file contains all the user specified characteristics of the MODIFY PLANT MODEL and MODIFY CONTROLLER selections. Upon retrieval, the state equations are computed for the saved transfer function with return to the Main menu. Save This selection is used to save all the configuration characteristics of the current PLANT MODEL and CONTROLLER. The user is requested to specify the file name, without an extension. In the event file already exists, the user is prompted to choose another name or rewrite the file. Directory This selection displays the current directory, including disc drive and subdirectories. A change in directory is accomplished by depressing "D" and entering desired new directory. "E" is depressed to return to the CONFIGURATION FILES menu. After a directory has been changed, all file actions, including data recording and playback will use the current directory until program termination. The default directory is where CONTROL-SIM is located. Exit Returns selection to the Main menu. 5.0 APPLICATIONS A few examples of CONTROL-SIM use are included as follows: CONTROL-SIM Users Manual Page 13 Tuning Although there are only 3 adjustments to make, selecting the proper values to obtain optimum performance can be tedious. Several techniques have been devised to at least get the controller into the 'ballpark'. At that point it may still come down to fine corrections by trial and error to get the desired result. One of the most popular and oldest techniques for tuning a PID controller is called the Zeigler and Nichols Method. First the integral (Ki) and derivative (Kd) constants are set to zero. Then the proportional gain factor is increased until the loop just becomes unstable and a small setpoint change results in sustained oscillation. The period of the oscillation (Pu), in seconds, is then noted along with the value of Kp (Ku). The constants are then set according to the following formulas depending on which type of control is desired (or maybe works best): Proportional only Kp = .5 Ku PI control Kp = .45 Ku, Ki = 1/(.8 Pu) PID control Kp = .6 Ku, Ki = 1/(.5 Pu), Kd = .125 Pu Open Loop Characteristics To determine the time response of a transfer function, enter the transfer function and select DISPLAY & PRINT DATA. Enter a file name, or just depress to output to printer, and then select Open Loop by depressing M. To determine a unit step response, enter 1 then depress S. Depress T to terminate when measurement value has settled. Note that some transfer functions are very unstable and the result will be quickly out of range. Examine the printout, or select Playback of the file, to see the time versus measurement values. Simulation of a Text Book Example The closed loop response of a transfer function can be determined by setting the PID controller proportional gain to 1 (Kp = 1, Ki = 0, Kd = 0) and examining the response to a setpoint change. For example, assume the following servo system G(s) is entered. .32 G(s) = ------- s(s + .6) Since the closed loop transfer function with unity feedback is G(s) .32 F(s) = ------------ = ------------- 1 + G(s)H(s) s^2 + .6s + .32 with the characteristic form s^2 + 2(zeta)(omega-n)s + (omega-n )^2 CONTROL-SIM Users Manual Page 14 omega-n, the natural frequency, is equal to 2(pi)f, or f = .09 hz and zeta, the damping ratio, is equal to .53. The damping ratio of a second order system is related to per cent overshoot by a standard curve (or calculation) which in this case is 15%. With the output range set to bipolar and quantization level set at 128 bits, CONTROL-SIM will respond to a setpoint change with a response which indicates 15% overshoot and approximately 11 seconds settling time. Select EXAMPL1.PLT. This technique then allows evaluation of many control systems text book examples and problems. Reset Controller and Plant Sometimes after modifying the plant or controller, the measurement or control signals may become saturated due to excessive internal calculations. It is easy to set up control loops that are not controllable. To reset the controller and plant, enter the Controller Selection menu and reselect the controller. This will return states to initial conditions. Example Configuration Files Name Type Order K Poles Zeros Td Ts Kp Ki Kd Bits Out PROCESS.PLT PID 2nd 1 -1 .2 .1 2.8 .25 2.5 12 Uni -.2 TEMP.PLT PID 1st .001 -.001 0 10 50 1.3 0 12 Uni SERVO.PLT PID 3rd .3 0 -1 0 .1 1 .5 .5 12 Bi -.2 -1.2 EXAMPL1.PLT PID 2nd .32 0 0 .25 1 0 0 128 Bi -.6 EXAMPL2.PLT PID 2nd 10 -1 0 .1 .8 1.2 .4 128 Bi -.5 COMPCKT.PLT PID 4th 63.5 0 -.015 0 .01 3 1.08 1 12 Bi -.012 -2 -20 FZVALVE.PLT FZ 1st 2 -1 0 .25 .2 .05 .005 12 Uni FZSERVO.PLT FZ 2nd .32 0 0 .1 1 .2 .02 12 Bi -.6 PIPDEL1.PLT PIP 1st 1 -1 5 .25 1 1 5 12 Uni PIPDEL2.PLT PIP 4th 64 -1 10 .1 1 1.5 11 12 Uni -2 -4 -8 CONTROL-SIM Users Manual Page 15 6.0 REFERENCES The following are references used by the author which the user should refer to for better understanding of control systems. Claggett, E. H., "Keep a Notebook of Digital Control Algorithms, Control Engineering", Cahners Publishing, Denver, CO, October 1980 Del Toro, V. and Parker, S. R., "Principles of Control Systems Engineering, McGraw-Hill, New York, NY 1960 De Silva, C. W. and Aronson, M. H., "Process Control, A Professional Course", Measurements and Data Corporation, Pittsburg, PA, 1976 Franklin, G. F. and Powell, J. D., "Digital Control of Dynamic Systems", Addison-Wesley, Reading, MA, 1980 Grabbe, E. M., Ramo S., and Woolridge D. E., "Handbook of Automation, Computation, and Control, Vol 3, Systems and Components", John Wiley & Sons, New York, NY, 1961 Haaglund, T., "A Predictive PI Controller for Processes with Long Dead Times, IEEE Control Systems Magazine", IEEE, New York, NY, February 1992 Hafer, C. R., "Electronics Engineering for Professional Engineers' Examinations", McGraw-Hill, New York, NY, 1989 Kuo, B. C., "Digital Control Systems", Holt, Rhinehart and Winston, New York, NY, 1980 CONTROL-SIM Users Manual Page 16 APPENDIX 1 ERROR MESSAGES Bad File Name or Number Enter correctly. Data Overflow Program automatically terminates due to excessively large calculated values. Re-enter program and check plant and controller settings. Settings may be unrealistic. Delay storage area Choice of delay time and sample rate exceeded requires more memory than allocated. Decrease delay or increase sample time. Disc Full Change drive path or exit program and make room. File Already Exists Determine whether to overwrite. File Not Found Non-existent file name. Invalid Value entered not allowed. Printer Not Ready Turn on printer or check paper. Redo From Start Entry out of sequence. Enter correctly or exit and re-enter.