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Chapter: Automation, Production Systems, and Computer Integrated Manufacturing - Numerical Control

Fundamentals of NC Technology

Fundamentals of NC Technology: a. Basic Components of an NC System b. NC Coordinate Systems c. Motion Control Systems

FUNDAMENTALS OF NC TECHNOLOGY


 Fundamentals of NC Technology: 

a. Basic Components of an NC System

b. NC Coordinate Systems

c. Motion Control Systems

 

To introduce NC technology, we First define the basic components of an NC system. This is followed by a description of NC coordinate systems in common use and types of motion controls used in NC.


Basic Components of an NC System


In modern NC technology, the machine controt unit (MeU) consists or a microcomputer and related control hardware that stores the program of instructions and executes it by converting each command into mechanical actions of the processing equipment, one command '1\ a time. The related hardware of the Mel) includes components to interface with the processing equipment and reedbuck control elements. I he MeU also includes one or more reading devices lor entering part programs into memory. The type of readers depends on the stoagc media used for part programs in the machine shop te.g., punched tape reader. magnetic tape reader, floppy disk drive). The MCU also includes control system software.calculation algorithms. and translation software to convert the NC part program into a usable format for the MCl), Because the Mel) is a computer, the term computer numerical control (CNC) is used to distinguish this type of NC from its technological predecessors that wen: based entirely on hardWired electronics. Today, virtually all new MCl)s are based on computer technology; hence. when we refer to NC in this chapter and elsewhere, we mean CNC

 

The third basic component of an NC system is the processing equipment that performs uscruf work, It accornplrshcs the processing sloops tu transform the stasttng workpiece into a completed parr.It, operation is directed by the MCl!, which in turn is driven by instruction, contained in the pun program. In the most common example of NC, machining, the processing equipment con,is!~ ofthe worktable and spindle as well as the motors and controls to drive them



          NC Coordinate  Systems

 

To program the NC processing equipment a standard axis system must be defined by which the position of the workhead relative to the workpart can be specified. There are two axis systems used in NC, one for flat and prismatic workparts and the other for rotational parts. Both axis systems are based on the Cartesian coordinate system.

 

The axis system for flat and prismatic parts consists of the three linear axes (x, y, z) in the Cartesian coordinate system, plus three rotational axes (Q, b, c), as shown in Figure 6.2(a). In most machine tool applications, the xand yaxes are used to move and position the worktable to which the part is attached, and the z-axis is used to control the vertical position of the cutting tool. Such a positioning scheme is adequate for simple NC applications such as drilling and punching of flat sheet metal. Programming of these machine tools consists of little more than specifying a sequence of xy coordinates.

 

The a, b, and crotational axes specify angular positions about the X, j, and zaxes, respectively, To distinguish positive from negative angies, the righthand rule is used. Using the right hand with the thumb pointing in the positive linear axis direction (+x, +y, or +z), the fingers of the hand are curled in the positive rotational direction. The rotational axes can be used for one or both of the following: (1) orientation of the workpart to present different surfaces for machining or (2) orientation of the tool or workhead at some angle relative to the part. These additional axes pennit machining of complex work.pan geometries.

 

Machine tools with rotational axis capability generally have either four or five axes: three linear axes plus one or two rotational axes. Most NC machine tool systems do not require all six axes.

 

The coordinate axes for a rotational NC system are illustrated in Figure 6.2(b). These systems are associated with NC lathes and turning centersAlthough the work rotates, this is not one of the controlled axes on most of these tuming machines. Consequently, the )'_ axis is not used. The path of the cutting tool relative to the rotating workpiece is defined

 

in the .rz plane, where the xaxis is the radial location of the tool, and the zaxis is parallei to the axis of rotation of the part.

The part programmer must decide where the origin of the coordinate axis system should be located. This decision is usually based on programming convenience. For example, the origin might be located at one of the comers of the part. If the workpart is sym


metrical, the zero point might be most conveniently defined at the center of symmetry. wherever the location. this zero point is communicated to the machine tool operator. At the beginning of the job, the operator must move the cutting tool under manual control to some target point on the worktable, where the tool can be easily and accurately positioned. The target point has been previously referenced to the origin of the coordinate axis system by the part programmer. When the tool has been accurately positioned at the target point, the operator indicates to the MCV where the origin is located for subsequent tool movements,

 

          Motion  Control Systems

 

Some NC processes are performed at discrete locations on the workpart (e.g., drilling and spot welding). Others are carried out while the workhead is moving (e.g., turning and continuous arc welding). If the workhead is moving, it may be required to follow a straight line path or a circular or other curvilinear path. These different types of movement are accomplished by the motion control system, whose features are explained below.

 

PointtoPoint Versus Continuous Path Control. Motion control systems for NC (and robotics, Chapter 7) can be divided into two types: (1) pointtopoint and (2) continuous path. Pointto·poimsystems, abo called positioning systems, move the worktable tu a programmed location without regard for the path taken to get to that location. Once the move has been completed, some processing action is accomplished by the workhead at the location. such as drilling or punching a hole. Thus, the program consists of a series of point locations at which operations are performed, as depicted in Figure 6.3.

 

Continuous path systems generally refer to systems that are capable of continuous simultaneous control of two or more axes. This provides control of the tool trajectory relative to the workpart. In this case, the tool performs the process while the worktable is moving, thus enabling the system to generate angular surfaces, twodimensional curves, or threedimensional contours in the workpart. This control mode is required in many milling and turning operations. A simple twodimensional profile milling operation is shown in Figure 6.4 to illustrate continuous path control. When continuous path control is utilized to move the tool parallel to only one of the major axes of the machine tool worktable, this is called straightcut NC. When continuous path control is used for simultaneous control of two or more axes in machining operations, the term contouring is used.


Interpolation Methods. One of the important aspects of contouring IS inrerpollJlion. The paths that a contouringtype NC system is required to generate often consist of circular arcs and other smooth nonlinear shapes. Some of these shapes can be defined mathematically by relatively simple geometric formulas (e.g., the equation for a circle is Xl + i = R2where R = the radius of the circle and the center of the circle is at the origin), whereas others cannot be mathematically defined except by approximation. In any case, a fundamental problem in generating these shapes using NC equipment is that they are continuous, whereas NC is digital. To cut along a circular path, the circle must be divided into a series of straight line segments that approximate the curve. The tool is commanded to machine each line segment in succession so that the machined surface closely matches the desired shape. The maximum error between the nominal (desired) surface and the actual (machined) surface can be controlled by the lengths of the individual line segments, as explained in Figure 6.5.

 

If the programmer were required to specify the endpoints for each of the line segments, the programming task would be extremely arduous and fraught with errors.Atso, the part program would be extremely long because of the large number of points. To ease the burden, interpolation routines have been developed that calculate the intermediate points to be followed by the cutter to generate a particular mathematically defined or approximated path.

 

A number of interpolation methods are available to deal with the various problems encountered in generating a smooth continuous path in contouring. They include: (1) linear interpolation, (2) circular interpolation, (3) helical interpolation, (4) parabolic interpolation, and (5) cubic interpolation. Each of these procedures, briefly described in Table 6.1, permits the programmer to generate machine instructions for linear or curvilinear paths using relatively few input parameters. The interpolation module in the MCU performs the calculations and directs the tool along the path. In CNC systems, the interpolator is generally accomplished by software. Linear and circular interpolators are almost always included in modem CNe systems, whereas helical interpolation is a common option. Parabolic and cubic interpolations are less common; they are only needed by machine shops that must produce complex surface contours.

 

Absolute Versus Incremental Positioning. Another aspect of motion control is concerned with whether positions are defined relative to the origin of the coordinate system


Figure 6.5 Approximation of a curved path in NC by a series of straight line segments. The accuracy of the approximation is controlled by the maximum deviation (called the tolerance) between the nominal (desired) curve and the straight line segments that are machined by the NC system. In (a) the tolerance is defined on only the inside of the nominal curve. In (b) the tolerance is defined on only the outside of the desired curve. In (c) the tolerance is defined on both the inside and outside of the desired curve.


Linear interpolation. This is the most basic and is used when a straight line path is to be generated in continuous path NC. Twoaxis and threeaxis linear interpolation routines are sometimes distinguished in practice, but conceptually they are th ••""me. The programmar specifies thc beginning point and end point of the straight line and the feed rate to be used along the straight line. The interpolator computes the feed rates for each of the two (or three) axes to achieve the specified feed rate.

 

Circular interpolation. This method permits programming of a circular arc by specifying the following parameters: (1) the coorotnetes of the starting point, (21 the coordinates of the endpoint, (31 either the center or radius of the arc, and (4) the direction of the cutter along the arc. The genarated tool path consists of a series of small straight nne segments (see Figure 6.5) calculated by the interpolation module. The cutter is directed to move along each line segment onebyone to generate the smooth circular path. A limitation of circular interpolation is that the plane in which the circular arc exists must be a plane defined by two axes of the NC svstern :«  y, x  Z, or y  Z)'


or relative  to the previous   location    of the tool. The two cases are called  absolute position ing and incremental positioning. In absolute positioning, the workhead locations are always defined with respect to the origin of the axis system. In incremental positioning, the next workhead position is defined relative to the present location. The difference is illustrated in Figure 6.6.

 

     COMPUTER  NUMERICAL  CONTROL

 

Since the introduction of NC in 1952, there have been dramatic advances in digital computer technology. The physical size and cost of 11digital computer have been significantly reduced at the same time that its computational capabilities have been substantially increased. It was logical for the makers of NC equipment to incorporate these advances in computer technology into their products, starting first with large mainframe computers in the 1960s, followed hy minicomputers in the 1970s,and microcomputers in the 1980s (Historical Note 6.2). Today, NC means computer numerical control. Computer numerical control (CNC) is defined as an NC system whose MeV is based on a dedicated microcomputer rather than on a hardwired controller.

 


Historical
 Note 6.2 Digital computers for  NC 

I'he development of NC has relied heavily on advances in digital computer technology. As computers evolved and their performance improved, producers of NC machines were quick to adopt the latest generation of com pUler technology, The first application of the digital computer for NC was 10 perform part programming. In 1956.MIT demonstrated the feasibility of a computeraided part programming system using its Whirlwind I computer (an early digital computer prototype developed at MIT). 

 


          Features  of CNC

 

Computer NC systems include additional features beyond what is feasible with conventional hardwired NC. These features, many of which are standard on most CNC MCVs whereas others are optional, include rhc following'

 

    Storage of more than one part program. With improvements in computer storage technology. newer CNC controllers have sufficient capacity to store multiple programs. Controller manufacturers generally offer one or more memory expansions as optionstothcMCU.

 

   Variomform.l of program input. Whereas conventional (hardwired) MCVs are limited to punched tape as the input medium for entering part programs. CNC controllers generally possess multiple data entry capabilities, such as punched tape (if the machine shop still uses punched tape), magnetic tape. floppy diskette. RS232 communications with external computers, and manual data input (operator entry of program).

Program editi'll{ at the machine tool. CNC permits a part program to be edited while it resides in the MCV computer memory. Hence, the process of testing and correcting a program can be done entirely at the machine sire, rather than returning to the programming office to,correct t~e tape. In addi:ion to part ~tograrn corrections.editing also permits optimizing cuumg conditions m the machining cycle. After correcting and optimizing the program. the revised version can be stored on punched tape or other media for future usc.

 

   Fixed cycles and programming subroutines. The increased memory capacity and the ability III I'lOgrmlllhe control cornpurer provide the opportunity 10store frequently

 

used machining cycles as mucros that can be called by the part program. Instead of writing the full instructions for the particular cycle into every program, a call statement is included in the part program to indicate that the macro cycle should be executed. These cycles often require that certain parameters be defined; for example. a bolt hole circle, in which the diameter ot the bolt circle, the spacing of the bolt holes, and other parameters must be specified.

 

   Interpolation. Some of the interpolation schemes described in Table 6.1 are normally executed only on a CNC system because of the computational requirements. Linear and circular interpolation arc sometimes hardwired into the control unit, but helical, parabolic, and cubic interpolations are usually executed in a stored program algorithm.

   Positioning features for setup. Setting up the machine tool for a givcn work part involves installing and aligning a fixture on the machine tool table. This must be accomplished so that the machine axes are estahlished with respect to the workpart, The alignment task can be facilitated using certain features made possible by software options in a CNC system. Position set is one of these features. With position set , the operator is not required to locate the fixture on the machine table with extreme accuracy. Instead, the machine tool axes are referenced to the location of the fixture by using a target point or set of target points on the work or fixture.

 

   Cutter length and size compensation. In older style controls, cutter dimensions had 10 be set very precisely to agree with the tool path defined in the part program. Alternative methods for ensuring accurate tool path definition have been incorporated into CNC controls. One method involves manually entering the actual 1001dimensions into the MCU. These actual dimensions may differ from those originally programmed Compensations arc then automatically made in the computed tool path. Another method involves use of a toollcngth sensor built into the machine. In this technique. the cutter is mounted in the spindle and the sensor measures its length. This measured value is then used to correct the programmed tool path.

 

   Acceleration and deceleration calculations, This feature is applicable when the cutter moves at high feed rates. It is designed to avoid tool marks on the work surface that would be generated due to machine tool dynamics when the cutter path changes abruptly. Instead, the feed rate is smoothly decelerated in anticipation of a tool path change and then accelerated back up to the programmed feed rate after the direction change.

   Communications interface. With the trend toward interfacing and networking in plants today, most modem CNC con/rollers are equipped with a standard RS232 or other communications interface to allow the machine to be linked to other computers and computerdriven devices. This is useful for various applications, such as: (1) downloading part programs from a central data file as in distributed NC; (2) collecting operational data such as workpiece counts, cycle times, and machine utilization; and

 

    interfacing  with peripheral  equipment.  such as robots  that load and unload  parts.

    Diagnostics. Many modern CNC systems possess an online diagnostics capability that monitors certain aspects of the machine tool to detect malfunctions or signs of impending malfunctions or to diagnose system breakdowns. Some of the common features of a CNC diagnostics system are listed in Table 6.2

          The Machine Control  Unit for CNC

 

The MCU is the hardware that distinguishes CNC from conventional NC. The general configuration of the MCV in a CNC system is illustrated in Figure 6.7. The MCV consists of the following components and subsystems: (I) central processing unit, (2) momery,

 

(3) lIO interface. (4) controls for machine tool axes and spindle speed. and (5) sequence controls for other machine tool functions. These subsystems are interconnected by means of a system bus. as indicated in the figure,


Centra! Processing Unit. The central processing unit (CPU) i~the brain of the Mev. It manages the other components in the Mel! based on software contained in main memory.Thc CPU can be divided into three sections: (1) control section, (2) arithmeticlogic unit, and (3) immediate access memory. The control section retrieves commands and data from memory and generates signals 10 activate other components in the Meu. In short, it sequences. coordinates. and regulates all of the activities of the Mrl J computer. The arithmeuciogtc unit (ALU) consists of the circuitry to perform various calculations (addition, subtraction, multiplication), counting. and logical functions required by software residing in memory. The immediate access memory provides a temporary storage for data being processed by the CPu. It is connected to main memory by means of the system data bus.

 

Memory. The immediate (lCCCSS memory in the CPU is not intended for storing CNC software, A much greater storage capacity is required for the various programs and data needed to operate the CNC system. As with most other computer systems, CNC memory can be divided into two categories: (I) main memory and (2) secondary memory. Main memory (also known as primary storaf;e) consists of RO\1 (readonly memory) and RAM (random access memory) devices. Operating system software and machine interface programs (Section 6.2.3) are generally stored in ROM. These programs are usually installed by the manufacturer of the MCU. Numerical control part programs are stored in RAM devices. Current programs in RAM can he erased and replaced by new programs as jobs are changed.

 

Highcapacity secondary memory (also called auxiliary storage or secondary storage) devices are used to store large programs and data files, which are transferred to main memory as needed. Common among the secondary memory devices are floppy diskettes and hard disks. Floppy diskettes are portable and have replaced much of the punched paper tape traditionally used 10 store part programs. Hard disks are highcapacity storage devices that are permanently installed in the CNC machine control unit. CNC secondary memory is used to store part programs, macros, and other software.

 

Input/Output Interface. The IIO interface provides communication between the various components of the CNC system, other computer systems. and the machine operator. As its name suggests, the 110 interface transmits and receives data and signals to and [rom external devices, several of which are indicated in Figure 6.7. The opera/or control panel is the basic interface by which the machine operator communicates to the CNC system. This is used to enter commands relating to part program editing, MeU operating mode (e.g .. program control vs. manual control),speeds and feeds, cutrtng fluid pump onloff, and similar functions. Either an alphanumeric keypad or keyboard is usuallv included in the operator control panel. The 110interface also includes a display (CRT or LED) for corn~unication o.fdata and information from the MCV to the machine operator. The display IS used to indicate current status of the program as it is being executed and to warn the operator of any malfunctions in the CNC system.

 

Also included in the 110 interrace are one or more means of entering the part program into storage. As indicated previously, NC part programs are stored in a variety of ways, including punched tape. magnetic tape, and floppy disks. Programs can also be entered manually by the machine operator or stored at a central computer site and transmitted via loea! area nl'T~'nrk (LAN) to the CNCsystem',Whichevcr mcans.isemployed by the plant, a suitable device must be included in the J/O interface 10 allow input of the program into MCU memory.


Sequence Controls for Other Machine Tool Functions. In addition to control of table position. feed rate. and spindle speed, several additional functions arc accomplished under pan program control.These auxiliary functions arc generally on/off (binary] actuations. interlocks. and discrete numerical data. A sampling of these functions is presented in Table 0.3. To avoid overloading the CPU, a prograrnrnable logic controller (Chapter H) is sometimes used to manage the 110 interface for these auxiliary functions

 

Personal Computers and the MCV. In growing numbers, personal computers (PCs) are being used in the factory to implement process control (Section 4.4.6), and CNC is no exception. Tho basic configurations are being applied [14J: (1) the PC is used as a separate frontend interface for the MCU, and (2) the PC contains the motion control board and other hardware required to operate the machine tool. In the second case, the CNC control board fits into a standard slot of the P'C. In either configuration, the advantage of using a PC for CNC is its flexibility to execute a variety of user software in addition


to and concurrently with controlling the machine tool operation. The user software might include programs for shopflour control. statistical process control, solid modeling,cutting tool management, and other computeraided manufacturing software. Other benefits include improved case of use compared with conventional CNC and ease of networking the PCs, Possible disadvantages include (1) lost time to retrofit the PC for OK. particularly when installing the rNr motion controls inside the PC and (2) current limitations in applications requinng complex fiveaxis control of the machine toolfor these application>, traditional CNC is still more efficient. II should he mentioned that advances in the technology of PCbased CNC are likely to reduce these disadvantages over time. Companies are demanding open architecture in CNC products, which permits components from different vendors to be used in the same system [7J

 

          CNC Software

 

The computer in CNC operates by means of software. There are three types of software programs used in CNC systems: (I) operating system software, (2) machine interface software, and (3) application software.

 

The principal  function  of the operating system software  is to interpret    the NC part pro

 

grams and generate the corresponding control signals to drive the machine tool axes. It is installed by the controller rnanuracturer and is stored in ROM in the MCU. The operating

system software consists of the following: (1) an editor, which permits the machine operator to input and edit NC part programs and perform other file management functions: (2) a control program, which decodes the part program instructions, performs interpolation and acceleration/deceleration calculations, and accomplishes other related functions to produce the coordinate control signals for each axis.and (3) an executive program, which manages the execution of the CNC software as well as the 1/0 operations of the Mev. The operating system software also includes the diagnostics routines that are available in the CNC svstem (Table 6.2).

 

The machine interface software is used to operate the communication link between the CPU and the machine tool to accomplish the CNC auxiliary functions (Table 6.3).As previously indicated, the I/O signals associated with the auxiliary functions arc sometimes im

plemented by means of a programmable logic controller interfaced to the MCU,and so the machine interface software is often written in tbe form of ladder logic diagrams (Section 8.2).

Finally, the application software consists of the NC part programs that are written for machining (or other) applications in the user's plant. We postpone the topic of part programming to Section 6.5.



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