A comparison of two command language user interfaces for a CNC machine

International Journal of Human Factors in Manufacturing, Vol. 1 (4) 351-363 (October 1991) 0 1991 John Wiley & Sons, Inc. CCC 1045-2699/91/040351-13S.00

A Comparison of Two Command Language User Interfaces for a CNC Machine

Octavio F. Torres-Chazaro,* Robert J. Beaton, and Michael P. Deisenroth Department of Industrial and Systems Engineering Virginia Polytechnic Institute and State University Blacksburg, VA 24061

ABSTRACT

Computer numerically controlled (CNC) machines are common pieces of equipment in manufacturing plants. In recent years, CNC-user interfaces have been evolving from CNC command languages to menu-based and direct-manipulation interfaces. However, there have been few efforts to evaluate CNC-user interfaces and to develop design strategies to improve their usability. An experiment is reported comparing the effectiveness of two user- interface layouts for a CNC-milling machine. One user-interface layout was a simulation of a popular interface for a CNC-milling machine. The second layout was designed after analyzing the command language characteristics, the button grouping, and the finger movements required to operate the first interface. Then, both layouts were compared in terms of time to complete a set of benchmark tasks, number of errors generated, and users’ preferences. Four factors were manipulated: (1) users’ level of expertise, (2) layouts, (3) tasks, and (4) trials. It was found that both groups of subjects completed the tasks faster using Layout 2 rather than Layout 1. Similarly, subjects’ responses to a questionnaire indicated that Layout 2 was preferred over Layout 1 in almost all of the usability categories studied.

1. INTRODUCTION

A recent report indicated that 56% of the U.S. market for machine-control products was dominated by computer numerically controlled (CNC) machines. The re- maining market segments comprised cell controllers, programmable logic controllers, robot controllers and relay equipment at 29%, lo%, 4%, and 1%, respectively (Machine control market to double, 1989). Similarly, Giffi and Roth (1989) found that CNC machines and programmable controllers were the most frequently mentioned devices by North American companies involved with production processes.

Giffi and Roth compared the responses of the managers to a similar survey they conducted in the 1987-1989 time period. These researchers found a steady decline in the number of companies expecting significant benefits from advanced manufacturing technology (AMT). In particular, U.S. executive managers had reduced expectations about the productivity, product quality, and profits that AMT could bring to the workplace.

These reports suggest that manufacturers increasingly suspect that further im- provements in manufacturing technology will not significantly contribute to the

* Now at RWD Technologies, Inc., 10480 Little Patuxent Parkway, Columbia, MD 21044-3530.

351

352 TORRES-CHAZARO et al.

success of their installations. It can be argued that a lack of consideration for human factors in the user interface has contributed to this situation. Companies face difficulties in training and retraining workers for increasingly complex machines that do not integrate easily with human operators or with other machines (e.g., Craven and Statter, 1988; Johnson and Wilson, 1988; Sinclair, 1985).

User interfaces for CNC machines have, been typically based on command languages that allow users to specify the path to be followed by a cutting tool. Initially, these interfaces were based on difficult to use command languages, However, in recent years, more powerful CNC command languages have a p peared. The purpose of these languages has been to increase operator productivity facilitating the programming, operation, and set-up of the machine.

Unfortunately, a powerful command language can still be difficult to use if there is no compatibility between the command language and its representation on the user interface. The typical user interface for CNC machines is composed of a control panel that is used to operate and program the machine on-line. The control panel typically integrates the command language used by the machine. Unfortu- nately, there are a lack of techniques focused on the design and evaluation of CNC control panels. A design technique must address issues such as the compatibility between the command language and its representation on the panel, command grouping, display location, and line-editing logic.

The purpose of this article is to compare two different layouts of the same command language for a CNC-milling machine. The evaluation procedure, rather than the design approach used, is emphasized. The design approach is described elsewhere (Torres-Chazaro, Deisenroth, and Beaton, l m a , l m b ) .

2. DESIGN OF USER-INTERFACES FOR CNC MACHINES

A CNC machine is a programmable device that implements machine tool movements according to the specifications contained in a program. A CNC program defines tool movements in a three-dimensional space.

A command language interface (CLI) for a CNC machine typically includes three types of commands: movement specification commands, line-editing commands, and program-manipulation commands. These commands allow the user to create, edit, store, read, and run CNC programs.

2.1. Design Procedures to Generate Control-Panel Layouts

The design of control panel layouts has been typically based on four design principles: (1) functional grouping, (2) sequence of use, (3) frequency of use, and (4) importance (Bonney and Williams, 1977; Kochlar and Barash, 1987; Sanders and McCormick, 1987).

These principles are often mentioned even though the supporting research has two limitations: (1) there are few empirical evaluations of interactions between the task performed with the control panel and the effectiveness of the design principle followed, and (2) there are few techniques to generate command/button groups based on the syntax and semantics of the control language.

A consequence of the first limitation is that a panel layout found usable to perform a routine task may be difficult to operate in an error recovery situation.

CNC-USER INTERFACES 353

LINE NUMBER

File Manipulation

PROGRAM RUN I I

Figure 1 Tree-graph of CNC-Dyna-language syntax.

However, this task-device interaction is rarely explored. The second limitation is closely related to the design of control panels for programmable equipment such as CNC machines and programmable controllers. These devices have a control language whose grammar, from a user’s point of view, is typically ignored in the final design of the control panel.

2.2. CNC Command Language Structure CNC commands can be classified according to the type of argument used by the command and the entity affected by the command. One classification of the CNC- DYNA commands is presented in Figures 1 and 2. The CNC-DYNA machine commands were grouped in six categories: Modes, References, Moves, Jumps, Headings, and Communications (see Torres-Chazaro et ai., 199Ob).

The Modes group determines the operational mode of the CNC machine, allowing the user to activate a restricted subset of commands. The other five groups belong to the programming mode, and categorize the arguments used by the commands. For example, some arguments define the direction of a move, while other arguments define the number of seconds that the machine should stay idle.

The Reference group includes the commands that define reference points in the machine work envelope. The Move group includes those commands that determine the specific path to be followed by the tool. The Jump group is composed of iteration and jump commands that allow the program to execute nonsequential segments of code within the CNC program. The Communication group includes those commands that allow the CNC to send audio or other output signals.

This command classification is not reflected in any obvious way in Layout 1, the reduced version of the CNC-DYNA control panel shown in Figure 3. Layout 2 is an alternative design based on a previous analysis reported elsewhere (Torres- Chazaro et al. 199Oa). Layout 2, shown in Figure 4, was designed to match the command classification presented in Figures 1 and 2. Notice that this layout does not use mode keys.


MODE KEYS ( 1 ) L I N E NUMBER MODE ( l i n e e d i t i n g and f i l e InanipUlatiOn) :

Next, Previous, I n s e r t , Delete, and Read/Write f i l e . (2 ) PROGRAM RUN ( sends program t o CNC machine ) (3)MANUAL ( d i r e c t and emergency moves commands):

(4)PROGRAM MODE ( a l l programming keys ind ica ted below).

Programming Keys: Headings Group

Go<upldown>, Goelef t I right>, Goccloser l fur ther>

S t a r t c in . I mm >< program number > Tool Diameter < diam i n inches I mm > Feed Rate c x I y I z >=< r p m >

Moves group Go abs

U> References group

z --> z CLEAR z --> z MAX XY --> REF 0 PROG REF

Skip t o < l i n e number > C a l l Subroutine c subrout ine number > Subroutine c subrout ine numher > Subroutine r e t u r n

Control < buzzer(1) I pulse o u t ( 2 ) I pu lse i n ( 3 ) > Spindle c off I on > Dwell < number of seconds >

Jumps group

communications

~

Figure 2 CNC-Dyna-language proposed structure.

Layout 2 is divided into four areas. The areas are: command language area, line editor area, numeric keypad, and program manipulation and emergency moves. There are some additional differences in the control logic of the interfaces. In particular, line insertion and deletion are each performed using a single key in Layout 2. However, Layout 1 requires a change of modes, which makes the insertion sequence more complicated.

The specific objective of this study was to compare the usability of two different user-interfaces for a CNC-milling machine. Two command language interfaces were compared in terms of: (1) task completion times, (2) errors, and (3) user preferences.

3. EXPERIMENT

We hypothesized that Layout 2 is a better interface because: (1) the command keys are organized by matching the proposed command language structure with the location of the command keys on the control panel, and (2) it has a simpler line editor than Layout 1.


Figure 3 Layout 1 : Reduced version of CNC-Dyna-control panel.

Thus, it was expected that the time to copy and modify a CNC program would be reduced using Layout 2 in comparison with Layout 1. Finally, it was predicted that subjects would prefer Layout 2 over Layout 1 in all the usability categories.

3.1. Method

3.1.1. Subjects. Two groups of eight subjects participated in this study. Subjects were classified as experts or novices using a paper-and-pencil classification test of CNC programming concepts. Subjects were recruited from the university popu- lation.

Figure 4 Layout 2: Redesigned control panel for CNC-Dyna.


3.1.2. Apparatus and Computer Programs. A computer program controlled the presentation of both layouts, using all the area of a touch-sensitive color monitor with a 32-cm diagonal. Both CLI interfaces were touch-sensitive devices. Users interacted with the CNC programs by manually touching the VDT touch-sensitive screen.

3.1.3. Training. A two-phase training period was administered at the beginning of the session. In phase 1, subjects received an explanation about basic concepts of CNC programming. The topics included: the X, Y,Z coordinate system, interpre- ting a CNC program to drill an array of holes, and cutting a rectangular frame. Phase 1 was concluded following participants’ completion of the classification test.

In phase 2, subjects were trained to operate the touch-screen layouts. Eight subjects initially worked with Layout 1 and the other eight started training with Layout 2. All subjects were given three typing exercises designed to familiarize them with the interfaces. Phase 2 was concluded when participants completed the training exercises.

3.1.4. Tasks. After completing training, subjects performed a copy and modify task three times. Task 1 required them to copy a CNC program from paper to screen using the touch-screen layouts. Task 2 was a program modification task in which participants were required to delete and insert lines into the program code copied during Task 1. Copy and modification tasks were designed to make subjects operate two user-interface command groups. The copy task could be performed using the programming commands only. The modification task required them to use the insertion and deletion commands in addition to the programming commands. Thus, in order to keep meaningful chunks of activity, subjects performed one copy task and then one modification task.

3.7.5. Experimental Procedure. Seven steps were followed during Experiment 1 after subjects read and signed a consent form. First, subjects were trained on Dyna-language programming. Second, subjects were classified as experts or novices using a classification test. Third, subjects were trained to use a layout per- forming three typing exercises. Fourth, subjects performed Task 1 (copy) and Task 2 (modify) three times. Fifth, subjects were trained to use the alternative layout using the same typing exeneises mentioned above. Sixth, similar tasks to those performed during step 4 were performed again with the alternative interface. Finally, subjects were given a usability evaluation questionnaire. The usability evaluation questionnaire was administered after the subjects had been exposed to both interfaces and performed all the tasks.

3.7.6. Experimental Design and Data Ana/ysis. The dependent variables were: (1) Task completion time (TCT), (2) errors, and (3) opinions about the interface. TCT was obtained in the following way. First, subjects were given a sheet of paper indicating the task to be performed, while the touch-sensitive screen remained blanked. Second, they were asked to touch the screen to access the CLI-layout whenever they felt ready to start. Third, subjects had to start copying the CNC program printed on the paper, or they had to access a text file containing a program to be modified according to the instructions. The computer recorded every touch-


input that the subject entered. Fourth, the computer started registering every touch-input from the beginning to the end of the trial. This procedure was repeated three times for the copy task, and three times for the modification task.

The computer generated a complete report of the experimental session. The experimenter then excluded all those data at the beginning or the end of the trial that were irrelevant for this study. All the touches between the beginning and end of the trial were kept as part of the experimental data. The data discarded included touches dealing with file reading at the beginning of the session. The TCT was the time difference between the first and last touch related to the target task. Thus, TCT could include some intervals when users were making errors.

The number of errors per trial was equal to the number of touches per trial minus the expected number if the task had been performed with a minimum number of steps. In order to analyze this dependent variable every touch was counted on a trial-by-trial basis. Then, the expected number of correct touches was subtracted from these numbers and analyzed using the same design used for TCT. Thus, subjects made zero errors when a task was performed using a minimum number of commands.

A 2 x 2 x 2 x 3(experience x layout x task x trial)mixedfactorialanalysis of variance (ANOVA) was used to analyze TCT and errors. Subject experience was a between-subjects factor, while layout, task, and trial were within-subject factors.

4. RESULTS

4.1. Task Completion Time

TCT was analyzed with a four-way mixed factorial ANOVA procedure. Table 1 presents the ANOVA summary for the analysis. The main effects of layout and trial were significant (p < 0.05)’ as was the two-factor interaction between layout and task (p < 0.05). The layout effect indicated that subjects finished the tasks faster with Layout 2 than Layout 1.

The main effect of trial was significant at p < 0.05. Subjects’ performance improved over trials in the experiment. Newman-Keuls’ multiple comparisons are presented in Table 2. According to this table, the difference between means for trials 1 and 3 was significant.

The two-factor interaction between layout and task is shown in Figure 5 . A Newman-Keuls multiple comparisons test indicated that both tasks were performed faster with Layout 2 as compared with Layout 1. The Copy task was performed faster than the Modification task when Layout 1 was’ used. However, the Modification task was performed faster with Layout 2.

4.2. Errors The ANOVA summary table presented in Table 3 indicated that the factor Task was the only significant effect on number of errors per trial. Figure 6 shows that subjects made twice as many errors modifying a program than copying the program from paper. The graph indicates that subjects made an average of four errors per copy trial and eight errors per modify trial, independent of the layout used.

358 TORRES-CHAZARO et el.

TABLE 1. ANOVA Summary Table for Task Completion Time Source 4f MS F P Between subjects

Within Subjects

Group (G) 1 14386.68 0.92 0.3533 G x Subjects (S) 14 15610.95

Layout (L) I 65934.18 21.26 O.o004* L x G I 12.00 0.00 0.9513 L x G x S 14 3 101.70 Task (T) 1 1 240.33 1.15 0.3013 T x G 1 567.18 0.53 0.4800 T x G x S 14 1076.89 Trial (Tr) 2 17 163.29 7.53 0.0024t Tr x G 2 2758.33 1.21 0.3131 T r x G X S 28 2277.89 L x T 1 19886.02 19.43 0.0oO6* L x T x G 1 176.33 0.17 0.6844 L x T x G x S 14 1023.45 L x Tr 2 2520.95 2.40 0.1091 L x T r x G 2 2505.01 2.39 0.1105 L x Tr x G x S 28 1050. I9 T x Tr 2 310.53 0.11 0.8926 T x T r x G 2 428.39 0.16 0.8552 T x Tr x G x S 28 2722.80

0.8342 L x T x T r 2 179.84 0.18 L x T x Tr x G 2 1521.22 1.54 0.2314 L x T x Tr x G x S 28 Total 191

985.84

* Indicates effects found to be statistically significant (p < 0.05). t Effects found significant using Huynh-Feldt correction ( p < 0.05).

This result emphasizes an additional advantage of Layout 2. Even though subjects made more errors during the Modification task, they modified the program faster when they used Layout 2 rather than Layout 1 (see Fig. 5). Thus, Layout 2 allowed subjects to recover faster from errors than did Layout 1.

4.3. Subjective Questlonnaire Results: Seven-Point Rating Scales

The first section of the questionnaire (10 usability attributes scaled from 1 to 7) was analyzed using a two-factor mixed ANOVA procedure in which group and layout were the independent variables. Group was a between-subjects factor, and layout was a within-subjects factor. Figure 7 is a general summary of this analysis.

Layout 2 was preferred over Layout 1 with respect to button labels (Attribute

TABLE 2. Newman-Keuls’ Results for Levels of Trial, Task Completion Time (TCT) in Seconds Trial 1 Trial 2 Trial 3

~~

138.4 (A) 120.8 (A, B) 105.7 (B)

Note. Means with the same letter are not signiACaatiy different ( p > 0.05).


200 Task

Completion

Time

(in secs.)

0 Layout 1 Layout2

1 Task 1 : Copy Task 2: Modify

Figure 5 Two-factor interaction between layout and task on task completion times.

TABLE 3. ANOVA Summary Table for Errors Source DF Group (G) Error Layout (L) L x G Error (L) Task (TI T x G Error (T) Trial (Tr) Tr x G Error (Tr) L x T L x T x G Error (L x T) L x Tr L x T r x G Error (L x Tr) T x Tr T x T r x G Error (T x Tr) L x T x T r L x T x Tr x G Error (L x T x Tr)

1 14 1 1

14 1 1

14 2 2

28 1 1

14 2 2

28 2 2

28 2 2

28

ss 0.021

2901.458 368.521

11.021 1550.125 660.083

3.00 1466.25 42.0104 694.885

3617.604 126.750 154.083

11 16.833 196.073 247.198

1802.562 331.698 142.406

3836.062 13.344

359.385 1916.104

MS 0.021

207.247 368.521

11.021 110.723 660.083

3.00 104.732 21.005

347.443 129.200 126.750 154.083 79.774 98.036

123.599 64.377

165.849 71.203

137.002 6.672

179.693 68.432

F 0.00

3.33 0.10

6.30 0.03

0.16 2.69

1.59 1.93

1.52 1.92

1.21 0.52

0.10 2.63

P 0.9921

0.089 0.757

0.025* 0.868

0.851 0.085

0.228 0.186

0.235 0.165

0.313 0.600

0.907 0.090

~

* Indicates effects found to be statistically significant ( p < 0.05).


8-

6-

4-

2-

Y m a

0-

E E w’ r 0

a P E a z

Task 1 : Copy Task 2: Modify Figure 6 Main effect of task on number of errors.

I), organization of buttons (Attribute 2), organization of editing buttons (Attribute 3), organization of programming buttons (Attribute 4), learning to operate the CNC machine (Attribute 8), remembering names and use of commands (Attribute 9), and easiness to perform a task (Attribute 10).

Factor layout was not significant with respect to spacing between buttons (Attri- bute 9, use of color (Attribute a), and button size (Attribute 7). This lack of effect is remarkable given the large differences in use of color in both layouts, and the larger button sizes used in Layout 1.

The Group factor was significantly different only for Attribute 9, which assessed preferences for remembering names and use of commands. In all other cases, the group factor was not statistically significant and was excluded from Figure 7.

4.4. Subjective Questionnaire Results: Forced Choice Questions

A binomial test was used to assess preferences for the touch-screen layouts mea- sured by the second section of the questionnaire (Attributes 11-16). As summa- rized in Figure 8, Layout 2 was preferred for inserting a line, deleting a line, finding a digit, finding a command, and overall preference. However, subjects did not prefer either layout with respect to the question, “Display the next line of a program.”

5. DISCUSSION

5.1. Performance: Task Completlon Time

According to Table 1, the main effect of group did not significantly affect TCT. There are two possible reasons underlying this finding. First, the tasks were typing tasks which are not related to the classification criterion. Thus, it may be irrelevant


0

6

4

2

1 2 3 4 5' 6' 7 ' 8 ' 9' 10

User Interface Attributes Figure 7 Usability rating scores for Layouts 1 and 2. The arrows indicate significant differences using independent ANOVA tests.

how much knowledge subjects have about CNC programming if the measures of performance are based on typing activities. Second, the classification test may be inadequate to differentiate between levels of expertise.

The layout effect on TCT indicated that Layout 2 improved subject performance. This outcome could be due to the greater simplicity of the line editor in Layout 2 which facilitated performance. Although both tasks were performed faster with Layout 2, the speed difference was amplified when the modify task was performed, since subjects were required to use the line insert and line delete keys.

Figure 5 indicates that a particular layout may improve some aspects of performance more than others. Thus, a usability evaluation of machine-user interfaces should include a comprehensive set of operators tasks, such as copying files, troubleshooting a program, creating programs, etc.

5.2. Opinions

The ANOVA performed on the rating scales indicated that novice and expert users had similar opinions about the interfaces. According to Figure 7, subjects perceived a better organization of buttons in Layout 2. They also found it easier to learn the interface operation, to remember command names, and to perform the tasks with Layout 2 rather than Layout 1. (An alternative approach to measure learning rate and recall performance is presented in Torres-Chazaro, Beaton, and Deisenroth, 1991 .)

There were no observed differences in opinions about the use of color and button size. Layout 1 used a blue color to indicate those buttons that required the operation of the shift key. Layout 2 did not incorporate a shift key, and it used four colored frames to indicate the four functional groups in the layout. It was originally thought that Layout 2 used color better than Layout 1. It appears, however, that the utility of color as a secondary highlighting cue had equivalent impact on performance in both layouts. Thus, there were no perceived advantages


20

10

a

m

d I :“a n ? i i

1 P E s E ii

- a 9 m

I 0 Layout 1 Layout 2

- 11 12 13 14 15 16

User Interface Attributes Figure 8 Forced choice preferences for Layout 1 vs. Layout 2. The arrows indicate significant differences using a binomial test.

in the use of color in either layout. Similarly, button size (i.e., touch-sensitive area size per key) did not produce differences in opinion, even though Layout 1 had wider buttons.

The forced-choice section of the questionnaire confirmed subject preferences for Layout 2 (see Fig. 8). Layout 2 line editor was preferred over Layout 1 in five categories-all except one. There were no significant differences between layouts with respect to line display. Subjects did not prefer the grouping of line editor and line display in the upper left corner of Layout 2, probably because of a common rejection to one-line displays.

5.3. The Number of Differences Between Layouts

It could be argued that the present experiment does not demonstrate the advantages of the design technique used because there were several differences between layouts which could explain the superiority of Layout 2, We contend that the superiority of Layout 2 resulted because our design approach pointed to more than one source of troubles for the original design. The differences in color, line editor logic, button grouping, etc., were in fact motivated by the previous analysis of the command language syntax and the finger-path motions presented elsewhere (see Torres-Chazaro et al., l W a , 199Ob).

The user-interface features presented in this study are relevant for command language interfaces. However, these features have been shown to be not as effec- tive as the graphical environments supported by direct manipulation interfaces (see Torres-Chazaro et al., 1991). Thus, designers are cautioned to consider the benefits and limitations of alternative user-interfaces for CNC machines.


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Craven, F.W. and Slatter, R.R., 1988, An overview of advanced manufacturing technology, Applied Ergonomics, 19( l), 9-16.

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Machine control market to double, 1989, Manufacturing Engineering, 103(3), 16. Sanders, M.K. and McCormick. E.J., 1987, Human Factors in Engineering and Design,

Sinclair, M.A., 1986, Ergonomics aspects of the automated factory, Ergonomics, 29( 12),

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Torres-Chazaro, O., Deisenroth, M., and Beaton, R., 1990b, A procedure for design and evaluation of control keypads. In Ergonomics of Hybrid Automated Systems-11, by W . Karwowski and M. Rahimi (eds.) (Elsevier, Amsterdam) pp. 633-640.

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(6th ed.) (McGraw-Hill, New York).

1507- 1523.

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A comparison of two command language user interfaces for a CNC machine