The advent and widespread use of the computer-generated image (CGI) device to simulate visual cues has a mixed impact on the realism and fidelity of flight simulators. On the plus side, CGIs provide greater flexibility in scene content than terrain boards and closed circuit television based visual systems, and they have the potential for a greater field of view. However, on the minus side, CGIs introduce into the visual simulation relatively long time delays. In many CGIs, this delay is as much as 200 ms, which is comparable to the inherent delay time of the pilot. Because most GCIs use multiloop processing and smoothing algorithms and are linked to a multiloop host computer, it is seldom possible to identify a unique throughput time delay, and it is therefore difficult to quantify the performance of the closed loop pilot simulator system relative to the real world task. A method to address these issues using the critical task tester is described. Some empirical results from applying the method are presented, and a novel technique for improving the performance of GCIs is discussed

Clement, W. F.

Jewell, W. F.

English

NASA Technical Reports Server

A METHOD FOR MEASURING THE EFFECTIVE THROUGHPUT TIME 
DELAY IN SIMULATED DISPLAYS INVOLVING MANUAL CONTROL 
Wayne F. Jewell, Senior Research Engineer 
Warren F. Clement, Principal Research Engineer 
Systems Technology, Inc. 
2672 Bayshore-Frontage Road, Suite .505 
Mountain View, California 94043 
SUMMARY 
The advent and widespread use of the computer-generated image (CGI) 
device to simulate visual cues has had a mixed impact on the realism and 
fidelity of flight simulators. On the plus side, CGIs can provide greater 
flexibility in scene content than terrain boards and closed-circuit tele-
vision-based visual systems can, and they have the potential for a greater 
field of view. However, on the minus side, CGIs introduce into the visual 
simulation relatively long time delays. In many state-of-the-art CGls, 
this delay is as much as 200 ms, which is comparable to the inherent delay 
time of the pilot. Because most GCls use mul tiloop processing and smooth-
ing algorithms and are linked to a mul tiloop host computer, it is seldom 
possible to identify a unique throughput time delay, and it is therefore 
difficul t to quantify the performance of the closed-loop pilot-simulator 
system relative to the "real world" task. This paper describes a method 
to address these issues using the STI-developed Critical Task Tester 
(CTT). Some empirical results from applying the method are presented, and 
a novel technique for improving the performance of CGIs is discussed. 
BACKGROUND AND INTRODUCTION 
Modern flight simulators usually employ a "host" digital computer in 
order to represent the mathematical model of the aircraft dynamics, the 
flight control system, the equations of motion, and the environmental 
disturbances. A "satellite" digital computer generates the dynamic exter-
nal visual field which is output to one or more cathode-ray tube (CRT) 
displays. The combined process of generating and displaying the external 
visual field is usually referred to as a computer-generated image (CGI). 
The pilot "flies" the aircraft by monitoring the CGI, and his control 
outputs, <j>, are inputs to the host computer. This closed-loop process is 
depicted in the block diagram shown in Fig. 1. 
In order to conserve computer resources and minimize digital delays, 
both the host and the CGI computers usually employ mul tiloop architec-
tures. In addition, the CGI computer uses smoothing algorithms in order 
to prevent the visual scene from "j umping" on the display. The data 
transfer between the host and CGI computers is almost always asynchronous. 
173 
https://ntrs.nasa.gov/search.jsp?R=19850006188 2020-03-22T17:48:34+00:00Z
... 
,. 
Host Digital Computer CGr Digital Computer 
5 
T1 Digital Model of T2 Computer-Generated Vehicle Dynamics x 
- ~ .... -Pilot .. Image of r and Equations r Visual Cues 
of Motion 
Figure 1. Functional Block Diagram of Architecture for Host and 
CGI Computers Used in Modern Flight Simulators 
Because of the complex architecture used in the computers of modern 
flight simulators, it is very difficult to identify the effective time 
delay of the overall system, or, more importantly, to identify how the 
performance of the simulator compares to that of the real world. In many 
state-of-the-art CGIs, this delay is as much as 200 ms (Ref. 1), which is 
comparable to the inherent delay time of the pilot. For some flight tasks 
(e.g., up-and-away flight), this much delay is tolerable. For others 
(e.g., precision hover or landing), it is intolerable and completely un-
realistic. When a pilot is unable to perform a task in a simulator, he 
often does not know exactly what is wrong; he knows only that he can per-
form the same task in the real world (Ref. 2). On the other hand, if the 
pilot can perform the task, he often complains that the workload is much 
higher than that in the real world. One explanation for both of these 
problems is that the pilot must generate lead in order to compensate for 
the lag in the CGI. 
The primary purpose of this paper is to describe a method which can 
both measure the effective time delay of a modern flight simulator and 
quantify the performance of the c1osed-:-loop pilot-simulator system rela-
tive to the "real world." Since this method is independent of the 
hardware and software of the computers used in the simulator, it offers a 
rational means for evaluating hardware and/or software changes to any part 
of the flight simulator. 
The remainder of this paper describes the proposed method and presents 
some empirical results of applying the method. A novel technique for 
improving the performance of visual simulators is also presented and 
discussed. 
174 
DESCRIPTION OF THE METHOD 
The proposed method is based on human operator (HO) performance degra-
dation in performing a manual control task. The particular manual control 
task is to stabilize an unstable controlled element using the critical 
tracking task (CTT, Refs. 3 and 4), as depicted in Fig. 2. The HO uses a 
manipulator, 0, to null the error, e, which is displayed on a CRT. The 
task is automatically paced in the sense that the unstable pole, A, in-
creases slowly with time, thus making the task progressively more 
difficult. At some point, the HO can no longer control the error, e, and 
the value of e exceeds a preset value. At this point, the task ends, and 
the corresponding final value of A is defined to be the critical task 
score, AC. 
The CTT has been used in numerous experiments involving the human 
operator. Most of these experiments have examined the performance degra-
dation of the HO due to exogenous effects such as alcohol, drugs, and 
prolonged bed rest (Refs. 5 through 8). It is also possible, however, to 
use the crT in order to examine the performance degradation of the HO due 
to divided attention (Refs. 9 and 10) and other causes within the display 
(Refs. 11 and 12), the manipulator (Ref. 13), or the order of the unstable 
controlled element itself (Refs. 13 and 14). It is the causes of HO per-
formance degradation between the operator's manipulator and a CGr display 
that forms the basis for the proposed method. 
Consider now the modified CTT block diagram shown in Fig. 3. A CGr 
with sample update period, Te , is now used to display the error signal, 
e. The control output of the HO, 0, is sampled at period To. (Te and To 
were equal but not synchronized for the results described herein.) 
Figure 3 represents the essential features of a modern flight simulator 
that uses one or more digital computers to sample and process the pilot's 
output and a CGr to display the state of the vehicle to the pilot in terms 
of a simulated appearance of the external field of view. Figure 2 can be 
thought of as the "rea1-world" counterpart of Fig. 3, where the display is 
continuous, and there is no delay due to sampled data effects. 
Because the variability in AC for. a well-trained subj ect is suffi-
ciently low (Refs. 3 and 15), the continuous (i.e., Fig. 2) and discrete 
(I.e., Fig. 3) versions of the CTT offer a unique means for comparing the 
effects of sampled data systems and CGr displays on the performance of the 
human operator. 
SOME EMPIRICAL RESULTS 
The resu1 ts of an initial inves tigation of the effects of Te and To 
from the sampled, first-order CTT described in Fig. 3 are shown in 
Fig. 4. Note that the mean score, AC' is a linear function of the sample 
periods, Te and To. Also note that there is low variability in the data, 
as evidenced by the low values of the standard deviations. Using the zero 
time delay score as a reference point (i.e., AC::!: 6.S rad/sec), the 
175 
to-' 
-...J 
(J'\ 
e 
.. 
.. 
Display 
CRT ... - .. 
1 = 1 + 0 i dt 
~ {0.20 rad/sec/sec 
1 
0.05 rad/sec/sec 
Human 
Operator 
y 
p 
f'or lei < ec 
f'or leI> ec 1 
.. 
--
1 
Controlled 
Element 
Kl 
s - 1 
.~ 
one-way switch 
Note: Task is automatically stopped when leI > e 
s 
Figure 2. Functional Block Diagram of' Critical Task Tester (CTT) 
e 
.. 
--
I-' 
-...J e 
-...J 
Visual Simulator 
"Computer-Generated 
Display" (CGI) 
Continuous Transfer Function 
of Algorithm. Wi thin 
Digital Processor 
~------------~-------------~ r , 
Human 
Te -i Mo I ., CRT H I 0 y P 
Sampled Data 
System 
,--------'~,------~ I r7ATT \ 
/.' - M T --. 0 0 
,.----- \ 
.. 
~ 
Controlled 
Element 
Kl 
-s' - 1 
1-
Note: The process of updating and holding a sample for one frame time, T, 
is modeled with a zero-order hold (ZOH): 
M = o 
-Ts (1 -e )/s 
where T is the time delay associated with either Te or To 
Figure 3. CTT with Computer-Generated Display and Sampled Data System 
e 
-
-po 
t-' 
~ 
(Xl 
A + aAC C-
(rad/sec) 
8 
6 
4 
2 
o 
o 0.01 
Each plotted value is comprised of nine samples 
0.05 0.10 0.20 
Te, To (sec) 
Figure 4. Effects of T , T~ on A 
e u c 
performance at 100 ms throughput delay is degraded by about 26 percent 
and, at 200 ms, by 49 percent! If we can extrapolate these resul ts to 
other flight tasks, it is no wonder that pilots complain that they cannot 
perform real-world tasks in a flight simulator. 
The discrete version of the first-order CTT in Fig. 3 thus offers a 
simple, convenient, and portable means for comparing the degradation in HO 
performance which accompanies throughput time delay and update rate be-
tween the pilot's manipulator and the CGI. It also meets the objectives 
stated at the beginning of this paper, i.e., to measure the effective time 
delay of a modern flight simulator and to quantify the performance of the 
closed-loop pilot-simulator system relative to the real world. The proce-
dure for doing this in any given flight simulator is as follows: 
1. Program the CTT algorithm in the host computer. Options for 
driving anyone of the six axes of the CGI should be 
provided. 
2. Establish a reference curve for Ac versus the cycle time of 
the host computer. A separate curve must be established for 
each controller-display-operator combination. 
3. Since the host computer will not be able to run at zero cycle 
time, each controller-display-operator combination mus t be 
extrapolated to the zero cycle time point. This point, Aco' 
will be used as the "real world" reference point. 
4. The effective throughput time delay of the total simulator, 
Le., host computer and CGI, is calculated.as follows: 
= 
where Act is the value of Ac at the normal operating point of 
the host computer. The above equation is based on the total 
throughput delay of the HO and digital computers being 
proportional to the inverse CTT score (Ref. 4). In general, 
the value of Teff will not be the same as the "exact" 
throughput time delay. Hence the name "effective throughput 
time delay" is given. 
Note that the procedure outlined above offers a rational means of 
evaluating the performance of a flight simulator. It als.o provides a 
method for evaluating changes to any component of the simulator. For 
example, the technique for improving the performance of a CGI that is 
discussed in the next section could be evaluated by this procedure. 
179 
~ 
A NOVEL TECHNIQUE FOR IKPROVINGTHE PERFORMANCE OF 
COHPUTER-GENERATED EMAGES 
One way to compensate for the lag due to time delay in the combined 
host computer and CGI is to use lead in the signals being sent to the 
CGI. There are limitations in doing this, because the host computer can-
not generate lead beyond the Nyquist frequency, and linear lead filters 
distort the. amplitude response at the expense of obtaining the correct 
phase response. To overcome the first of these restrictions, the hybrid 
approach shown in Fig. 5/cou1d be used. 
Host Digital Computer 
Digital Model of 
Low-Frequency ~LF 
Vehicle Dynamics 
and Equations CGI Digital 'Computer 
of Motion Combine ~LF· and 
0 ~ to Form~, 
-Pilot Computer-Generated 
Host Analog Computer 
Image of 
Analog Model of Visual Cues 
High-Frequency ~HF 
• .. Vehicle Dynamics, Equations of 
Motions, and T3 Compensation 
. 
Figure 5. Functional Block Diagram of Advanced Hybrid Architecture 
Proposed for Host and CGI Computers 
Note that there are two "host" computers, one digital and one analog 
(hence the name "hybrid"). The host digital computer simulates the low-
frequency vehicle dynamics (!LF) , where "low frequency" means up to the 
Nyquist frequency, 'IT/T 1; the host analog computer simulates the high-
frequency vehicle dynamics (~F)' where "high frequency" means above 'IT/T 1; 
and it compensates ~F to account for lags in the CGI digital computer. 
The CGI digital computer then combines !LF and ~F via "complimentary 
filtering" in order to form the final vehicle states, x, which are dis-
played to the HO. A simple first-order complimentary fil ter is shown 
below, 
180 
I--
a 
~LF s + a 
..... - ...... x 
s 
xHF--------------~~ s + a 
where the break frequency, a, is chosen to be just below the Nyquistfre..,. 
quency, 1f!Tr • 
The compensation technique we propose to implement in the host analog 
computer was first reported in Ref. 16 and then later in Ref. 17. The 
technique, called the Split Path Nonlinear Filter or SPAN is shown in 
Fig. 6. The advantages of SPAN are that (1) it provides conditionally 
independent magnitude and phase angle specification (e.g., it can generate 
phase lead without amplitude distortion!) and (2) it is not dependent on 
input signal amplitude. On the other hand, the possible disadvantages of 
SPAN are that (1) the output will contain harmonic distortion which may 
need to be attenuated and (2) if the linear filter in the magnitude con-
trol path (Fro) contributes phase shift, it will be reflected in the 
output, hence the magnitude control path is conditionally independent. 
Magnitude Control Path 
.. F 1~1 
-
m 
- Absolute 
Linear Filter Value 
" Inpu t Outp 
Multiplier .. 
ut 
1T .-
al SJ.g Sign nal 
~I' 
+1 
-,:::1 .. F .. P r ... Bistable 
Linear Filter Function 
Phase Control Path 
Figure 6. Flight Simulator Delay Compensation by Means of 
Split-Path Nonlinear Independent Magnitude and Phase Filters 
181 
The procedure outlined above needs to be tested under carefully con-
trolled conditions. The CTT method described in this paper offers a 
unique way of quantitatively evaluating this novel technique for improving 
the performance of visual simulators. 
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183 


A Method for Measuring the Effective Throughput Time Delay in Simulated Displays Involving Manual Control

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