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DIN ...sto-/ 

PAGE of 

COPY 7 of 

PIR 

DATE: May 20, 1968 

TO: J. Frey 

FROM: W. Montgomery 

LIMITED SEARCH MODE CAPABILITIES OF THE ATS 

The present SPDR specifies the ATS shall have limited search capability. 

Discussions with the customer have led to the interpretation of this as, "the 

capability which enables the operator to scan the earth with the system, while 

controlling the scan rate or position with the manual control stick". 

There seems to exist the belief that the search rates and accuracies which 

can be controlled by the operator can be just about anything that he desires. 

At the present time there are no time or accuracy requirements in the SPDR to 

define performance of the system in this mode of operation. This PIR defines 

the search mode performance in terms of the capability of the operator and ATS 

system to: 

1. Change the LOS rate with a specified accuracy within a specified time. 

2. Reposition the image in the field of view with a specified accuracy 

and within a specified time. 

The limited search mode performance shall be defined as: 

The capability to make the LOS rate change 1000 u radians per second with 

an accuracy of + 17 u radians per second within 2.5 seconds after the operator 

initiates the change via the control stick. 

The capability to reposition the image from the edge of the field of view 

to the center of the field of view + 2 6711 within 2.5 seconds after the operator 

initiates the command via the control stick. The magnification shall be in thehigh range (63 1/2 to 127x). 

zO4K' 

To reposition the image to the center of the field of view and to reduce 

the rate error after the repositioning maneuver to less than + 17 radians will 

require at least four seconds. 

Simulations have been run for the various stick configurations and assumed 

man models and the results are discussed in then PIR; however, it should be 

emphasized that these are only assumed man models. With this in mind, it is felt 

the times given for changing rate, repositioning and repositioning and rate 

nulling are the shortest that GE should agree to accept as SPDR requirements 

for limited search operation, until more work is done with the crewmen 

and the simulator to tie down the man's transfer function. 

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C. 

S +-t) 

K (X5+1)e' 

5 (5/10.1.0(5w, tI) 

A to 6,/ See. 

DISCUSSION 

The system capabilities given are a function of the operator's capabilities 

and are based on an assumed operator model. The systeds search capability is 

presently limited by the operator's capabilities and the computer sampling rates 

and not by the scanner hardware. The scanner torquers and control loops are sized 

to operate in the slew mode and thus are capable of high search rates. A 17400 

u -radian per second step command can be input to the scanner and the rate will be 

changed by this amount + 17 u radians per second in > 0.5 seconds without the 

man in the loop. 

The variables which affect the limited search mode performance are: 

1. Man's transfer function 

2. Control Stick transfer function 

3. Computer Sampling Rate 

4. Scanner Drive Servo Configuration 

Variable (1) is not known at this time, thus in attempting to determine the 

time required to change the rate or position of the ATS is necessary to assume 

a man model. Based on information given in Krendel and MeMter the following 

transfer functions were used in simulating the limited search problem. 

A. K e-" 0116 5Cc 

B. 

0,o16 Lo 

A 0.0K, to O. /.6-see. 

(io+I)(AO -f-i) 

e

-As 

Transfer function (A) was the basic transfer function used. 

Transfer function (B) was used to determine the difference in system 

performance when the integral plus proportional characteristics were removed from 

the control stick and incorporated in the man. 

Transfer function (C) was the basic man model modified to enable him to 

make the repositioning maneuver using an integral plus proportional control stick. 

It can be seen that the required increase -in the man's capabilities (change in 

transfer function) precludes the use of an integral plus proportional stick for 

repositioning. 

DUNG ..1 r if 

• 
The servo characteristics were: 

0.4 e-A5 

(S4r, 1) Vtoo +I) 

Operator 0.01s- to 0.15 

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The control stick transfer function can be varied by changing constants in 

the digital computer. The only constraint on the control stick was that only 

44>? 433 quantum levels were available at maximum stick deflection. This constraint 

requires that a gain change be incorporated in the computer when the ratio of the 

maximum stick output)to resolution per quantum level requirement exceeds 433. 

Control stick transfer functions used were: 

1. Proportional = K 

2. Integral plus proportional, • ( I'', Ar ) 

6 

The computer sampling rate was fixed at ten times per second. The initial 

sample time was varied relative to the start of the settling problem in the 

simulation. 

The Scanner Drive Servo Configuration was the baseline servo with the 30 

radians per second break in the D/A buffer removed. It was felt that the effect 

of this break on the simulation results would be minimal in that the phase 

margin of the man loop would be decreased slightly but the ten cps effects of the 

sampler would also be decreased. The net result would be little change in the 

time required to settly a specified accuracy. 

RATE CHANGE CAPABILITIES 

1. Proportional plus Integral Control Stick 

The rate change problem was simulated on the digital computer with the 

control stick in the proportional plus integral configuration. 

Stick Scale Factor = one u radian per second quantum maximumoutput 

At D/A Converter 433 u radians per second. 

1-1 

I-7- 

Computer Configuration stick I,pvf/I 

Scanner Servo 

Closed Loop Frequency 

Response 

Figure (1) shows the 

( 5!is-41) ( . ;c0-1-1) 

(5/5.4- i-1)( 7“-o i 1 )( 56ystii(7 (tz g)' 

servo diagram of the simulation. 

1- ___(o.s-01.1 

I28 I 

This configuration was simulated with the operator's delay varied from 

0.015 to 0.15 seconds. The results indicate that the settling time to a rate 

change increases as the operators delay increases. The time of the first sample 

relative to the start of the rate change by the operator was also varied. The 

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results indicate that the settling time is increased slightly as the time of the 

first sample is increased relative to the start of the problem. 

Figure (2) shows the sampling profile for the computer output when the 

operator's delay is equal to 0.15 seconds and the initial sample occurs 0.05 and 

0.10 seconds respectively after the rate change starts. 

Figures (3) and (4) show the final settling profiles for the two initial 

sample times. 

The simulation results indicate that with this stick configuration and the 

assumed operator characteristics, the "worst case" settling time = 1.95 seconds. 

Recent information from Aerospace indicates the man's delay may be greater than 

0.20 seconds. On this basis, an acceptable SPDR time requirement to change the 

LOS rate 1000 ± 17 u radians per seconds using this stick configuration would 

be 2.5 seconds. 

2. Proportional Stick 

A simulation was run with a proportional control stick. The integration 

accomplished by proportional and integral control stick of the previous simulation 

was now assumed to be done by the operator, thus the (s/5+1)/S characteristic 

of the proportional stick was incorporated into the man mode. 

First simulation results indicated that for the same man stick gain, the 

overshoot and setting time were increased when compared to the simulation with the 

proportional and integral stick configuration. This was a result of the increased 

phase shift (90 degrees) in the control loop due to the integration being made 

continuously in the man model. When the integration was made in the computer as 

was the case for the proportional and integral stick simulation, the phase 

resulting from this operation was somewhat less than (90 degrees). It can be 

shown that the phase shift resulting from digital integration is a function of 

the loop crossover frequency Wn and the sampling frequency Ws. For Wn<Ws, the 

phase is less than 90 degrees. To optimize the loop response, the man-stick gain 

was decreased from 4 to 2.5. This is justified since the man will adapt and 

decrease his gain when faced with the same conditions (decreased damping). 

The servo characteristics of the simulation were changed to: 

Operator 0.25 (5/6"-h e A S A 0.0 IS ZOO

= 

5 010 (Y/ 00 + 

Scanner Servo Closed Loop = 

Frequency Response 

3 u radians per second quantum 

maximum out 433 quantums 

Stick 3~ff Loop Command 

(5/8-4- 4-1)(Y‘40 ÷)(KVS- ( s zi)1 t (6.- )+ I ) 

123 

Stick Scale Factor 

At D/A Conlierter 

Computer Configuration 

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Figure (5) shows the servo configuration for this simulation. 

With the reduced stick gain (2.5 versus 4.0), the simulation was rerun and 

the operator's delay varied from 0.015 seconds to 0.15 seconds. The results again 

indicated that settling time was directly proportional to the operator's delay. 

The results showed that the time of the first sample relative to the start of the 

rate change was not critical. Figures 6, 7, and 8 show the sampler profiles and 

the final settling profiles. With the operators delay set equal to 0.15 seconds, 

the worst case settling time was equal to 1.6 seconds. This is somewhat better 

than with the time determined for the proportional and integral stick simulation; 

however, it should be noted here that the man was assumed to be able to do the 

integration operation in this simulation. If the man-stick gain had been lowered 

slightly for proportional and integral stick simulation, the settling times would 

have been approximately equal and the two stick configurations would appear to 

allow the same transient performance. The integral plus proportional stick 

allows for better stick resolution (one u radian per second) that does the proportional stick (three u radians per second). The integration operation in the 

computer rather than in the man allows this improvement. 

3. Proportional and End Stop Stick 

Recent simulation results indicate that the crewmen perfer this stick 

configuration. A simulation is being run presently to determine effects of this 

configuration with respect to the limited search capabilities of the ATS. 

Assuming the same man model as for the proportional stick simulation, the 

worst settling time for Proportional and End Stop could be determined 

.1. i i' 

to 

,,1-1._ IT 

---(11 -41 -4" ' 

---- rE 

A ---.. ' I !,_ 4, t : 

1--1-T 

*----- 1 Ce c —g: 1 T 

1.5Se c1 1 ---- 

1.44 on----4J 

by letting the stick have the following scale factor. 

A = 0.15 sec = Man's Delay 

B = 0.1 sec = Computer Delay 

E = Error Due to Delays 

= 225 u rad/sec 

1 u rad/sec for inputs 4-300 u rad/sec 

300 (1+2/5 u rad/sec) for input 7 300 u radians 

For these stick gains and worst case delays of 0.15 for the man and 0.1 

seconds for the computer total settling time will be: 

Time = - 0.15 initial man delay 

0.10 computer delay 

1.00 time to reach 900 u rad/sec 

0.15 man delay in removing end stop input 

0.10 computer delay in removing end stop input 

0.97 time to settle from 225 u rad/sec to 17 u rad/sec 

with proportional stick 

2.47 seconds 

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This is a worst case analysis and training of the man will help him to determine 

the optimum time to remove the end stop input. After training, this stick 

configuration will probably allow the rate change to be made in approximately 

two seconds assuming the man's delay does not exceed 0.15 seconds. 

REPOSITIONING CAPABILITY 

1: Proportional plus Integral Control Stick 

A simulation was run to determine the repositioning capabilities of the 

ATS using a proportional plus integral control stick. With the man as a position 

sensor, it becomes necessary to incorporate a low frequency lead into the man 

model to keep the man-loop stable when an integral plus proportional stick is used. 

Figure (9) shows simulation servo configuration. When this lead is put into the 

man model it means the man's capabilities are greatly increased and are not 

realistic. This can be seen in figure (10) which shows the sampler profile. The 

lead in the man means he reacts almost instantaneously and displaces the control 

stick by a very large amount. If the man could have the transfer function 

needed, it would be undesireable to use the proportional and integral stick in the 

repositioning problem due to the large stick range which would be necessary 

and due to large variation in settling time as a function of the time of the 

first sample. This also can be seen in figure (10). 

The stick configuration would not be used if the image were to be repositioned. 

2. Proportional Stick 

The repositioning problem was simulated using a proportional stick and the 

basic transfer function of the man given in Krendel and McMuer. 

The servo characteristics used in this simulation were: 

= 0 . 2C C - A -- 0, 01r; to 0.15"0 S-eCStick Scale Factor 40 u rad/sec/quantum 

At D/A Converter 

Computer Configuration Stick Output H_ Loop Command 

Frequency Response (5-71 .1 ) (?/- i 'h -5 , 

Scanner Servo Closed Loop = 

____ ____ .. _____. _ ,-c., 

(SX-01)5D 74 )NrY-fir,(125)t4 '2Ji2i'P)') ---77--- 

The image was repositioned within the two are minutes tolerance in less 

than two seconds assuming the man's delay to be 0.15 seconds. Should the delay 

be longer, then the settling time will be longer. 

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To make the repositioning maneuver, the control stick scaling must be 

40 u radians per quantum to insure that there is sufficient stick range. If at 

the conclusion of the repositioning it is desireable to reduce the rate error, it 

will be necessary to change the stick scaling via a constant in the computer. 

This will induce a transient and will increase to time necessary to reduce the 

rate error. The rate error is large when the position error is with the two 

min tolerance, hence to reposition and reduce rate error to less than 17 u 

radians per second requires considerable more time. The servo configuration 

of the simulation is shown in figure (11). 

The rate and position errors are shown in figure (12). 

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