Circuit is used in spin-rate computing unit of control system for solar pointing rocket during its acquisition mode. Direction cosines from magnetometers and coarse sun sensors derive function that is approximately roll-rate times sum of absolute value of direction cosines

Cooke, W. A.

English

NASA Technical Reports Server

June 1971
	 Brief 71-10204 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Coarse Roll-Rate Gain-Control Circuit 
A coarse roll-rate gain-control circuit is used in the 
spin-rate computing unit of the control system for a 
solar-pointing rocket during its acquisition mode. 
Direction cosines from magnetometers and coarse 
sun sensors are used to derive a function that is ap-
proximately the roll-rate times the sum of the abso-
lute values of the direction cosines. The roll-rate cir-
cuit takes the resulting signal and divides it by the 
sum of the absolute values of the direction cosines to 
produce a signal that is proportional to the roll rate.
Division in the coarse roll-rate gain-control circuit 
is accomplished by including a multiplier in the feed-
back loop of a high gain operational amplifier, AR3 
(see diagram). The multiplier circuit includes a 
signal source (AR1), matched differential transistors 
(Qi), and a summing amplifier (AR2). 
The absolute value of the direction cosines is taken 
at the input of amplifier AM; the positive input volt-
ages are sent to the negative amplifier input while the 
negative inputs are coupled to the positive amplifier 
As	
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input. Any increase in the absolute value of the sum 
of the direction cosines causes the AR1 amplifier out-
put voltage to swing in a negative direction which in-
creases the bias current to the multiplier transistors 
(Qi). Resistor RII is needed to balance the AR1 
input voltage; otherwise, the negative 0.6-volt base 
emitter drop of Qi would cause the AR1 amplifier to 
be offset by the same amount since the drop is fed 
back on the positive input. Resistor R12 is necessary 
to prevent the division result from going to infinity 
(saturation) when the amplitude and rate inputs go 
to zero; it causes a small bias current to flow in multi-
plierQl. 
The multiplier supply voltage is reduced to 6.2 
volts by CR9 so that it is maintained within the com-
mon mode range of AR2. The multiplier input voltage 
is divided by R14 and R15 to keep the Qi input volt-
ages very small (approx. 20 mV); this is required so 
that the small-signal equivalent characteristics of the 
multiplier are maintained for satisfactory multiplying 
accuracy. 
The AR2 amplifier sums the Qi differential collec-
tor currents and provides a single-end output voltage.
Amplifier ARS requires a small capacitor (C5) to 
prevent high-frequency oscillation. The rate inputs 
come from a rate-switching circuit composed of a 
capacitor in series with a resistor. With several rate 
function input signals (not shown) connected to each 
input terminal (positive and negative), amplifier AR8 
sums and divides at the same time. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B71-10204 
Patent status: 
No patent action is contemplated by NASA 
Source: Walter A. Cooke of 
Lockheed Missiles & Space Company 
under contract to 
Ames Research Center 
(ARC-10064) 
Brief 71-10204	 Category 01


Coarse roll-rate gain-control circuit

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