8-29] SEARCH LOOP MECHANIZATION 451 



network C, possibly in combination with D, might be used in a tachometer 

 feedback path around the antenna, actuator, and power ampHfier. 



To use the RC compensating networks, it is necessary to convert the a-c 

 error signal to a d-c signal with a demodulator.^" A peak demodulator is 

 often used in the sear.ch loop because it has (1) less high-frequency noise 

 and (2) a smaller time constant than an averaging demodulator. After the 

 error signal is demodulated and passed through d-c networks and a power 

 amplifier, it is applied to the power actuator which moves the antenna. 

 Generally, the actuator is a two-phase a-c electric motor or a hydraulic 

 actuator controlled with an electrically operated valve. If an a-c motor 

 is used, the d-c actuating signal from the compensating network must be 

 modulated before it is applied to the motor through a power amplifier. This 

 may be done electronically with vacuum tubes or with magnetic amplifiers 

 which provide amplification, modulation, and power in one reliable unit. 

 However, if a hydraulic actuator with a control valve is used, the d-c 

 actuating signal may be used to control the valve. Some signal amplifi- 

 cation may be provided with tubes or with transistors. 



There are advantages and disadvantages in both types of antenna drives. 

 The electric motor is cheaper and lighter than the hydraulic actuator; it 

 does not need oil lines or rotary joints with oil seals; but it does not run as 

 smoothly at low speeds, it is much less efficient, and it cannot produce as 

 much steady-state torque or velocity as a hydraulic actuator of the same 

 size. In addition, gear trains -with troublesome backlash and friction are 

 needed with electric drives, whereas they are not used with hydraulic 

 actuators.^' 



Regardless of its type, an actuator must be selected which will provide 

 the required search-pattern velocities and accelerations. The output power 

 of the actuator must be greater than the power required to move the 

 antenna inertia along the search-pattern paths in space in the presence of 

 antenna friction and unbalance as well as aircraft pitch and roll motion 

 which may be directly opposed to the desired antenna space motion. In 

 fact, aircraft motion adds considerably to the required actuator torque, 

 velocity, and power because the actuators move with respect to the airframe 

 to generate a pattern in space rather than with respect to the airframe; thus 

 the expected aircraft motion must be combined with the required search- 

 pattern velocities and accelerations to determine the actuator performance 

 characteristics. Note that the antenna inertia and unbalance in elevation 

 may be less than in azimuth and the actuator may be correspondingly 



'"Simple RC networks can be directly approximated with "notch" networks or resonant 

 filters for a-c signals; but for airborne systems, this is usually not practical because of the 

 accuracy required and because the carrier frequency varies. 



'lit is assumed that the hydraulic actuator discussed here consists of a shaft with a vane 

 enclosed in a housing through which oil may be ported to either side of the vane to produce 

 a shaft rotation. Of course, 360° rotation is not possible with this type of actuator. 



