10 



« 



The accelerations are measured by pickup devices which are essen- 

 tially identical. These accelerometers utilize a "linear variable differen- 

 tial transformer" (LVDT) as the sensing element. 



This transformer consists of a secondary and two primary windings 

 wound on the same axis, with the secondary located between the primaries. The 

 primaries are connected in series so that the voltages they induce oppose each 

 other. When an iron core is placed on the axis of the LVDT and symmetrically 

 located with respect to the primaries, the output of the secondary is a mini- 

 mum (practically zero). This is the null position. When the core is moved in 

 one direction from the null position, the voltage induced in the secondary by 

 one primary will increase while that induced by the other primary will de- 

 crease. The net result will be an increase in output voltage directly propor- 

 tional to the core displacement, and the phase of this voltage will depend on 

 the direction of the displacement from the null position. To operate as an 

 accelerometer the displacement of the core is made proportional to accelera- 

 tion. A signal proportional to angular acceleration is provided by connecting 

 the output of two linear accelerometers in series opposition. On the Coast 

 Guard weather ship, two linear pickups were spaced l6.1 feet apart to give the 

 desired angular acceleration signal. 



A signal proportional to the rolling and pitching angles may be ob- 

 tained either directly from the output of a stable element, or, alternatively 

 from a potentiometer driven by a servo system which in turn is actuated by a 

 stable element. For greater accuracy more refined methods can be devised. 



It should be noted that the transducers as well as the recorder and 

 amplifiers are designed for 60-cycle 110-v a-c operation. They could just as 

 well have been designed for an a-c supply of higher frequency. 



The accelerometers were obtained from the Schaevitz Engineering Co.; 

 their specifications are given in Table 2. 



PRINCIPLE OF OPERATION 



In the schematic diagram of Figure 8, it is seen that the outputs of 

 the secondaries of two differential transformers (LVDT) are connected in 

 series opposition. One LVDT is located in the recorder, see Figure 4. Each 

 produces a 60-cycle a-c output. 



The sum of the outputs of the two LVDT's is amplified, its phase is 

 shifted electrically 90 degrees, and then impressed on one phase of a two- 

 phase motor; the other phase of the motor is directly excited from the a-c 

 power supply. This two-phase motor in turn drives both the pen and the core 

 of the balance LVDT until the output of the balance transformer's secondary is 

 equal to that of the accelerometer ' s secondary, resulting in zero voltage 



