The sensor system has to represent the rotational frequency of both propellers in an electrical format that can be used in later subsystems. We decided on an approach in which the rotation of the propeller creates square voltage pulses whose frequency is identical to that of the propeller. Initially we hoped to reflect a red laser beam off the back of the propeller whenever it passed a certain point in its rotation. This reflection would be sensed by a photo transistor, creating a square pulse train. However the geometrical logistics of reflecting the laser into a small photo transistor proved too complicated for our time constraints. We finally alighted on a system in which the propeller interrupts a red LED beam. This approach is much more resistant to vibration, which was a main cause of headache in the laser approach. The signal conditioning circuitry required a voltage pulse of 100 mv peak voltage. This was achieved using the red LED approach. The setup concept is shown in Figure 1.
Figure 1
Signal Conditioning System
The signal conditioning system has to provide a clean 0-5 volt square voltage pulse train to the UFDC-1 frequency sensing chip. This is accomplished in two stages, amplification and filtering. The amplification stage is performed by a non inverting op-amp configuration of gain 20. The incoming pulse train from the photo transistor is thus amplified to the proper voltage range. However the problem still exists of additional noise in the signal that prevents the UFDC-1 from being able to read the frequency. In order to clean the signal up a voltage comparator is used. The voltage comparator only outputs HIGH when the incoming pulse exceeds a certain threshold value. Since the noise amplitude is much lower than the amplitude of the pulse caused by the propeller motion a properly chosen threshold will "ignore" the low amplitude noise. Figure 2 shows the progression of the signal through the signal conditioning system.
Figure 2
Control System
The control system consists of an Arduino Micro controller and a UFDC-1 frequency sensing chip. The UFDC-1 reads the frequency difference of the signals from the signal conditioning system and communicates that information to the Micro controller. The Micro controller then implements a control algorithm that sends a control signal to the slave motor adjusting its frequency. The two chips communicate serially. The control algorithm is a proportional integral approximation. The micro controller responds at regular time intervals and only has a limited number of control signals to output, thus it is a discrete time digital controller. The proportional term in the algorithm responds in proportion to the instantaneous frequency difference. The integral term responds to the integral of the frequency difference. Both terms are summated and mapped to the proper control signal range. Figure 3 shows the mathematical description of a true PI algorithm.
Figure 3
Driver System
Hardware
System Flowcharts