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C.  Mach Trim Circuit Operation
(1)  Signal Flow (See figure 2.)
(a)  
The air data computer (ADC) supplies parallel dc signals from two mach potentiometers. The two signals are applied to the mach trim coupler command channel and comparator channel respectively. These channels operate in a similar manner.

(b)  
The detector circuit receives signal voltage from the ADC mach potentiometer which is a function of mach number (speed). The received signal is monitored and compared for an amplitude that is in the mach tuck region. When in the mach region, one output from the detector is applied to a logic circuit for control of power to the motor and brake. A second output is applied to the limiter which removes a ground from the input signal to summing point 1. The third output is the mach signal which is fed to the function generator.

(c)  
The function generator amplifies and shapes the signal to provide proper gain and magnitude which is proportional to the signal level.

(d)  
The limiter amplifies, limits, and controls the input signal. One function of the limiter is to monitor the ADC mach potentiometer output for control of mach tuck upper voltage limits. The second function of the limiter is to ground the output signal when the output of the detector is below mach tuck region. The limiter also shapes and amplifies the output signal which is then applied to summing point 1.

(e)  
The output signal from summing point 1 goes through two limit switches in the actuator. When the actuator rod (jackscrew) is driven by the gear train to either fully extended or retracted position, the appropriate switch opens and stops the motor. Diodes then allow a signal of opposite polarity to drive the motor in the opposite direction. When below the mach tuck region, the actuator is in the fully extended position.

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H28730
500  Mach Trim Block Diagram 
Aug 15/68  Figure 2  22-21-0 
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(f)  
The signal from the limit switches is applied to an amplifier. The signal is also modulated and chopped at 90 degree phase angle with respect to the actuator motor excitation. The resultant output has a pulse width which is a function of the magnitude of the dc mach signal input. The dc mach signal input polarity determines the phase of the pulse train.

(g)  
The motor amplifier has two split secondary transformers operating on the input signal to produce a switching signal. This signal applies 40 volts dc to the motor control winding for the pulse duration and also switches the 40 volt dc polarity at the pulse frequency.

(h)  
An ac position feedback from the synchro is routed to the demodulator. The demodulator chops the synchro signal at opposing phases and amplifies to supply a dc output. This output is used in the comparator, the rate feedback network, and at summing point 1. The rate feedback network acts as a damper to the mach signal by providing an input to summing point 1. The composite signal from summing point 1 is used to drive the actuator motor.

(i)  
The comparator circuit operates from information received from the demodulator and the limiter. This circuit supplies a 12 volts dc signal to a logic circuit when the difference between its two input signals is less than a predetermined level.

(j)  
The logic circuit requires mach region good input, mach good input from ADC, and comparator good input in order to complete the logic necessary to provide voltage to the brake and motor. The logic circuit monitors for 28 volts dc as brake power good from the brake circuit. With this input and the other inputs good, the logic circuit provides a warning logic good output to the warning circuit.

(2)  Logic and Interlocks
(a)  Power to the actuator motor and brake is provided through the power relay contacts in the coupler. (See figure 3.) The relay is operated by the following logic requirements. 1) The 28 volt dc and 115 volt ac mach trim circuit breakers are closed. 2) Airplane is operating in mach region as monitored through mach trim coupler
command channel.
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H28732
500  Mach Trim Engage and Warning Logic Circuit 
Nov 15/68  Figure 3  22-21-0 
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