You have just spent $10,000 and put in hundreds of hours to build the latest true turbojet powered radio controlled scale model of an F104 Star Fighter, a model that weighs 30 pounds and that will fly at 250 miles an hour. What you don't need is an engine flame out due to throttle mismanagement. The circuit presented controls the rate at which a miniature turbojet engine can be throttled. Increasing or decreasing the throttle too quickly on a miniature turbojet engine, or any jet engine, can lead to quick failure in flight causing expensive repairs and potential safety problems due to flameouts. The described circuit can be used as an analog backup circuit in a microcomputer based throttle controller or as the primary throttle rate controller. The response rate, servo direction, CW and CCW gain (end
points) and servo centering parameters are adjustable.
Servo circuit for miniature turbojet engines
This circuit takes a received radio control incoming positive going pulse which varies from 1 to 2ms, the standard for most aircraft radio control systems, at a fixed frame rate of 20ms, integrates it over time, and uses the output of the integrator to control the output pulse width of a 555 based monostable. The rate of pulse width change is determined by the effective integrator time constant, R24 and VR1 in series, C1, and the duty cycle of the incoming pulse stream. Q1 is switched on for the duration of the positive going pulse aapplying +5V to the integrator input resistor, R24 and VR1 in series. The circuit is designed such that a point will be reached where the effective charge rate, based on the input pulse width, and the reset/discharge rate of integration capacitor C1 by R1 balances for each positive pulse duration, and the output of integrator U3 stabilizes at a DC value as a function of the received pulse width. An LTC2054 Zero-Drift Op Amp is used for the integrator. The LTC2054 has an ultra low input bias current, 1pA typical 150pA Max, offset voltage of 3microVand a drift spec of 30nV/C Max. An open-loop gain of 140dB typical, a PSRR and CMRR of 130dB typical, and a low noise spec of 1.6microVP-P typical all add up to an excellent Op Amp for the integration function.
The LT1120A low voltage regulator includes a reset output used to hold-off any output from U5 until the integrator output has had time to reach an output after one full time constant. The low dropout regulator delivers a well-regulated 4V from a 4.8V NiCad battery even when the battery voltage is pulled down to 4.2V under heavy loads such as using high current digital servos.
U4: A provides gain and the ability to independently adjust the CW and CCW endpoint travel. Diodes D1 and D2 effectively splits the feedback path when the input signal is above or below the 2.35V Ref level on pin 5. This circuit is a basic precision rectifier used as a gain splitter. U4: B buffers and sums the CW and CCW signals. U4: C, a 1 gain amp, adds the servo reverse function as will as providing a servo center position adjustment.
Op Amp U4: D and resistors R11 and R12 generates the 2.35V pseudo-ground reference for the single supply op amps. The 2.35V value is chosen to be centered within the Op Amp's input common mode range to give symmetrical output swing. The centering adjustment VR4 sets the input control voltage of the 555 Timer to its input control voltage mid point. An LM334 constant current source set to 16ua, charges timing capacitor C11 with a constant current to achieve linear response to the input pulse. D3 and R14 are used to negate the 10mV/°C temperature coefficient of U6 the LM334. Two LMOS TC7S14F Schmitt inverters for signal buffering rounds out the circuit.
This circuit provides a very smooth adjustable servo response without having to use a high value electrolytic capacitor or very large value resistors. Servo center adjustment, servo direction and independent end-point adjustments are included.
Jim Mahoney is Associate Design Engineer by Linear Technology