6.101 Final Project

Brandon MotesGo Kart Controller

April 17, 2020

Abstract

This goal of this project is to construct a purely analogy controller for a 7R-7S motorcycleengine to be used on a go-kart. This entails all electronics from the crank shaft sensor tothe input to the spark plugs. It will include steps such as signal conditioning, derivatives,integrals, voltage memory units, and a flyback transformer. For this project success is arobustly operational design across all expected frequency input, and will be demonstratedby simulation. An extended goal of this is to fabricate a working prototype of this systemand achieve stable operation of the real engine.

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Topology Overview

Figure 1: Overview Block Diagram

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Cyclic Pointer (Red, Figure 1.)

The purpose of this module is to identify a unique point in the crank shaft sensor outputand output a consistent pulse to use to synchronize the spark plugs to the piston cycles. Thelong falling edge from a peak to a trough (highlighted in red, Figure 2) was selected for thisdesign, as it only occurs once in the cycle and is invariant to the frequency of the input.

Figure 2: Crankshaft Sensor Output

The signal is first conditioned by centering the incoming AC at 2.5V. Then a large gainis applied to any deviation from 2.5V. This gives a signal with clean square positive andnegative pulses relative to 2.5V. The signal is then fed into a differentiation unit. Theoutput of this unit is negative to the slope of the incoming signal. The differentiation outputis then rectified and integrated. This method is robust to changes in frequency as it calculatesthe total negative movement of each falling edge. This is useful because the falling edge isnow confined to two behaviors. A full rail-to-rail 5V drop or a 2.5 volt drop. As mentionedearlier, this 5V drop takes place only once in the cycle.In order to only integrate of the negative drop, the integrator capacitor (named the activecapacitor) is connected to ground unless the output of the differentiator is positive (negativeintegral). Therefore, the active capacitor is reset between every negative drop. Consequently,one of two charged values are expected on the active capacitor, one proportional to the halfdrop and another proportional to the full drop, which is approximately double that of thehalf.The voltage of the charged active capacitor is then stored on a second capacitor, called thememory capacitor, and is held through the next charging sequence. This allows the activecapacitor voltage to be compared to the previous one. And because there is only one timein which there is a full step voltage, there is only one time in which the voltage on the activecapacitor will be higher than the memory. During the period in which the active capacitoris charging and is over the memory capacitor, the trigger comparator is outputting a highsignal.

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Square Wave Generator (Blue, Figure 1.)

The purpose of the square wave generator is to create a half duty-cycle square wave syn-chronized to the trigger pulses and then double the frequency to output a wave with fourevenly spaces edges, two rising and two falling. This block falls into the realm of digital asall signals are binary, excluding the error signal.The first square wave will be generated with a 555 chip configured in a one-shot mode. Thechip will be triggered every time a pulse comes in. The output is fed though an integratorthat provides the control voltage on the duration of the one-shot in order to create a halfduty cycle wave.A common way to construct a frequency multiplier is to pass the wave into a filter in orderto obtain a harmonic, however two challenges are posed here. The first is that the secondharmonic is needed, and the square wave does not contain any of its second harmonic. Thesecond, which also rules other common methods out, is that the frequency doubler must havea relatively large bandwidth for the range it is dealing with. Therefore, an altered version ofthe previous method is employed. A single-shot configured 555 is connected to the incomingsquare wave, and another to the inverted wave. Then both chips’ outputs are connected to acommon output through a diode. The common output is then pulled low with a pull-downresistor. The 555 chips are given feedback from their individual outputs to bring them toa quarter duty cycle. This common output now outputs a half duty cycle square wave attwice the frequency of the input.Between the duty cycle equalizer and the frequency multiplier, there is a delay component.This component is necessary, because it is unlikely that the natural connections of thesecircuits will fire the sparks at the appropriate time in the cycle. A delay must be added toallow the user to adjust when the spark plugs are actually fired relative to the marker fromthe crankshaft sensor. This same feature will also allowed the engine to be tuned to be moreor less aggressive with the timing.

Ignitions Coils (Green, Figure 1.)

This final section takes the square wave from the generator and delivers a high-voltage pulseto the appropriate spark plug.The design only uses two coils despite there being four spark plugs. This can be done becauseit is fine to fire a spark plug on the exhaust stroke of an engine. Consequently, you can attachalternating spark plugs to the same ignition coil. With only two coils, one can be triggeredfrom falling edges and the other from rising.The coil is a based on a a flyback transformer architecture. An externally adjustable 555one-shot timer will allow the user to set the amount of energy being delivered to the sparkplug. The longer the primary is allowed to charge for, the more energy will be forced intothe secondary.

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