Engine control circuit including speed monitor and governor

#The present invention relates to control devices for internal combustion engines and particularly small engines. The usual type of controller is a PID controller in which a signal proportional to engine RPM is combined with a derivative signal and an integral signal to generate an error voltage used to regulate a throttle or other engine speed control device.

#With particular reference to smaller engines, there is a greater potential for damage to the engine in the event that there is a failure in the control electronics. Indeed, there are several types of failures to be dealt with in order to fully protect the engine from damage. For example, if the magnetic pick up device used to monitor engine RPM, fails, an ordinary speed governor may interpret the absence of such signals as a below set point speed error and, therefore, continually increase engine speed until damage occurs. In reality, such a condition may be nothing more than a broken pickup coil or associated problem. It is necessary, therefore, to provide a fail safe circuit which will not respond in this manner in the absence of magnetic pickup signals.

#Similarly, in the event that a governor output transistor becomes shorted, the governor loses its ability to shut down the engine and, again, over speed conditions can occur which will cause damage. A third problem frequently encountered is damage to the potentiometer by which the desired speed reference value is set. If the potentiometer fails, the governor would be provided with an incorrect reference voltage also leading to the engine's damage.

#FIG. 1 is a simplified schematic diagram of the invention illustrating the important features thereof.

#FIG. 2 is a detailed schematic of the invention showing the circuitry in detail.

#Referring to FIG. 1, a control circuit is illustrated. The control circuit consists of two sections, a speed governor section 10 and an engine speed monitor section 12. Referring first to the governor section, a magnetic pickup signal is obtained in the usual manner from a shaft encoder or other magnetic pick up device associated with the engine to be governed. As is well known in this art, such a magnetic pickup can produce a waveform (usually sinusoidal), the frequency of which is directly related to engine RPM. The pickup signal is provided to a network 14 which buffers the signal, converts it to a series of DC levels (square waves) and which contains fail safe circuitry to shut down the engine in the absence of a magnetic pickup signal. The details concerning network 14 are described hereafter in connection with FIG. 2. The output of network 14 is provided as one input to a comparator 16, the other (plus) input of which is obtained from a potentiometer 18. Potentiometer 18 is the speed reference potentiometer. Its value determines the speed at which the governor will permit the engine to run during normal operation. The output of the comparator 16 is a voltage, V.sub.E which is the speed error obtained by subtracting the run voltage, V.sub.RUN from the set point voltage V.sub.SET. V.sub.E is normally equal to zero when the engine running speed is equal to the set speed.

#The speed error signal, V.sub.E is operated on by the PID section, including operation amplifiers 20 and 22, in a manner well known in this art. Specifically, the proportional and derivative of V.sub.E are combined with the integral of the signal from amplifier 22. When the error signal is zero, the output of the amplifier 20 is forced to zero. If the output amplifier 20 is not zero, it is indicative of a speed error. This output is a DC voltage. It is used to limit the integrator 22 by means of the limit network 24 to keep the integrator in a desired operating region. This prevents saturation of the integrator while permitting the use of a low gain response for the governor to insure precise operation. The PID gain is set by a potentiometer 26.

#A significant feature of the present invention is the ability to alter the PID gain during engine start up. During such start up it is desired to provide a high gain so that the circuit can quickly adjust the engine speed to the desired set point. This is accomplished by means of diode 30 and resistor 32 connected between the input of the amplifier 20 and the output of amplifier 16. These components produce a large PD gain whenever there is a large error signal (at least 10-20%). With error signals of 10 to 20%, this combination causes the gain to go very high as is desired during engine start up. This result obtains only during a start up condition due to the polarity of the speed error signal under these circumstances.