1. Field of the Invention
The present invention relates generally to remote control devices and more particularly to a remotely controlled motor speed control circuit for use in remotely controlled model vehicles.
2. Description of the Prior Art
The remote control of model cars, boats and airplanes is typically accomplished by the use of a radio control system such as the one shown diagrammatically in FIG. 1. The system includes a plurality of user adjustable controls, or joysticks, 10 which are adjustable over a range of positions in one or two planes and correspond to the desired setting of one or more control parameters in the model, e.g., the position of the rudder, ailerons and/or engine throttle.
An encoder 12 in the base unit generates a series of pulses each having an average width of 11/2 milliseconds and a basic frame length of from 10 to 16 milliseconds. In response to the position of a given joystick, assigned pulses within the series are caused to vary over a range of from 1 to 2 milliseconds in width. The pulses generated by the encoder are then used to modulate a radio transmitter 14 having a carrier frequency which is usually within one of the three bands of frequencies located near 27, 50 and 72 megahertz.
The transmitted signal is received by a radio receiver 16 located within the model. The receiver detects the transmitted signal, recovering the continuous series of pulses which are then used to drive a decoder 18 which multiplexes the pulses, assigning a particular pulse of the series to a corresponding one of several control servos 20. In response to the width of its assigned pulse each servo drives one of the controls of the model connected thereto.
A model typically includes one or more DC motors, such as the motor 22, to provide power to be used, for example, in driving the model. The speed of the motor is controlled by a motor speed control 24 in response to the width of an assigned pulse from decoder 18.
It will be appreciated that the remote radio control of models presents some unique problems due in part to the great premium on space, and usually weight. Such restrictions severely limit the quantity of batteries which the model can carry. Since the principal source of power consumption within the model is usually the DC motors, their efficiency significantly affects the performance of the model. The small DC motors also present unusual problems due to their series resistance which varies from a very low value when the motor is stopped, or running slowly, to a much higher value at full speed. The low starting resistance of the motor results in a very high starting current which contributes greatly to the limited starting torque available.
Heretofore, control of the speed of DC motors used in models has primarily been accomplished by the use of servos, similar to those used to drive the other controls, which servos are used to drive rheostats or operate switches which vary the resistance in series with the motor. Not only is the use of series resistance to control motor speed very wasteful of the limited power available, but it aggravates the poor response inherent in such small motors. The series resistance also limits the starting current, resulting in poor low speed control, especially under varying load conditions.
One prior art device for electronically controlling the speed of a DC motor is described in the April 1976 issue of the magazine Radio Controlled Modeler at pages 6 and 66. This device includes a pulse width demodulator for receiving input control pulses. When a particular input pulse exceeds a reference width, the demodulator generates an output pulse having a width proportional to the excess width. The output is then fed through a Darlington circuit to control the motor speed. Although effective in generating a high motor starting current, the device is inefficient in controlling the motor at moderate and high speeds because of the large voltage loss occurring across the saturated Darlington transistors.
An electronic bidirectional device for controlling the speed of a DC motor is disclosed in the May 1976 issue of Radio Controlled Modeler at pages 12, 147-148, 151 and 154. This device uses a similar pulse width demodulator for generating first and second output pulses when the input pulse has a width greater than or less than the reference. The two output pulses are used to drive two inputs of a bridge type motor driving circuit. This device suffers from the disadvantage of requiring two separate batteries, i.e., one for powering the demodulator and another for powering the motor drive circuit so as to isolate the high current inductive load from the demodulator.