Electropneumatic servoactuators are used in control systems where it is desired to convert a varying electrical input control signal to a mechanical position output which is controlled by the electrical input signal. This type of servoactuator derives its source of energy for the force output from connections to a source of fluid pressure and, in the case of the electropneumatic servo, the source of fluid pressure may either be compressed air or other gas or vacuum. Servoactuators of this type are used for dynamic control situations as, for example, the operation of the movable control surfaces such as ailerons, elevators and flaps of an aircraft or for throttle control of a powered vehicle. In this latter type of application, it has been found particularly convenient to use an electropneumatic servoactuator connected to a source of subatmospheric pressure, or vacuum, for controlling the throttle of a motor vehicle in the operation of vehicle road speed regulators or cruise controls, as they are commonly called.
In vehicle speed regulator systems of the type known as "closed loop response" to those skilled in the art, it has been determined that a force balance output servoactuator cannot accommodate the spring forces, hysteresis, and friction present in the throttle linkage systems, which vary widely between vehicles. Where attempts have been made to utilize a force output fluid pressure servoactuator, the output has varied with variations in the fluid supply pressure, with altitude and with variations in electrical power to the electrical and electromagnetic components of the servo.
For vehicle speed regulator systems having "closed loop" logic control response, the criteria for satisfactory servoactuator operation are: (a) the ability to position accurately and repeatedly in response to a given input control signal in a manner independent of the throttle and within the capability of the servo.
In motor vehicles powered by naturally aspirated internal combustion engines, a source of subatmospheric pressure or vacuum is readily available from the induction manifold for feeding the combustible charge to the respective combustion chambers. Therefore an electropneumatic throttle servoactuator of the type operating from such a source of subatmospheric pressure has been found particularly suitable for use in controlling throttle of a motor vehicle during operation of a cruise control for the vehicle.
In providing a cruise control system for a vehicle, those skilled in the art of servomechanisms and control systems have recognized that, in order to provide accurate position output, yet eliminate the need for a prohibitively high degree of gain in the control system, it is desirable to provide some means of feedback of the throttle position to the control input in order to provide the closed loop response. Heretofore, the throttle position has been detected electrically and an electrical signal fed back to the input control signal generator for modifying electrical control input signal to the servo. However, it is also known to provide mechanical feedback of the throttle position within the servoactuator to directly control the fluid pressure acting upon the output force actuator. Examples of this latter type of feedback are described in U.S. Pat. No. 3,298,482 to R. S. Mueller et al., and in a copending application assigned to the assignee of the present invention, Ser. No. 671,539, now U.S. Pat. No. 4,072,206, filed Mar. 29, 1976 to Larson et al., and a copending application assigned to the assignee of the present invention, Ser. No. 660,290, now U.S. Pat. No. 4,046,213, filed Feb. 23, 1976 to G. L. Larson. In the Larson references a servoactuator is shown as having a control valve member alternating between a fluid pressure port and a port venting the fluid pressure chamber to the atmosphere, in a manner alternately closing the fluid port for controlling the fluid pressure in the chamber. The control valve is driven by the magnetomotive force (mmf) of an electromagnetic means responsive to the input electrical signal. Such an arrangement is known as a duty cycle control, or "flapper," valve; and, the mmf is typically produced by the input control signal having a series of width modulated pulses applied to the electromagnetic means, usually a coil. The alternating movement of the control valve flapper between the vent and fluid pressure supply port means in accordance with the modulation of the control signal determines the fluid pressure within the servo chamber at any given time. The pressure in the chamber acting on the actuator means produces the desired position output.
In control systems where closed looped control is desirable for providing a position output as, for example, in vehicle speed regulation, it has been found particularly useful to employ the throttle position feedback technique mentioned above. One such technique for employing direct mechanical throttle position feedback is that shown and described in copending application Ser. No. 671,539 described above in which a pivotable member senses motion of the actuator means and by its pivotal motion alters the bias of the spring providing the return force on the control valve flapper, thereby altering the duty cycle movement of the flapper in response to the control signal. This type of throttle position feedback directly within the servoactuator, by varying the force on the control valve flapper, affects the response of the flapper to the mmf and alters the movement of the flapper and affects the dynamic characteristic of the control valve flapper by changing the air gap relationship of the flapper to the ferromagnetic core employed with the coil. This change in air gap caused by the feedback complicates the design of the control valve flapper and undesirably limits the response characteristics of the flapper to the control signal.
In the design and manufacture of such duty cycle type pneumatic servoactuators it has also been found desirable to provide an auxiliary means of relieving fluid pressure from the servoactuator chamber in emergency situations. For example, where the servoactuator is used for vehicle speed control system and connected to the vehicle throttle, it is necessary that the actuator means release the force on the throttle almost instantaneously upon activation of the vehicle service brakes. In order to accomplish such rapid release, it has been found necessary to provide an auxiliary vent or dump valve in the servo chamber. Previous means of providing auxiliary venting or dumping of the servo chamber have been that of a separate electromagnetically actuated valve member covering an auxiliary vent or dump port. Such an electromagnetic dump valve may then be actuated by a switch attached to the vehicle brake pedal as is known in the art.
However, in providing an auxiliary dump valve for the servoactuator for fluid pressure chamber, it has been found necessary in previous systems to employ a separate electromagnetic coil for actuating the dump valve. The addition of a second electromagnetic coil circuit to the servoactuator has proven to be costly and also requires additional power to the servoactuator.