Blocking oscillator for a reciprocating electromagnetic actuator

A blocking oscillator for a reciprocating electromagnetic actuator having a solenoid coil, a detection coil for generating a control signal in response to a charge in the magnetic field generated by the solenoid coil, a silicon Darlington amplifier responsive to the control signal to control the current through said solenoid coil, and a diode connected in series with the collector of the Darlington amplifier rendering the Darlington amplifier immune to reverse voltage and transient signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to blocking oscillators for reciprocating 
electromagnetic actuators and in particular to a blocking oscillator for 
an electromagnetic fluid pump. 
2. Description of the Prior Art 
Electromagnetically actuated actuators such as electromagnetic fluid pumps 
are used for a wide variety of applications. These devices are required to 
operate over a wide range of temperature and are subject to relatively 
large voltage fluctuations. Because they are often used in relatively 
inaccessible places they must be reliable and trouble free. 
Many of these electromagnetic actuators embody a blocking oscillator 
generally of the type disclosed by H. P. Wertheimer et al in U.S. Pat. No. 
3,386,616 (May, 1968). The present invention is an improved blocking 
oscillator circuit, having superior reliability and performance over the 
blocking oscillators disclosed in my prior patents U.S. Pat. No. 3,629,674 
(December, 1971) and U.S. Pat. No. 4,080,552 (March, 1978). 
SUMMARY OF THE INVENTION 
The present invention comtemplates a blocking oscillator having a solenoid 
coil reciprocating a biased armature such as the piston in an 
electromagnetic fluid pump and a detection coil magnetically linked to the 
solenoid coil. The solenoid coil has an input end receiving electrical 
power from the positive terminal of an external source. A high gain 
silicon transistor, connected in series with the solenoid coil, controls 
the current flow through the solenoid coil. 
The silicon transistor has an emitter connected to the output end of the 
solenoid coil, a base, and a collector. The collector is connected to the 
negative terminal of the source of electrical power through a diode. 
The detection coil has one end connected to the output end of the solenoid 
coil and the other end connected to the base of the silicon transistor 
through a pair of serially connected resistances. A bias resistance is 
connected from between the pair of serially connected resistances to the 
negative terminal of the external source. A first serially connected diode 
and resistance are connected between the input and output ends of the 
solenoid coil, and a second serially connected diode and resistance are 
connected between the input end of the solenoid coil and the other end of 
the detection coil. In the preferred embodiment the silicon transistor is 
a high gain silicon Darlington amplifier. 
The advantage of the blocking oscillator is that the diode connected 
between the negative terminal of the external source and the collector of 
the silicon transistor reduces the loop gain and prevents high voltage 
reverse currents from being applied across the silicon transistor 
rendering it immune to reverse voltage and transient signals. 
Another advantage of the blocking oscillator is that it is compatible with 
a coil assembly having the solenoid coil wound over the detection coil. 
These and other advantages of the blocking oscillator will become more 
apparent from a reading of the detailed description in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 there is shown a circuit diagram of the blocking 
oscillator having a solenoid coil 10 and a detection coil 12. The solenoid 
coil 10 generates a magnetic field which produces a force 14 urging an 
armature 16 against the force of a resilient member such as a spring 18. 
The armature 16 may be a biased piston of an electromagnetic fluid piston 
pump as disclosed in my prior patent, U.S. Pat. No. 4,080,552. 
The input end of the solenoid coil 10 is connected to the positive terminal 
of a source of electrical power illustrated as a battery 24. The output 
end of the solenoid coil 10 is connected to one end of the detection coil 
12 and to the emitter of a transistor 20. The collector of the transistor 
20 is connected to the anode of a diode 22 having its cathode connected to 
the negative terminal of a source of the battery 24. Those skilled in the 
art will recognize that the transistor 20 may be replaced by a high gain 
Darlington transistor without departing from the spirit of the invention. 
The other end of the detection coil 12 is connected to the base of the 
transistor 20 through serially connected resistances 26 and 28 and to the 
negative terminal of the battery 24 through resistances 26 and 30. With 
this arrangement the diode 22 provides for a unidirectional flow of 
collector current from the transistor 20, lowers the loop gain of the 
circuit, and protects the transistor 20 from accidental reversal of 
voltages to the circuit and transient signals. 
A first serially connected resistance 34 and the diode 36 are connected 
between the input and the output ends of the solenoid coil 10, to provide 
a low resistance path for the current induced in the solenoid coil 10 by 
the collapsing magnetic field when the transistor 20 is turned off. 
A second serially connected resistance 38 and a diode 40 are connected 
between the input end of the solenoid coil 10 and the junction 32 between 
the detection coil 12 and resistance 26. The serially connected resistance 
38 and the diode 40, in conjunction with the serially connected resistance 
34 and the diode 36, control the rate of the collapsing magnetic field of 
the solenoid coil 10 which is a major factor in reducing electromagnetic 
interference (EMI) emissions. 
OPERATION 
The operation of the circuit is as follows. Upon the application of 
electrical power, the resistances 26 and 30 form a voltage divider 
producing reduced potential applied to the base of the transistor 20 
through the resistance 28. This initiates a current flow from the base of 
the transistor 20 making it conductive, initiating a current flow through 
the solenoid coil 10. 
An increasing current flow through the solenoid coil 10 induces a current 
flow away from the junction 32 in the detection coil 12. This decreases 
the potential cross the resistances 26 and 30 increases the base current 
flow of the transistor 20 until it becomes saturated. With the transistor 
20 saturated, the magnetic field generated by the solenoid coil 10 
produces a magnetic force 14 on the armature 16 sufficient to move the 
armature 16 against the force of the spring 18 producing the desired 
mechanical motion. 
Saturation of the transistor 20 and diode also terminates the expansion of 
the magnetic field produced by the solenoid coil 10 and terminates the 
current induced in the detection coil 12. This causes the potential at the 
junction 32 and the base of the transistor 20 to increase, reducing the 
conductance of the transistor 20, which in turn reduces the current flow 
through the solenoid coil 10. Reduction of the current flow through the 
solenoid coil 10 causes the generated magentic field to start to collapse. 
The collapsing magnetic field of the solenoid coil 10 induces a current 
flow towards the junction 32 in the detection coil 12. The increased 
current flow towards the junction 32 increases the current flow through 
the resistances 26 and 30 further increasing the potential at the junction 
32 which in turn further reduces the base current flow from the transistor 
20. This continues until the transistor 20 is turned completely off, 
terminating the magnetic field generated by the solenoid coil 10 and 
restoring the circuit to its original condition. Termination of the 
magnetic field generated by the solenoid coil terminates the magnetic 
force acting on the armature 16 and the resilient force produced by the 
spring 18 returns the armature to its starting position. 
By appropriate selection of the turns ratio between the solenoid coil 10 
and the detection coil 12, as well as the values or the resistances 26 and 
30, the oscillation frequency of the circuit may be controlled over a 
fairly broad range as is known in the art. 
The blocking oscillator circuit has the further advantage in that the 
silicon transistor, or silicon Darlington amplifier, have higher gains 
than conventional transistors. This significantly reduces the magnitude of 
the base current required to drive them into saturation and permits the 
detection coil to be wound with a much finer (smaller diameter) wire. In 
particular, the detection coil 12 may be wound with a wire size of 40 AWG 
or smaller, significantly reducing the volume of the coil assembly 
including the solenoid coil 10 and the detection coil 12. 
Because of this factor, the blocking oscillator lends itself to the coil 
configuration in which the detection coil 12 is wound on the inner 
diameter of a spool 42 with the solenoid coil 10 wound over the detection 
coil 12 as shown in FIG. 2. This configuration in combination with the 
blocking oscillator has several advantages which shall be explained 
hereinafter. 
First, the fine wire of the detection coil 12 is buried under the coarser 
wire of the solenoid coil 10 which makes the coil assembly less 
susceptible to physical damage. Secondly, it enables the use of more turns 
in the solenoid coil which increases the ampere turns and reduces the 
start voltage of the oscillator. Because the solenoid coil has a larger 
mean diameter, the length of wire is increased thereby increasing its 
resistance. As a result of this increased resistance the E/R 
(Voltage/Resistance) current of the solenoid coil is limited to a value 
within the silicon transistor range. This factor further permits the diode 
22 to be inserted in the collector circuit of the transistor providing the 
circuit with significantly improved reverse voltage immunity yet still 
permitting the blocking oscillator to start and sustain operation at a 
reduced voltage. For example, at room temperature, a circuit designed to 
operate at twelve volts, was able to start and sustain operation at a 
potential as low as six volts. 
Another characteristic of the blocking oscillator with the detection coil 
wound inside of the solenoid coil is that the oscillatory frequency is an 
inverse function of the load. In particular, the frequency of a blocking 
oscillator incorporated in an electromechanical fluid pump of the type 
disclosed in U.S. Pat. No. 4,080,552 with the detection coil wound inside 
of the solenoid coil was found to decrease with increased output pressure 
indicative of zero fluid flow. This feature of the blocking oscillator 
broadens it's applicability because the lower frequency reduces power 
consumption and is accompanied by a corresponding reduction in heat 
generated by the solenoid coil. 
It is not intended that the blocking oscillator be limited to the specific 
electrical components and coil assembly shown in the drawings and 
discussed in the specification. Those skilled in the art will recognize 
that changes may be made without departing from the spirit of the 
invention as set forth in the appended claims.