Force Limited

Principles and Benefits

In the selection of a clutch for use in servo-actuators, a wide variety of torque controlling devices is available. In many applications, devices such as the slip clutch, the solenoid operated disc clutch, the spring clutch, and the magnetic particle clutch have been used. Of these sundry devices, the two most frequently used in servo-actuators, are the spring clutch and the magnetic particle clutch. The spring clutch is an on-off device offering extremely high response times in the order of 4 to 5 milliseconds. Spring clutches have been used frequently in small missile control systems. They require intricate adjustments and are not linear over a long life cycle.

The magnetic particle clutch, with its output torque proportional to its input current characteristic and its long slip operation, is the only proportional torque device ideally suited for application in proportional servo-actuator control systems. Different types of magnetic particle clutches have been developed over the years for aerospace applications. Two broad categories are the Stationary Coil and the Rotating Coil devices. In the Stationary Coil Clutch, the flux is required to pass through an additional mechanical interface not present in the rotating coil clutch. For this reason, in clutches which have been manufactured in the past, the useful flux generated in the clutch will be less in the Stationary Coil device than in the Rotating Coil device of equivalent size having the same current and ampere turns input, since the rotating coil offers the shortest magnetic flux path. In the past this has resulted in the single disadvantage of the Stationary Coil Clutch, since the rotating coil clutch provided a higher output torque for given clutch diameter than a stationary coil clutch. However, Force Limited engineers have now developed and tested a Stationary Coil Clutch which essentially matches the output torque characteristics of the rotating coil clutch. This has enabled us to take advantage of the obvious advantages of the stationary coil clutch without sacrificing the inherent low-output disadvantage of previously designed clutches.

Some of the specific advantages of a Stationary Coil Clutch may be summarized as follows:

Longer Life. Slip-rings and brushes limit the life of a Rotating Coil design.
Higher Reliability. Fewer parts.
No R. F. Filter required.
Less susceptible to vibration.
Better Storage Life. No slip rings or brushes to tarnish.
Lack of Contamination. No brush or slip ring wear particles
Better Hard Vacuum Potential. No metal-to-metal contact, slip member to vacuum weld.
Easier "Spares" Housekeeping. No loose parts (brush blocks) to store.
Better High Temperature Potential. No solder joints or epoxies used in critical heat areas.
Less Sensitive to Shorting. Electrical parts unexposed to moisture.
Smoother Torque Output. -Not subject to brush bounce at high RPM.
Easier Servo Assembly. No brush alignment or assembly required.

The operation of a magnetic particle clutch in servo-actuator applications can be described simply as follows:

A motor drives the clutch input gear at a relatively constant velocity. During zero-command current conditions the output is static. Under these conditions the clutch housing rotates about the shaft end and is supported on the internal bearings. The drag cup is rigidly attached to the shaft. Magnetic particles are contained in the powder cavity and are in intimate contact with the rotary section of the drag cup, under the influence of inertial forces. As a command-current is applied to the coils, through their being energized by slip rings (in the rotary coil concept) or by direct wiring (in the stationary coil concept), a magnetic flux is set up in the air gap. This flux establishes shear strength in the magnetic particles and permits torque to be transmitted to the output shaft by the drag cup. Torque output will be roughly proportional to coil current.

The magnetic particle mixture utilized in the clutch being described, is a proprietary development of Force Limited. The mixture has properties expressly developed to minimize zero current friction while maximizing null sensitivity. The shape of the particles also provides the clutch with the ability to withstand rigid environmental requirements. Other ingredients are included in the powder mixture to reduce the probability of magnetic particle oxidation. Special machining and heat treatment processes are required to reduce hysteresis and provide for maximum null sensitivity. These advantages can be seen instantly when one realizes that the closer the torque breakaway point is to zero current, the lower the value of the quiescent current is. This lower continuous power drain on the missile power supply is a considerable advantage in this age of miniaturization. Electrical and torque time constants have been reduced to a minimum by exercising design controls over the clutch coil configuration and winding. This has resulted in minimizing coil inductances.

All of these design and performance advantages are offered to confirm our claim that clutches manufactured by Force Limited are superior in the areas of output torque, time constant, null sensitivity and dynamic performance.

This photo shows an example of a combination clutch-brake. An input drive shaft enters at the base of the shell. An output shaft extends beyond the mounting plate. Input leads to both the clutch coil and the brake coil will be connected to the control system circuitry.

A different clutch brake assembly can be seen here in the form of a mechanical drawing of its external dimensions.