I have been trying to remember as much as I can about my work at Autonetics. I am undertaking this effort with the encouragement of two, self identified, long time collectors of aerospace memorabilia. We met (sort of, they live on the east coast) when they commented on some of my posts to this Blog. Their encouraging words resonated with my need to write. I have been trying to remember the details of the Electronics Module of the Autonetics G9 gyro. That is how I came to recall the “fuzz button”. The “fuzz button” ts a small bit of compressed wire that has been knitted on a machine used to make socks (Stay with me, this is not a joke). These little, pill like, pieces of wire made the G9 gyro electronics module a practical design and thus solve some tough packaging problems. I did not have a part to play in the original design of the G9 but I became the Senior Responsible Engineer for the gyro for ten years. My memories of the “fuzz button” are from my experiences with the “fuzz button” as we put the G9 Gyro into production soon after we signed the contract to build the INS for the FB-111 bomber for the U.S. Air force.
The G9 gyro was a small, case rotated, version of the Autonetics G6 gyro. The fact that that the G9 gyro, except for the stationary torquer assembly, was rotated continuously made it not practical to use the very reliable pickoff transformer used on the G6 gyro. It was much too large in size. The G9 pickoff circuit was designed around an Autonetics designed current amplifier, the ‘MIDCA’. This amplifier required +/-12 volt power and the G6 transformer did not. The case rotation feature of the G9 brought with it the need for a resolver to “de–rotate” the gyro pick-off signals as well as a slip-ring assembly. All of this was required to be packaged within the electronics module. Space for the electronics module was limited by the requirement the module must fit inside the case rotation bearing of the gyro. The electronics module was required to plug and unplug into the gyro case quickly and easily without special tools. This must have seemed an impossible situation to our designer as there was no space in the electronics module for standard design connectors. A new G9 gyro specific connector would introduce more complexity and higher cost into an already tough situation. I do not know the circumstances, but the designer somehow became aware of the “fuzz button” and like magic his problems with the connectivity of the module melted away. The heart of any ordinary connector is a metal pin inserted into a metal spring – like split sleeve that makes electrical contact between the pin and sleeve. A connector assembly is an array of these pin and sleeve devices. The pin and sleeve devices are held in fixed positions by insulating blocks. The pin and sleeve arrays are known as male and female connectors respectively. The force necessary to make or break the electrical connection is the sum of the force necessary insert or withdraw a single pin. A greater number of pins and sleeves equates to a greater force for connector insertion and withdrawal. To avoid damage to the connector assembly, the connectors must be well aligned as the connectors are mated . So, how does a “fuzz button” work and why did it enable he designer of the electronics module to achieve a successful module design?
The “fuzz button” is a pill-like compressed single piece of knitted wire. The amount of knitted wire in a given pill size is carefully controlled so that after the wire pill is inserted into its cup-like holder, it will remain compliant, I.E., tht is, act like little springs. an electrical connection is made when a rounded-end pin is inserted into the metal mesh pill. Since the diameter of the metal pill is substantially larger than the pin, only nominal alignment is required at insertion. The force necessary to make contact is the force necessary to compress the pill. This force is less than the insertion force for an ordinary pin and sleeve type connector. There is no withdrawal force associated with the “fuzz button”. Think of the implications of what is stated above. The use of the “fuzz button” makes it possible to design an module that only needs to be nominally aligned, requires virtually no insertion force and has no withdrawal force. The “fuzz button” is held in place by a simple cup-like holder. Bulky ordinary connectors are replaced by a simple unobtrusive array of cup-like ‘fuzz button holders that can be designed to be in the pattern most advantageous to the design of the module. It is my assertion that the use of the “fuzz button” made the design of the G9 gyro possible.
So, if the “fuzz button” was so beneficial to the design of the G9 gyro, why do I harbor mixed feelings about my experiences with it? It is simple really. The “fuzz button”, as seemingly simple as it seemed, required the “fuzz button” user to be ever vigilant concerning possible errors in the manufacturing process. For instance, the compliance of the “fuzz button” was dependent on having the precise amount of knitted wire before compression into the pill. We developed a method of measuring the density of each pill before use to guard against too much or too little mesh in each pill. We learned the hard way to not allow the “fuzz buttons” to shipped in envelopes the allowed commingling. We were once forced to untangle a batch with tweezers to avoid a delay in the production of modules. There were some unhappy technicians during that episode. I must concede that the availability of the “fuzz button” was a positive factor in the success of the Autonetics G9 gyro – in spite of these problems.