MAGNETOSTRICTIVE ACTUATOR TECHNOLOGY
On the periodic table of elements, terbium (element number 65) inseparably couples magnetic with mechanical effects. This phenomenon is indestructible because it is quantum mechanical. (For a description of this effect, please refer to Chapter 1 of "Handbook of Giant Magnetostrictive Materials".) Noticing this, the U.S. Navy combined it with dysprosium (element number 66) and iron to make an intermetallic alloy useful in mechatronic actuators for ocean sonar. GPDT's continuous direct control of diesel fuel injector needle lift is but one more use of this durable mechatronic / electro-mechanical actuator technology.
The diagram is a simplified schematic showing how this terbium-dysprosium-iron alloy rod works inside an actuator. Within the solid body of each rod exist magnetic domains, shown here as ovals each with North-South orientation.
Alloy and Mechatronic Actuator Properties
- No Stress, No Field. The rod is in its free, as-manufactured state. In this state, it is not ferromagnetic -- the effects of magnetic domains cancel out each other.
- Compressed, No Field. A compressive preload has been applied to the rod. In a diesel fuel injector, this preload is ideally applied by fuel pressure. Since nothing is infinitely stiff, the rod contracts axially due to this force. The domains rotate as shown but still cancel each other magnetically.
- Compressed, Moderate Field. A moderate magnetic field aligns the domains. The magnetic field is the result of electrical current circulating around the rod in a coil (not shown for clarity).
- Compressed, Strong Field. An increase in current increases the magnetic field, causing the domains to rotate toward closer alignment with the magnetic field. This expands the alloy rod, even against the applied compressive preload. This mechanical expansion is the useful output harnessed by GPDT to directly lift the injector needle.
These properties create an actuator superior to piezoelectric products.
- Because the quantum mechanical coupling of elastic to magnetic effects is an indestructible atomic effect, there can be no fatigue limit. To confirm this, fatigue tests of terbium-dysprosium-iron alloy rods have been run to billions of cycles with no detectable performance degradation.
- The present formulation optimizes performance at ocean temperatures. Displacement will decrease with increasing temperature but is fully restored once the rod cools.
- The alloy itself is inert to non-ionic hydrocarbons such as liquid hydrocarbon fuels. There is no need to isolate the actuator from the fuel, eliminating that cost and complexity.
- The coil of wire that supplies the magnetic field does not contact the rod. Therefore, it cannot be fatigued by actuation.
- Hardened caps bonded to rod ends have proven to ruggedize it against abuse.
- Alloy force and displacement output are analogous to thermal expansion, except electrically controllable.