Unique Magnetics Deliver Cutting Edge Performance

Unique Magnetics Deliver Cutting Edge Performance

Electro-mechanical devices are designed to move a mass using either electric or magnetic fields. In macroscopic devices the magnetic field is the overwhelming choice. The goals can range from ultra-efficient use of available electrical current, as in space flight devices, to ultra-fast motion without limits on the current. In either case, the materials and geometry of the magnetics are key to performance. All of the desirable engineering functions must be addressed since they are highly convolved and cannot be optimized individually. The electric motor industry has nearly perfected the complicated task of optimizing the many functions for converting current flow into rotary motion. Reliability, torque, inductive response time, service cycles, thermal control, pole cogging, outgassing and many other features are quite mature design elements over the nearly 140 years of evolution. But what about linear motion?

Many linear motion devices such as magnetic rails, using multiple magnetic elements, are mature and designed for automated machinery. Typically they are capable of moving modest masses over significant distances, such as a meter. Motors driving a lead screw to produce linear motion are still in the rotary motion class of magnetics. When you review small electro-magnetic devices designed for millimeters of motion, there are only a few cases. Typically these are relays, solenoid plunger valves, resonant vibration devices, and laser/optical shutter products. Relays and solenoids usually do not require extremely fast reaction times; their design emphasis is low cost and reliability. Resonant devices are fixed frequency, ultra-efficient and do not push the envelope for magnetic performance. Mechanical shutters that are electro-magnetically driven push the limits of material science in this space between micro electrostatic device motion and common motor/rail automated machinery.

So what are some of the limits we run up against?

If we are designing for electrical efficiency and not switching speed, such as in a safety shutter, magnetic design is focused on efficiency and reliability. Efficiency is chosen to minimize resistive heat in the magnetic winding and the resulting temperature rise. This is desirable for optical instruments with low optical power and no capability to sink heat. Heat sources affect sensitive optical instruments via wave-front modification thru air gradients, thermal expansion of mechanical elements, etc. A magnetic flexure with optic attached can be mated with an air-gapped, wound magnetic core, to close the air gap when energized with current. This creates a long toroid geometry, extremely efficient, with well contained field and the reliability advantage of a flexure. In practice, the toroid geometry core has flat surfaces for heat flow to a high thermal conductivity shutter body and the copper wire windings are wet wound with thermal epoxy to route resistive heat of the wires to the core (also high thermal conductivity).

Polymer outgassing is a function of temperature so keeping things cool removes potential contamination film generation. Magnet wire is polymer coated. The magnetic efficiency and thermal transfer achievements ensure the shutter can operate open, closed, or cycling indefinitely….with no limits, and without creating thermal gradients in the beam path. In safety shutters where the average optical energy is higher, as in lasers > ~5W, the customer has already planned a thermal path in their mounting to dump the absorbed optical power. The efficient magnetics play a minor role, and the efficient thermal path of the optical absorbers play the major role. Small table top instruments, airborne and portable instruments, and any application with tight current budgets benefit from efficient, high reliability magnetics.

Now, what about High-Speed Magnetics?

We have some of the same performance interests in high-speed shutters as in safety shutters. We obviously want to use every flowing electron in the current to do mechanical work as fast as possible, so efficient, fast current escalation is required. How do we get fast work? Well we know F=ma, and our mass is optimized as low as possible for reliability considerations, so all we can do to get the acceleration up is to create more force. In air-gapped electro-magnets, the forces are generated by the (number of inductive turns)x(Amperes of current), or A-T. There are practical limits for the current. If you need 1000 A-T, you could use 1000 T and 1 Amp, or 1 T and 1000 Amps. Obviously, available power supplies for instruments are in the 10 Amp and less range. But bear in mind EV car motors do draw 1000A into fast motor electromagnets, and your average gasoline car starter motor can draw up to 200 A for a short time. So getting back to practical power supply values, the world has converged around a maximum of about 3-24 Volts and 10 Amps maximum for most instruments. Now build the fastest electromagnets around these parameters. What are we up against? Inductance delays, heat generation from copper wire resistance, clever thermal control, toleration of the magnetic system to shock and vibration, fast electrical damping current surge techniques, are critical.

Unique Benefit: High-Speed Magnetically driven shutters with no cycle limitations (due to thermal control) deliver Rapid Life Test results, for example billions of cycles in a few months. This is crucial for OEM reliability evaluation and integration.

Foremost in high-speed magnetics is thermal control. We have currents being generated rapidly and repetitively. We have to couple this wire heat to the magnetic core, then to the shutter body, then to the “infinite heat sink”. There are limits for thermal control. Heat is the (resistance) x (current squared), so as we push for more current the heat generation is squared. A diminishing return is found for any one design, setting a heat load limit.

We also have to overcome mechanical impact recoil, or “bounce” when speeds are too high for effective current damping. Inductance limits how fast you can apply a damping force. In these cases we have to use slightly longer magneto-motive force Impulses to overcome recoil, and use mechanical material science to damp. Custom catenary contours are designed on the magnetic poles. Special materials for impact durability and temperature are chosen. Heat loads are slightly greater to mitigate recoil.

Inductance increases with the square of the number of turns, so we want low inductance to make rapid speed motions, including damping. Inductance resists fast current changes. We’ll need rapid current impulses, yet need to have more turns for more force! How can we have both? We have to optimize our minimum and maximum parameters.

Minimize: Heat Generation, Inductance, Thermal Path Length to Cooling, Ferromagnetic Flexure/mirror Mass, Mechanical Impact Shock from Flexure

Maximize: Number of Magnetic Turns, Current, Winding Geometry Space, Magnetic Core and Flexure Permeability, Thermal Conductivity from Winding to Cooling, Cross Sectional Area of Thermal Path to Cooling, Mechanical Damping

All of the design properties are inter-related, or convolved, so they cannot be independently optimized. Achieving maximum switching speed performance and sustained repetition rates for a particular aperture size, mechanical package, or current constraint requires a full understanding of how to optimize these physical parameters. Mother Nature’s material science has placed limits on some of the parameters; the others are highly refined at NML, understood and implemented in designs, standard and custom, for rapid evaluation and integration into OEM instruments and to support scientific research facilities.

Author: NM Laser Engineering Team
April 26, 2021