Shutter Drive Electronics – Current Vs Voltage And Motion Damping

Shutter Drive Electronics – Current Vs Voltage And Motion Damping

Most electromechanical shutters are driven with low voltage power supplies, typically in the range 3.3 – 24 VDC. Some are direct DC drive at constant voltage and some use more power efficient two level drives, with a higher voltage impulse to open, then a lower voltage to hold long term. More complex waveforms can be generated to provide de-acceleration at the open and closed stop positions, known as damping. Depending on switching speeds required, the damping can be partial (faster switching) or complete critical damping (slower, low noise/shock).

In a magnetically driven flexure design, stiction and friction are virtually eliminated. Wear is absent up to hundreds of millions of cycles. The ferromagnetic flexure optical blade is constrained between a Normally Closed rest surface and the energized poles of a fixed position electromagnet (open position). Stresses in the flexure are repeatable and there is no change over time. The mechanical forces don’t change at all over the product lifetime. So what can change? What influences repeatability and the ability to critically damp?

Current! Only the current can change, and the Force produced by the electromagnet is the product of (# winding turns) x (Amperes). Certainly the turns don’t change, so how could the current change? The answer is temperature change. Copper wire resistance changes with temperature. So for a fixed voltage to open, or hold long term, the current will go down with temperature. The “simple to implement” voltage source now allows for current changes and subsequent force changes as the electromagnet gets hotter or colder. Now we get variations in force and resultant switching speed changes. Repeatability of exposures changes, jitter becomes pronounced at large temp excursions. If we are trying to do critical damping, we need to know exactly when the moving element is at location. We can’t do this accurately if speeds are varying.

Enter current control. Instead of sending a voltage to the shutter and letting the shutter draw as much current as it wants based on temperature, we deliver a fixed amount of current with more sophisticated electronics that sense the current tens of thousands of time per second and maintain a preset current feed waveform, when opening, holding, and damping. Forces stay constant over any temperature. Performance is maximized, for any gravity position, simply by loading a library preset to the controller board. Our Model CX4000 has these features.

So we have lots of advantages with the current control design, but what are the disadvantages? Potentially HEAT, if not managed…so manage it ! With current control it is imperative to design an excellent thermal path for the electromagnet heat to transfer to the cooling base in order to prevent a temperature escalation should the shutter see ambient temp swings. We are already converting a high wattage laser beam to heat in our absorber and transferring it to the cooling base, so we send the electromagnet heat there too! Most shutter manufacturers have extremely poor thermal paths for their electromagnets, restricting continuous cycling of the shutter, else burning the windings. If you allow too much heat in the electromagnet you increase chances for wire shorts, polymer outgassing, thermal gradients in the optical air cavity, etc. Our highly advanced magnetic design includes excellent thermal conductivity to a convenient mounting surface. All shutters manufactured by NML use advanced magnetics for use with current controlled drive electronics.

Author: NM Laser Engineering Team
March 24, 2021