Low Currents - Higher Voltages

The Ren-Sci motors were designed to have much lower capacitance than conventional solutions for cryogenic motors. Competitor motors run lower voltages (30-70V) and higher currents (10s of Amps). We chose to do the opposite; our voltages are 50-600 V and source only 100s of mA. 

At room temperature, our motors run at 70V fairly slowly, they're pretty zippy at 125 V though.  At 4K they run at similar speeds to the leading vendor if we drive them at  150V-200V. Competitors can run lower voltages (30V-70V) pretty fast, but they put rather large piezo stacks in, so their capacitances are ~5-10 uF whereas ours are 5-10 nF (3 orders of magnitude lower). This is by design though for several reasons. For one, the drive currents we use are much smaller, so the I^2 *R heating is lower. It is typical for cryogenic wiring from feedthrough to experiment to have several Ohms resistance since it's usually wire that is intentionally thermally resistive (e.g. phosphor bronze, manganin). So the Joule heating from drive currents is non-negligible, especially on cryostats with low cooling power on the second stage. 

Zooming out, on a fundamental level this is a cryostat, so we're trying to get the job done with minimal energy input and the 3 orders of magnitude lower capacitance is a huge effect. Compare a typical leading vendor scenario, 60 V, 1 kHz, 5 uF, so the  power pumped into that capacitor is CV^2/2 times frequency which is ~  9W. The analogous situation for our motors is 200 V, 1 kHz, 5 nF, which is 0.1  W, so using 90x less energy to get the same motion (and thus 90x less heat dissipation). 

In both cases the majority of the energy into the circuit is distributed between kinetic energy of the carrier (mv^2/2) and the  friction losses from the slip-stick motion. Since the two scenarios above result in similar carrier velocities, it's fairly easy to  prove that we're dumping a lot less parasitic heat into the cryostat. This has large ramifications, especially when going below 4K where heat lift decreases rapidly.  

Think of the piezo actuator as the engine of the nanopositioner. Piezo actuators die. A lot. And it's usually not the fault of the user nor the vendor who sold the motor - it's physics. We have taken the approach that the piezo actuator should be thought of as a pseudo-disposable. As such, we have intentionally designed our motors to have user exchangeable actuators and we supply 2 extra actuators per axis. So, each axis should easily last 7-10 years, even under heavy use. But even if you go through those sooner, just call us and we'll overnight you another actuator. Your days of waiting 6 weeks for a repair are over!   

Several suppliers for ambient piezo based motors will simply try to plop it in a cryostat and hope for the best. Our motors were carefully engineered with extreme attention to thermal contraction, capacitance losses in cryo, minimal friction at the bearing interfaces, and low current operation as described above. In addition, we engineered a very strong thermal pathway directly beneath the bearing surfaces and the actuator so that heat lift from the motion is swift. 

We've had experiences previously as customers of some of these vendors where a motor would seize up at 4K, we called the vendor and they asked us to increase the voltage and/or frequency to try to self-heat its way out of the problem. Our motors never seize at 4K. 

All Ren-Sci motors are UHV compatible, bakeable to 120 C, and made with non-magnetic materials. In addition, we've integrated all of these UHV and non-magnetic components into our production, so there's no 'up-charge' for having these features. No lubricants are used, solid-state or otherwise. Instead, we use diamond polished surfaces and extremely high precision on our machined parts (sub 2um parallelism on every part), so that there is no need for the use of lubricants.