SENSOR TECHNOLOGY
What type of sensor does PIEZOCONCEPT use in its piezostage ? And why does it outperform other sensor technologies ?
PIEZOCONCEPT uses a monocrystal Silicon sensor, called Silicon HR sensor. Even though it is part of the large family of the strain gauges sensor, it outperforms the 2 other technologies that are commonly used (capacitive sensor and metallic strain gauge). These 2 other competitive technologies have their own specific drawbacks.
Comparison between capacitive sensors and PIEZOCONCEPT Silicon HR sensors
Capacitive sensors are quite commonly used. They offer decent performances but they are sensitive to :
Air pressure changes : indeed, permittivity of the air depends on air pressure. The capacity measurement will be affected by any pressure changes.
Temperature changes : same thing, the permittivity of air will change with temperature
Presence of pollutants
All the above will cause some instabilities at the nanometer level so you need to take them into account if you want to achieve true subnanometric stability. Even though a correction can be done for air pressure and temperature, it is not possible to correct the effect of other factors (pollutants, outgasing). This explains the poor performances of capacitive sensor in vacuum environment. Additionally, capacitive sensor are quite costly and bulky. Hence it is not possible to have very thin stage like the BIO3/LT3 with capacitive sensor, at least without sacrifying even further the performances in terms of stability.
Because it is a solid-state technology, the resistance of the Silicon HR sensor is not dependent of air pressure or presence of pollutant. Secondly, temperature changes have an impact on the measurement (because of the thermal expansion of the material mainly) but this can be corrected by using an array of sensors. Basically, we use 2 silicon sensors in parallel for each axis - one is used for the measurement whereas the other one is used for taking account of the material expansion due to the temperature changes.
Comparison between metallic strain gauge and PIEZOCONCEPT Silicon HR technology.
The differences between metallic strain gauges and our Silicon HR technology (which are also a strain gauge) are even higher. There are 2 huge differences between metallic and silicon sensor strain gauge.
Competitors try to say that all the strain gauges have the same performances because they measure a strain. This is not correct. Semiconductor strain gauge are very different from metallic strain gauges in terms of stability.
The first difference between the metallic strain gauge and the silicon HR sensor (that PIEZOCONCEPT uses) is the gauge factor : the gauge factor is approximatively 100 times higher for the semiconductor strain gauge (Silicon HR) than for the metallic one. An higher gauge factor leads to an higher Signal-To-Noise ratio, which in the end leads to an higher stability.
More importantly, the second difference is that the metallic strain gauge can't be mounted directly on the flexure itself (that is where the movement is realized) : the metallic strain gauge has to be mounted on some sort of "backing". As a conseuence, it has to be mounted on the actuators itself because you won't have enough space to accomodate it on the flexure. The problem of measuring on the actuator only is that the piezoactuator is plenty of defects..... Phenomena like Creep or Hysteresis are present. As a result, since the piezoactuator doesn't expend uniformly, measuring only a part of the elongation of the actuator doesn't allow you to deduct precisely its complete elongation. By doing the measurement on the flexure itself, we don't have this "non-uniformity" problem.
Because of the above, there is a huge difference in terms of Signal-To-Noise ratio and stability if you compare the strain gauge (metallic one) and the PIEZOCONCEPT 's single silicon crystal.