![]() This is inevitable, since the fingers must be preloaded against the ceramic, and yet must move relative to it. The dominant issue is that there is significant friction to overcome. Despite this, the system has a number of disadvantages when compared to direct drive methods using permanent magnets and coils. When teamed with an appropriate position feedback device, servo loop controller, and linear or rotary bearing guideway, the result is a compact and reasonably effective long-range piezo actuator. The ceramic spar sets the length of travel, and the system has the advantage that when no motion is commanded, the fingers act as a brake and prevent stage motion. The piezo elements are parts of a high-Q, high-frequency, resonant LC tank circuit. The combination of extension and bending create an elliptical motion of the tip, which can be used to impart motion to the stage. In this design, one or more ceramic fingers are preloaded against a ceramic spar, and piezo elements are used to drive the fingers in an oscillatory motion against the spar. This type of piezo device offers extended travel (from millimeters to meters) with moderate resolution, and is the most practical implementation of the piezo effect for travels beyond the range of stack actuators. In general, piezo stages with internal preload and closed loop position feedback can be useful positioners, but will provide limited travel, may be longer and more expensive than desired, and will be limited to moderate servo bandwidths unless the payload mass is quite small. The high Q of this resonance can complicate efforts to notch it out, and the end result is often that a lower than desired servo bandwidth must be accepted. The payload mass and the stiffness of the piezo stack produce a high Q resonance, the natural frequency of which can be surprisingly low for realistic tooling and part masses. When operated in closed loop mode, an additional concern arises. In addition to hysteresis, there is a drift problem, in which motion continues to slowly creep after a position step. While the use of a linear position feedback transducer and a closed loop servo control can overcome hysteresis and drift, compared to the inherent simplicity of an open loop voltage commanded piezo stack, this comes at a considerable added cost and complexity. This can be as much as 15% of full travel. Another limitation is a significant amount of hysteresis, or variation of travel versus applied voltage between forward and reverse movement. A stack capable of 200 microns of motion (the most common upper limit among commercially available devices) will typically be about 200 mm long. The number of plates grows with increased travel, increasing both stack length and cost as travel increases. Typical piezo stages range in travel from 10 to 200 microns. The most obvious limitation is that of limited travel. Housed piezo stages with internal preload springs are useful positioning actuators, but suffer from several limitations. These limitations can be significantly reduced if the stack is mounted within a housing and if powerful springs are used to produce a high internal preload. Piezo stages by themselves are fragile and unable to produce force in both directions. Depending on the nature of the piezo material (soft versus hard), the applied voltage range is typically either +/- 1000 volts for hard or high voltage stacks, or +/- 120 volts for soft or low voltage stacks. The voltage applied to the piezo stack is distributed to each of the plates within the stack in a parallel configuration. ![]() The ratio L/L (change in length to length) for an applied voltage is quite small for a single piezoceramic element, and so most actuators are composed of a large number of thin plates, referred to as a piezo stack. Today, the most common materials used for piezo stages are either barium titanate or PZT (lead zirconate titanate) ceramics. The inverse piezoelectric effect was first discovered by Pierre and Jacques Curie in 1881.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |