Hysteresis, Creep and Non-linearity
Hysteresis, creep, and non-linearity are critical factors that significantly impact the performance and precision of piezoelectric actuators and other smart materials. Hysteresis refers to the lag between the input and output, which can cause inaccuracies in positioning and control. Creep denotes the time-dependent deformation under a constant load, which can lead to drift and instability over time. Non-linearity indicates the non-proportional relationship between input and output, complicating the prediction and control of the actuator's behavior. Understanding and mitigating these phenomena are essential to ensure accurate, stable, and repeatable performance in applications such as precision engineering, micro-positioning, and adaptive optics.
We strive to eliminate errors like hysteresis, creep, and non-linearity in a system because they compromise its accuracy, reliability, and performance. Hysteresis can lead to positioning inaccuracies, creep causes drift over time, and non-linearity makes it difficult to predict and control system behavior. These errors can result in inefficient operation, increased wear and tear, and even system failure. In precision applications, such as scientific instruments, medical devices, and aerospace engineering, minimizing these errors is crucial to achieving optimal functionality and ensuring safety.
Left: Hysteresis in a piezoactuator, Right: Creep in a piezoactuator
Piezoelectric actuators are nonlinear by nature. That means their displacement response is not linear with the voltage applied. This happens the relationship between the electric field applied to the piezoelectric material and its resulting mechanical deformation is not perfectly proportional. This non-linear response complicates precise control of the actuator's movement. They also have hysteresis, their response depends on the previous state, because the polarization of the piezoelectric material doesn't immediately follow the applied electric field. This lagging effect results in a difference between the input signal and the actuator's response.
Creep is another phenomenon observed in piezoelectric actuators. This phenomenon is due to the slow reorientation of the material's internal dipoles and the inherent viscoelastic nature of the material, which causes it to experience slow, time-dependent deformation under sustained stress. As a result, the actuator's position drifts over time, affecting its precision and stability in long-term applications. Understanding and mitigating creep is essential for ensuring reliable performance in precision systems.
One method to increase linearity is using charge control for driving piezoelectric actuators. Single crystal piezoeelctric actuators are also exhibit close to linear behaviour. But both of these solutions are expensive so rarely used. Closed-loop control is more commonly used to overcome those errors. Ulsis produce strain gauge bonded piezoactuators to get accurate response from the systems.
A piezoactuator with strain gauge bonded to increase precision
Although measuring the piezo strain with strain gauge can increase the accuracy, the actuator itself is not the only source of non-linearity. Sometimes the strain gauge itself has significant error, especially when it is not properly bonded. Maintaining the correct bonding between the actuator and the strain gauge is essential to get an accurate measurement. Selecting the correct type of gauge material is also crucial to neutralize the effect of temperature changes on measurements.
There are many other source of errors in precision motion systems: geometric non-linearities when large deformation is present, viscoelastic properties of the construction materials, nonlinear behaviour of Hertzian contact surfaces, effects of temperature changes etc. When not properly designed, electronic control systems may also have non-linear errors. Some of these errors can be compensated in software if they are predictable, but it may require additional computation which is unwanted in high frequency systems.
All in all, dealing with non-linearity, creep and hysteresis is challenging in precision motion systems and it requires the extensive knowledge about the subsystems. You can always contact us to discuss about your problem with an expert team.