Monolithic Multilayer Actuators

 

Smart Material is distributing stack actuators which are manufactured by Ceramtec AG, Germany.

Monolithic multilayer actuators (stack actuators) are consisting of up to 500 layers of ceramic material only 0.1 mm thick, manufactured by using a specially developed and patented stacking and sintering technology.

 

The required field strength to gain a deformation of 1,5 to 1,7 ‰ (1500-1700ppm) of the length of the actuator is approx. 2000 V/mm. By using a large number of thin layers the maximum displacement can be achieved by applying a relatively low voltage of only 200 V

 

Force

Actuators can produce very high forces (several kN) within a very short periods of time (a few μs). The associated acceleration rates can destroy the actuator. Thus, if an unrestrained charged actuator is short-circuited, it will contract faster than its inert mass is able to follow. The actuator will immediately disintegrated!

A mechanical pre-stressing device must prevent the actuator from being loaded with tensile stress. Even in static applications, a pre-stressing force of approx. 0.5 of respective blocking force is highly recommended to
stabilize the mechanical assembly.

Actuators must not operate faster than it is permitted by the resonance frequency of the actuator and the driven mechanical system. Shorter control pulses are possible but will require careful design of the control system, actuator, and driven mechanical assembly.

Capacitance

In terms of electrical properties, actuators resemble large capacitors. Their capacitance depends on the temperature and on the voltage applied. The low-signal capacitance we measured at 1 V and 100 Hz may double under real-life
operating conditions (120 °C, 200 V). In pulse mode, charging currents of up to 50 A may be encountered. Electronic control circuitry must be designed to be able to handle these characteristics.

 

The displacement (s) of an actuator follows the received charge(Q) with good linearity. Therefore the flowing current (I = dQ/dt) is an equivalent to the velocity of the actuator endplates (v = ds/dt). The steepness (slewrate) of fluctuations in the current (dI/dt) are then an equivalent to the acceleration (a = dv/dt) of the actuator endplates. Voltage controlled switching amplifiers often destroy actuators by sending bursts of charge causing extremely high acceleration rates. We recommend the use of slewrate controlled amplifiers or at least a strictly limited output current.

 

Resonant mode

The power dissipation rate can be significantly reduced by operating the actuator in resonance with the driven mechanical assembly. In this case, a small control voltage will result in a high stroke amplitude.
Thus, if the configuration has a mechanical Qfactor of 10, which corresponds to that of a “normal” mechanical lever system, the rated actuator amplitude can be achieved with as little as 20 V. Power dissipation will remain low even at high frequencies (several kHz).
If mechanical power is drawn from the system, damping will occur. This results in a reduced oscillation amplitude and a shift in resonant frequency. Therefore the power output of such systems is limited. In many applications this operating mode is being used successfully.

 

 

 

Test conditions

The product life of an actuator is highly dependent upon the individual application. Our endurance tests are performed under the following conditions:

Pre-stress:	0.5 Blocking Force
Control signal:	200 Vp-p, trapezoid, unipolar
Rise time:	140µs
Holding time:	1ms
Fall time:	140µs
Frequency:	200Hz
Surface temp:	120 °C

Under these conditions, the product life of an actuator exceeds 109 cycles. Our longest running tests have reached over 6 x 10^9 cycles, corresponding to a permanent operating time of approx. 2 years. This value exceeds the product life of most other electrical and mechanical components in the application environment.

 

Resonant mode

The product life of actuators operating in resonant
mode will be enhanced. 10^11 cycles have been reached under the following conditions:

Pre-stress:	0.5 of Blocking Force
Control signal:	40 Vp-p, sine-wave, unipolar
Frequency:	14 kHz
Surface temp:	60 °C

 

D.C. operation:

Under D.C. excitation, actuators can reach a product life of 3000 hrs. at 200 V as long as the air humidity does not exceed 5% rH.