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CEO: Dr. Bernt Götz • Phone +493641/66880 • Fax +493641/668866 • www.piezosystem.com
The PID-controller can work in two different modes: The open loop mode (OL) and the closed loop
mode (CL). In open loop mode, the piezo actuator is controlled by a voltage between -20 and +180
volt,
however, limited to the admissible voltage range of the connected piezo actuator. The fi
lters “slew rate”
and
“low pass” can be used in this case. The resulting stroke depends on the characteristic of the piezo
actuator and is affected by piezo-typical creeping and hysteresis behavior.
In closed loop mode (CL), these effects will be compensated by the digital loop controller. The functional
groups for CL mode will be explained now:
The setpoint can be given by USB, Ethernet, arbitrary waveform generator or analog input. The slew
rate can be limit by the slew rate limiter and/ or by using the 4
th
order low pass filter. These filters can
be used to avoid excitation of system resonances or to limit the setpoint dynamics in order to provide a
trackable motion profile to the PID-controller and the power amplifier. The position sensor value can
also be filtered with an adjustable 1
st
order low pass filter in order to reduce the measurement noise
spectrum.
The PID-controller calculates the control error
𝑒
between the (filtered and rate limited) reference input
𝑟
and position measurement
𝑦̂
. The PID algorithm with feed forward control is implemented in the form
𝑢 =
𝑘
PCF
𝑟
⏟
Feed forward term
+
𝑘
P
𝑒
⏟
Proportinal term
+ 𝑘
I
𝑇
S
2 𝑒 + 𝑢
i,old
⏟
Integral term
+
𝑘
D
𝑇
S
(𝑒 − 𝑒
old
)
⏟
Differential term
giving
𝑢
as the controller output prior to notch filtering. Using the subsequent notch filter the excitation
of the first resonance frequency of the actuator can be reduced by an order of magnitude.
In open loop mode, the PID-controller is bridged and the command input directly controls the amplifier.
In open loop mode you can also use slew rate, setpoint low pass and notch filter to reduce the
stimulation of unwanted vibrations.
The controller-
specific parameters are stored in the actuator’s ID-chip. The default parameters were
determined in the
piezosystem jena
laboratory and ensure safe function of the actuator and will work
for most applications. Depending on your application, additional load, setpoint trajectory and motion
requirements, fine tuning of the control parameters will most likely improve your results.
When any actuator made by
piezosystem jena
is connected to the
NV200/D NET
its specific values
are read from the actuator’s ID-chip. The controller is then automatically configured with these values.
There are no PID parameters for actuators without a measurement system because the closed loop
mode is disabled in this configuration.
To adapt the controller properties to your special application please start with the default values. In
general, change the parameters in small steps and, depending on the actuator's reaction, slightly
increase the step size. If the system begins to oscillate, switch to open loop mode (
cl,0
) immediately,
then reset to stable values again.
For filtering the set point, a slew rate limiter is implemented. The slew rate is defined in % / ms. For
example, a value of 1 %/ms would realize, that a jump off the full range (100
%) is converted into a ramp
with 100
ms duration. Independent from the slew rate value set in the controller, the limitation given by
the high voltage amplifier cannot be exceeded, since the piezo voltage slew rate is limited by the
maximum output current and the actuators capacity.
To filter higher frequency parts on the input signal (in the region to the resonant frequency or higher), a
low pass filter of 4
th
order is implemented.
