dSPACE - Magazine 2016/01

PAGE

Controller Requirements

The requirements for the open and

closed loop control system are high:

During testing, the model’s rotor spins

at 1050 rpm. Therefore, a planned

blade control frequency from the first

to sixth rotor harmonic results in an

actuator frequency range from 0 Hz

(static positioning) to 105 Hz, in

which a high control accuracy must

be achieved (approx. 0.05 mm). The

maximum stroke of the actuators is

±4 mm, which corresponds to a pitch

angle of approx. ±3.7° at the rotor

blade. The pitch angle of a rotor blade

always depends on the current azi-

muth angle of the rotor. Therefore,

an angle encoder at the rotor mast

is used to create trigger signals that

provide information about the cur-

rent azimuth angle and are used by

all open and closed loop controls and

measurement systems of the rotor

test rig. To achieve the desired con-

trol accuracy, the actuators of the

multiple-swashplate system are con-

trolled 256 times per revolution. At

a rotor frequency of 17.5 Hz, this

results in a clock speed of almost

4.5 kHz for the control system. This

means that all computations for the

control of six actuators and their

closed loop control have to be calcu-

lated at this speed, including signal

processing and analysis as well as

filters and feedforward control.

Developing the Controller Model

First, the entire system kinematics

were modeled in MATLAB

/Simulink

to derive the real-time-capable con-

trol laws. The actuator movements

required for the desired control case

are computed via control matrices

(some with > 50 columns) that con-

vert individual pitch angle modifica-

tions (and coupling terms) into the

corresponding control signals for the

actuators. The subsequent actuator

control consists of a PID controller

with a feedforward loop. Because this

“With the powerful dSPACE real-time system, we were able to extensively

test the algorithms of our active rotor control and successfully prove the

functionality of our multiple-swashplate concept.”

Philip Küfmann, DLR

feedforward control contains a com-

plete harmonic signal analysis and a

digital 8th-order low-pass filter, the

simultaneous computation of a con-

trol signal for six actuators is also very

demanding. After an actuator model

DLR BRAUNSCHWEIG

Prototype construction of the multiple-swashplate system.

Actuator pivot points

Gimbal support

Gimbal of outer

swashplate

Scissors for inner and

outer swashplate

Pitch links

Inner

swashplate

Mechanical construction of the multiple-swashplate system.

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