Chapter 3 ‒ Functional Principle 5
3.1 Piezo LEGS Linear and Rotary Motors
The PMD206 / PMD236 driver unit is made to control PiezoMotor’s linear and rotary Piezo LEGS motors. The
Piezo LEGS technology is characterized by its outstanding precision and high resolution. The motors are backlash free
because of the friction based direct drive. There is no need for gearboxes or linear screws, and the motor is very strong
compared to its size.
The performance of a Piezo LEGS motor is different from that of a DC or stepper motor in several aspects. A
Piezo LEGS motor is friction based, meaning motion is transferred through contact friction between the drive leg and
the drive rod (Piezo LEGS Linear), or between the drive leg and drive disc (Piezo LEGS Rotary). You cannot rely on
each step being equal to the next. This is especially true if the motor is operated under varying loads/torques.
3.2 Motor Drive Principle
You need to feed 4 electrical drive signals to the motor in order to activate the legs and start moving. The actuation
of the legs will lift and push the drive rod forward, or in case of a rotary motor, lift and spin the drive disc and axle
around. For each waveform signal period the legs will complete one full step (see illustration below). To alter the
speed of the motor the frequency of the electrical signals is changed. The motor speed will depend on the magnitude
of external force/torque. The maximum speed is limited to a few thousand steps per second, which translates to
movements in the range of millimeters per second (linear motion).
1 When all legs are electrically
activated they are elongated
and bending. As we shall see
below, alternate legs move as
pairs. Arrows show the direction
of motion of the tip of each leg.
2 The rst pair of legs
maintains contact with the rod
and moves towards the right.
The second pair retracts and
their tips begin to move left.
3The second pair of legs has
now extended and repositioned
in contact with the rod. Their
tips begin moving right. The
rst pair retracts and their tips
begin to move left.
4 The second pair of legs has
moved right. The rst pair
begins to elongate and move up
towards the rod.
Denition
One waveform-step (wfm-step) is the step taken by the drive legs for each waveform signal period.
Note!
The wfm-step length/angle is not constant. Step length/angle will depend on external force/torque, and
therefore the drive rod/disc will not always travel the same distance for one wfm-step.
Example
Piezo LEGS Linear: When taking a wfm-step the drive rod will move several micrometers.
Piezo LEGS Rotary: When taking a wfm-step the drive disc will rotate. The step angle depends on the
diameter of the drive disc.
If there is an external force/torque present and the force/torque is opposing the direction of motion, the wfm-step
length/angle will be decreased (shorter step). If the force/torque is working in the same direction as the direction of
motion, the wfm-step length/angle will be increased (longer step). The speed of a Piezo LEGS motor is a product of
the actual wfm-step length/angle (long or short) and the frequency of the drive signals. For varying loads/torques,
using constant waveform frequency will not give a constant speed, simply because the wfm-step length is not constant.
3 ‒ Functional Principle
