Quartz 4

RDL VL 12 Actuators

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lecture12-actuators-0.pdf

Terminology

  • Bus System – Communication System between a controller and several attached devices e.g. Can

  • Load – a load connected to the actual actuator [Watt]

  • Piston – an pneumatic or hydraulic actuator for linear movements • Position control – moving an actuator by position informations

  • Rotor – moving part of an electric motor (anchor)

  • Rpm – revolutions per minute

  • Speed control – moving an actuator by actual speed values

  • Stator – Static part of an electric motor (coil)

  • Shaft – connection of a rotating or a linear actuator to their environment

  • Torque – M = r * F / torque = position vector x force vector [kg m^2 s^-2]

Motor

Example

Direct current motors (DC-motors)

  • Up to 98% efficiency
  • Power cord or battery

Brush motor:

  • cheap
  • Mature technology
  • Sparking / high-frequency interferences
  • Life limit – carbon brushes

Brushless motor:

  • More efficient / less heat
  • Less wear
  • More power per weight
  • Requires a controller

Stepper Motors

  • Revolutions will be made in an amount of steps (degree), not as a constant movement, exact positions can be achieved

  • Bipolar: 2 coils 4 connections (more power per motor-volume)

  • Unipolar: at least 5 connections, simpler control

  • Reluctance motor

    • toothed soft iron structured rotor
    • No permanent magnets
    • Free magnetic flow, no magnetic field after switched off

  • Permanentmagnet motor

    • Permanentmagnet on the shaft / Stator made of soft iron
    • Moment of rest
    • Lower resolution in comparison to ther reluktance motor

  • Hybridmotor

    • Permanentmagnet and toothed soft iron core on the shaft

Servos

  • Analog servos
    • Low power consumption
    • Low price
  • Digital servos
    • Faster positioning time
    • Higher resolution
    • Partly programmable
  • PWM-control

    • Common for hobby servos

    • Servo elektronics regulate the actor (potentiometer) against the motor position

    • The pulse width of the control signal regulates the target position

    • Various variations on pulse widths and travel ranges

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Hydraulics

Energy density:

210 bar (Industry standard)
500 bar

Advantages

  • High power density
  • Good controllability
  • Good timing behaviour due to low inertia
  • Simple and reliable protection against overload
  • Good energy transmition over medium distances
  • Good lubrication and dissipation of the heat loss through the pressure transmission medium

Disadvantages

  • High energy consumption
  • High weight of drive and control elements
  • Losses from friction and internal leakage
  • Sensitive to dirt
  • Fire hazard

Motor

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Pneumatics

Advantages

  • The forces and speeds of the cylinders are infinitely variable.
  • High achievable working speeds (standard cylinder 1500 mm / s; high-performance cylinder 3000 mm / s, engines up to 100.000 min-1)
  • Compressed air devices can be overloaded to a standstill without damage.
  • Compressed air can be stored in pressure tanks.
  • With pneumatics, waste heat is only generated centrally at the compressor, not at decentralized electric drive units.
  • Air is free of charge and always available (however, energy consumption at the compressor is required to compress the air; the efficiency is comparatively low).
  • Clean, environmentally friendly medium.
  • The exhaust air can escape directly into the environment, return lines can be omitted.
  • Explosion safety of the medium is guaranteed.
  • Compressed air is insensitive to magnetic impulses.
  • Sealing and throttling technology possible (sine cylinder).

Disadvantages

  • Without fixed stops, precise positions are not possible due to the compressibility of the air.
  • Compressed air escapes causing noise. Countermeasures are silencers.
  • Compressed air treatment is required to remove dirt and moisture.
  • Gases are compressible. Bursting pneumatic accumulators release large gas volumes. This can have a devastating effect, especially in closed rooms. For this reason, pneumatic containers are subject to regular inspection (costs) from a certain size.
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