Lecture 4 - Sensors and Actuators Flashcards
How are electrical biosignals transferred to data processing?
Electrodes, SQUIDs
How are non-electrical biosignals transferred to data processing?
Sensors, transducers
Define what a sensor is.
A sensor is an element of a mechatronic or measurement system which produces a measurable change in response to a physical condition.
Define what a transducer is.
A transducer is an active element of a sensor that transforms a signal from a physical domain to another physical domain.
How can sensor systems be classified? Give examples.
- By Modality
- position & motion recording
- force & moment recording
- physiological data recording - By Attachment
- body mounted
- desktop/ground/wall mounted
- contact-free, non-intrusive
Give examples of sensors for physiolocal data recordings.
- muscle activities = EMG
- nerve activities = ENG, EEG
- cardiovascular function = ECG, blood pressure cuff
- metabolic function = blood gas concentration
- respiration = oxygen concentration
Give potential uses for EMG data.
- Gait analysis
- Input signal to drive arm prostheses
- Input signal to control another device (e.g. facial interface)
Give potential uses for EEG data.
- Diagnostics
- Brain Computer Interface
State the different segments of an ECG wave.
P wave - atrial depolarization
QRS complex - ventricular depolarization
T wave - ventricular repolarization
Give 5 approaches to position and motion recording.
- Resistive sensors, i.e. Potentiometers (change of resistance)
- Capacitive sensors (change of capacity)
- Inductive sensors (position change inductance or motion induces voltage)
- Optical methods (cameras, photoelectrical methods)
- Ultrasound methods (exploiting running time differences or Doppler effect)
Describe what a Goniometer is.
A goniometer is a device used to measure joint angle or ROM. The degree between endpoints represents the entire range of motion. It can be triangular or flexible.
What is an IMU?
Intertial Measurement Unit. It is comprised of:
- accelerometers
- gyroscopes
- (magnometer, etc.)
What is the difference between a gyroscope and an accelerometer?
Gyroscope: recording of angular speed (rate)
Accelerometer: recording of linear accelerations
Give advantages and disadvantages of accelerometers and gyroscopes.
+ cheap
+ can be used to determine relative positions
+ can be used to determine orientation (exploiting earth gravity field)
- singularities are possible
- very prone to noise & drift
Compare the accuracy of optical vs. acoustic vs. magnetic motion tracking systems. (All are contact free!)
Optical: <1mm
Acoustic: ~2mm
Magnetic: ~10mm
Compare the DOF of optical vs. acoustic vs. magnetic motion tracking systems. (All are contact free!)
Optical: 2-3
Acoustic: 3
Magnetic: 6
Give one advantage and one disadvantage of optical vs. acoustic vs. magnetic motion tracking systems. (All are contact free!)
Optical: \+ cable-free - challenging tracking Acoustic: \+ cheap - sound reflections = noise Magnetic: \+ no occlusion at obstacles - field deformations
Give possible approaches to force and moment recording.
- Resistive methods based on strain gauges
- Piezoelectric methods
- Optical markers/system
- Capacitive method
Define what an actuator is.
An actuator is a mechanical device which coverts energy into some kind of force and/or motion.
State the mechanical chain of an actuator system.
Actuator - Gear - Clutch - Load
What is the role of an actuator in an actuator system?
Transform (electric) energy mu into mechanical energy.
What is the role of a gear in an actuator system?
Change speed, force/torque & direction.
What is the role of a clutch in an actuator system?
Transmit power and motion.
What is the role of the load in an actuator system?
It is the final action of the mechanical system.
What are 4 different actuation principles?
- Electromagnetic actuators
- Fluidic actuators (Pneumatic + Hydraulic)
- Thermomechanical actuators
- EAPs - Electroactive polymers
Give advantages and disadvantages of EM actuators.
\+ cheap \+ clean \+ quiet \+ easy design, control and installation - friction due to eddy currents and bearings - low-medium power to weight ratio
What is the basic principle of fluidic actuation?
Pressurised air or liquid is controlled by servo-valves to move a linear or rotary unit.
How do pneumatic actuator properties typically compare to hydraulic actuator properties?
Pneumatics: soft, compliant
Hydralics: stiff, high inertia
What are the pros of using pneumatic actuation?
\+ simple \+ high power to weight ratio \+ light-weight \+ quiet \+ easy maintenance \+ fast \+ cheap \+ good force accuracy
What are the cons of using pneumatic actuation?
- air leakage
- less accurate due to compliance of air
- friction
- non-linear
- not quite portable
What are the pros of using hydraulic actuation?
+ high power to weight ratio
+ high output stiffness
+ larger payload
+ good position accuracy
What are the cons of using hydraulic actuation?
- leakage results in hygienic problems
- high inertia (slower)
- complex structure, bulky
- noisy
- expensive
- can be dangerous
Describe how thermomechanical actuators work.
They are based on the principle of Shape Memory Alloys (SMA) which are a unique class of metal alloys that can be stable in 2 different phases (2 diff. temperatures):
- high-temp phase “austenite”
- low-temp phase “mantensite”
State change is due to temperature change
Give and advantage and 3 disadvantages of thermomechanical actuators.
+ high power to weight ratio
- low speed
- limited bandwidth
- cooling difficulties
Explain the concept of Electroactive Polymers (EAPs) for actuation.
EAPS show a significant change of shape or size of the polymer in response to electrical currents or voltages.
Give advantages of EAPs for actuation compared to SMAs.
+ higher response speed
+ lower density
+ better resilience
Name 8 gear mechanism types.
- Spur
- Worm
- Bevel, spiral bevel, hypoid
- Rack & pinion
- Epicyclic (sun & planetary gear)
- Harmonic drive
- Pulley drive
- Belt drive
Give the formula for torque.
Torque = Force x length of lever arm
Give the formula for motor torque.
Torque_motor = Force_tensile x radius_shaft
Give the formula for motor power.
Power_motor = Torque_motor x ω_motor
Describe how actuators can be classified by kinematic structure.
Parallel Kinematics: - Stewart-Platt forms - Tripod systems - Delta systems - Pantographs Serial Kinematics: - Linear axes - Rotary axes - Combinations
What are 2 types of mechanical interaction approaches?
- End-effector based
2. Exoskeleton
Give pros and cons of end-effector based interaction approaches.
+ easy to adjust to patient
+ simple structure and control
- limb posture not fully determined
- risk of injury due to singularity
Give pros and cons of exoskeleton interaction approaches.
+ joint axes fully determined
+ physiological movements
+ force and position data for each joint available
- need to align robotic and anatomical axes, which may be challenging
- longer set-up times
- challenging anatomical constraints, e.g. shoulder
What is the main challenge of end-effector based interaction approaches?
Control of posture and movements.
What is the main challenge of exoskeleton interaction approaches?
Alignment of robotic and anatomical joint axes
List position-based controllers of actuators.
- feedforward control
- gravity and friction compensation
- computed torque
List force controllers of actuators.
- impedance control
- admittance control
List possible safety features of robotic actuators.
- passive mechanical stops
- barriers at the edge of the device workspace
- magnetic or pneumatic overload protection at end-effector
- redundant sensors
- hardware control of position, velocity, force
- software control of position, velocity, force
- generated reduced forces to limit acceleration and fatigue
- emergency stops and deadman button
