Module II – Exteroceptive / External Sensors

External vs. Internal Sensors

• Robots are equipped with two broad categories of sensors.
  • Internal (interoceptive): measure variables internal to the manipulator (position, velocity, acceleration, joint torque, etc.).
  • External (exteroceptive): measure the robot’s environment; the focus of this module.
• WHY external sensors matter
  • Enable interaction with work-cell equipment, work-pieces, operators, safety devices.
  • Provide feedback for control strategies (e.g.a0stop motion at hard limits, adjust grip force, detect presence of parts).

Taxonomy of External Sensors

• Contact-type
  • Force / limit switches
  • Tactile (touch) sensors: pressure, vibration, temperature, torque/force
• Non-contact-type
  • Proximity sensors
  • Inductive (metallic targets only)
  • Capacitive (metallic + non-metallic)
  • Other distance/displacement devices
  • Semiconductor displacement sensors (laser + PSD)
  • Microwave, ultrasonic, laser, vision, etc.

Contact-Type Sensors

1.a0Limit Switch (Electro-Mechanical Force Sensor)

• Construction & operation
  • Pressure-sensitive mechanical arm actuates an internal switch.
  • May incorporate a magnet on the moving object to close the contact without direct mechanical pressure.
• Electrical behaviour
  • Pull-up resistor keeps signal at $+V$. When arm is pressed, circuit goes to ground.
  • Configurations
  • Normally Open (NO): continuity only when pressed.
  • Normally Closed (NC): continuity until pressed.
  • Single-pole vs.a0multi-pole versions for one or many simultaneous circuits.
• Performance / limitations
  • Subject to wear, mechanical failure; low mean-time-between-failures compared to solid-state sensors.
  • Switching speed much lower; photo-electric micro-sensors can be up to $3000$× faster.
• Robotic use case
  • Detect extreme joint positions; once triggered, controller disables the corresponding actuator to avoid structural damage.

2.a0Tactile (Touch) Sensors

• Definition: capture & document physical contact; respond to contact, pressure, or force.
• Sub-classes by transduction principle
  • Vibration (accelerometers)
  • Pressure / force / torque (multi-axis arrays)
  • Temperature (thermistors, IC sensors)
• Three dominant working principles
  1. Capacitance
  • Two conductive plates separated by a dielectric. Pressure changes spacing $d$ or effective area $A$ → capacitance varies.
  • Formula $C = A \cdot \varepsilon<em>0 \cdot \dfrac{\varepsilon</em>r}{d}$
  1. Piezo-resistivity
  • Resistance of certain materials (e.g.a0piezoresistive silicon, conductive polymers) changes under stress.
  1. Piezoelectricity
  • Deformation of crystal lattice produces a voltage proportional to applied force.
• Design example: Capacitive taxel array printed on flexible PCB; can localise pressure distribution for hand/gripper feedback.

Non-Contact Proximity Sensors

General concept

• Detect presence/absence of an object without physical contact.
• Replace limit switches where wear, contamination, or high speed is an issue.

1.a0Inductive Proximity Sensor

• Detects conductive / metallic objects via electromagnetic induction.
• Core elements
  1. Sensor coil + ferrite core
  2. RF oscillator
  3. Detector (amplitude demodulator + threshold)
  4. Solid-state output switch (transistor, relay driver, etc.)
• Operating sequence
  1. Oscillator creates RF magnetic field at sensor face.
  2. Metal target enters field → eddy currents generated.
  3. Eddy currents create opposing field → damping (amplitude ↓).
  4. Detector senses amplitude drop at predefined point → output ON ("damped state").
  5. When target leaves, amplitude rises → detector resets → output OFF.
• Sensing range
  • Typical: $10\text{--}15\;\text{mm}$.
  • Extended models: up to $100\;\text{mm}$ (large coil, lower frequency).
• Influencing factors
  • Target material (steel > aluminium > copper)
  • Target size & orientation
  • Temperature, surrounding metal (mounting flush vs.a0non-flush)

2.a0Capacitive Proximity Sensor

• Detects objects that alter electric field capacitance.
• Physical analogue = plate capacitor: Sensor face (plate 1), target (plate 2), air/non-metal wall (dielectric).
• Internal blocks
  1. High-frequency RC/LC oscillator
  2. Trigger circuit (Schmitt comparator)
  3. Output stage + indicator LED
• Working principle
  • Object enters fringe field → capacitance increases.
  • When $C$ exceeds operating point, oscillator starts or changes amplitude → trigger flips output.
  • As object retreats, capacitance returns, oscillator stops/returns → output resets.
• Range
  • Standard: few mm up to $1\;\text{inch} (\approx 25\;\text{mm})$.
  • Extended: up to $2\;\text{inch}$.
• Target types
  • Conductive AND non-conductive (wood, plastic, glass, liquids, powders), as long as dielectric constant differs from air.
• Mechanical parts
  • Body, sensing face, indicator LED, cable/connector.
• Advantages
  • True contactless; versatile material detection; can sense through non-metal walls; adjustable sensitivity; no moving parts → long life; industrial-grade.
• Disadvantages
  • Shorter range than some inductive designs; typically more expensive.

Inductive vs.a0Capacitive Quick Comparison

• Field type: Magnetic vs.a0Electric
• Target conductivity required: Yes vs.a0No
• Typical range: $\le 15\;\text{mm}$ vs.a0$\le 25\;\text{mm}$ (extendable)
• Cost: Lower vs.a0Higher (for equal performance)

Semiconductor (Laser) Displacement Sensor

• Architecture
  1. Light source: semiconductor LED or laser diode
  2. Focusing lens projects spot onto target surface
  3. Reflected light re-imaged onto Position Sensitive Detector (PSD)
• Principle
  • As target moves (displacement $\Delta x$), reflected spot shifts on PSD.
  • PSD outputs two photocurrents; centre‐of-gravity calculation yields linear displacement reading.
• Applications in robotics
  • Non-contact gauge for part location, surface profiling, vibration measurement, closed-loop micro-positioning.
• Advantages
  • Micron-level precision, high sampling rate, no mechanical load.
• Considerations
  • Surface reflectivity, ambient light tolerance, speckle with coherent laser.

Practical / Ethical / Safety Implications

• Contact sensors (limit switches) provide hard safety stops; mandatory in standards (ISO 10218) to avoid over-travelling arms.
• Non-contact sensors enable higher throughput (faster response) and reduce maintenance downtime caused by wear.
• Capacitive sensors that detect operators through panel walls can enhance collaborative robot safety but must be tuned to avoid false positives (e.g.a0dust, humidity).
• Laser displacement sensors involve eye-safety classifications (IEC 60825); Class 2 or 3R often acceptable with proper guarding.

Key Formulas & Numerical Data

• Capacitance of parallel plates
  $C = A \cdot \varepsilon<em>0 \cdot \dfrac{\varepsilon</em>r}{d}$
• Typical inductive sensor range: $10\text{--}15\;\text{mm}$ (up to $100\;\text{mm}$ special).
• Capacitive sensor range: up to $25\;\text{mm}$ (standard), $50\;\text{mm}$ extended.
• Contact vs.a0photo-electric switching speed: photo-electric up to $3000$× faster.

Study Tips / Connections

• Relate external sensors to control loops: sensor → controller → actuator → environment.
• Compare internal sensors (encoders, resolvers) to external ones; understand how combined feedback increases accuracy and safety.
• For exam problems, memorise characteristic detection ranges & target materials for inductive vs.a0capacitive.
• Practice drawing block diagrams (oscillator, detector, output) & labelling signal transitions (damped vs.a0clear state).