Sensors & Measurement · Study deck

Braitenberg Transfer Tuning

A reactive rule only works if the sensor curve and motor response are understood.

Physics Phoebe is your guide for this deck.

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After studying this chapter

Learning objectives

You will be able to:

Explain why a sensor transfer function can change a Braitenberg vehicle's apparent behavior.
Predict where LDR compression, saturation, and response lag will cause steering errors.
Tune divider, dead-band, scaling, and motor mapping choices from bench evidence.
Implement and test multi-mode reactive behavior before escalating to AI/ML.

Major section

Transfer Function as Behavior

A light sensor that responds steeply in dim conditions but barely at all in bright ones will make a vehicle lunge toward a distant lamp yet crawl once it is close.

The shape of the curve — where it is steep, where it flattens — is the design.
Intuition: the transfer function is the vehicle's reflexes.
For a light-following robot, the curve is part of the steering loop.
If the left light sensor climbs from 10 to 100 lux while the right sensor stays near 10 lux, the left-side signal changes strongly and the vehicle turns.

Logarithmic chart showing light-dependent resistor resistance decreasing as light intensity increases.

Major section

Photoresistor Compression Loop

Each 10x jump in light multiplies R by the same 0.20, so the resistance CHANGE shrinks: 8 kΩ then 1.6 kΩ.

Sensitivity is the local slope of the transfer function, and for many real sensors that slope changes along the curve.
A photoresistor (LDR/CdS cell) is a classic example: its resistance follows a power law in illuminance, so it is compressive.
The sensor is far more responsive in dim light than bright.

Major section

Photoresistor Compression Loop (continued)

A data-sheet gamma may be specified only over a narrower illuminance interval (for example, 10-100 lux); the 1,000 lux point is an extrapolation that must be checked against the selected LDR.

The first 10x light increase moves the signal by about 1.10 V; the next 10x increase moves it by only 0.42 V.
A microcontroller or motor driver sees those voltages, not the lux directly.
So the effective vehicle rule is not "turn proportional to light"; it is "turn proportional to the conditioned voltage curve.".

Major section

Common Pitfalls

An excitatory connection increases motor speed when the sensor signal increases; an inhibitory connection decreases motor speed.

Swapping these causes vehicles to move opposite to intended — approach instead of avoid.
If the sensor output range drives motor speed directly without scaling, the motor may always run at maximum or minimum speed with little intermediate behavior.
Physical motors have inertia and time constants (50-500 ms) that lag behind rapidly changing sensor inputs, causing jittery oscillatory motion.

Deck summary

Key takeaways

A light sensor that responds steeply in dim conditions but barely at all in bright ones will make a vehicle lunge toward a distant lamp yet crawl once it is close.

Each 10x jump in light multiplies R by the same 0.20, so the resistance CHANGE shrinks: 8 kΩ then 1.6 kΩ.
A data-sheet gamma may be specified only over a narrower illuminance interval (for example, 10-100 lux); the 1,000 lux point is an extrapolation that must be checked against the selected LDR.

Retrieval practice

Recall check 1 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q1In a Braitenberg vehicle where a light sensor drives a motor directly, what does the sensor's transfer function determine?

AOnly the sensor's supply voltage, not its response.

BHow stimulus maps to signal and behaviour.

CThe colour of the light, and nothing about the motion.

DNothing; behaviour depends only on the motor.

Show answer

Answer: B With the sensor wired to the motor, the shape of the stimulus-to-signal curve becomes the behaviour.

Retrieval practice

Recall check 2 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q2A photoresistor's resistance changes a lot per lux in dim light but very little per lux in bright light. What does this say about its sensitivity?

ASensitivity is constant when measured per tenfold increase in light.

BThe sensor is saturating and should be replaced for this response.

CSensitivity varies: high in dim light, low in bright light.

DSensitivity is highest in bright light, opposite to what is described.

Show answer

Answer: C A compressive curve has a steep slope at low levels and a shallow slope at high levels, so sensitivity varies along it.

Retrieval practice

Recall check 3 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q3A light-seeking Braitenberg vehicle behaves well far from a lamp but stops responding to changes once it is close. The sensor curve flattens at high light. What is happening?

AThe motor loses electrical power only at short lamp distances.

BThe light sensor saturates and loses sensitivity near the lamp.

CThe vehicle detects too little light close to a bright source.

DSaturation increases sensitivity and makes the robot overreact.

Show answer

Answer: B A flattening (saturating) curve has near-zero local slope, so the driven behaviour stalls at high light.

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Answers

Answer key.

B · With the sensor wired to the motor, the shape of the stimulus-to-signal curve becomes the behaviour.
C · A compressive curve has a steep slope at low levels and a shallow slope at high levels, so sensitivity varies along it.
B · A flattening (saturating) curve has near-zero local slope, so the driven behaviour stalls at high light.