Glossary

108 terms from programming, computer science and robotics, each defined in plain words and linked to the free lesson that teaches it.

abstraction Leaving out detail that does not matter to the problem, so that what remains is simpler to think about. A…
actuator A device that turns a program's command into movement or an effect on the world: a motor, a servo, an LED.
admissible heuristic A heuristic that never overestimates the true remaining cost to the goal. A* search with an admissible…
algorithm A precise sequence of steps that solves a problem or carries out a task. A program is an algorithm written…
assignment Giving a variable a value with =, as in x = 10. The right-hand side is worked out first, then stored under…
block A group of lines that belong together and run as one unit: the body of an if, a loop or a function. In…
blocking call A function that does not return until its job is finished, such as forward with a distance. The program…
body frame The coordinate frame fixed to the robot, which turns as the robot turns. On the BugBot its x axis points to…
Boolean A value that is either True or False. Comparisons produce Booleans, and if and while decide what to do from…
broadcast Sending one message that every device on the network receives, rather than to one named receiver.
classifier A model that answers with a category: which of several labels this input is most like.
closed loop Control with measurement: read a sensor, compare with what you wanted, adjust, repeat. The measurement…
comment A note in the code for people, not the computer. In Python it starts with # and the computer ignores…
comparison operator An operator that compares two values and gives True or False: == (equal), != (not equal), <, >, <= and >=.
complementary filter A way of fusing two sensors that fail in opposite ways: trust the one that is smooth but drifts (an…
condition An expression that is either true or false, used by selection and iteration to decide what happens next:…
configuration space The space of every possible configuration of the robot, such as (x, y, heading), with the obstacles grown by…
control error In control, the difference between the target and the measurement: target minus actual. Its sign says which…
controller The part of a program that decides what command to send from the difference between what was wanted and what…
covariance A measure of how two uncertain quantities vary together. A covariance matrix holds each variable's variance…
cross-track error How far the robot is from its path, measured at right angles to the path and signed to say which side it is…
data type The kind of value a piece of data is: integer, real (float), Boolean, character or string. The type decides…
dead band A range of small commands or small errors that produce no action at all, for example motor commands under…
dead reckoning Working out where the robot is by starting from a known pose and adding up its measured speeds and turns…
decomposition Breaking a problem into smaller parts that are easier to solve, then solving each part. Writing one function…
deterministic Always giving the same result from the same starting point. The simulator is deterministic, which is what…
domain randomisation Tuning or training across many randomly varied versions of the world, such as different noise levels, drive…
effective sample size A count of how many particles are still doing useful work: 1 divided by the sum of the squared normalised…
feature One of the numbers a model is given as input. Choosing which numbers describe the situation is called…
feedforward The part of a command worked out from a model of the system and the reference alone, before any error has…
float A real number with a fractional part, such as 3.5 or -0.25. Python calls the type float; exam papers say real.
focal length In the pinhole camera model, the distance from the pinhole to the image plane, given in pixels for vision…
forward kinematics Working out how a robot moves from the commands sent to it: command to velocity in the body frame, rotated…
frontier In an occupancy grid, a free cell with at least one unknown neighbour: a place the robot can reach that…
function A named block of code that does one job and can be called from anywhere in a program. It may take parameters…
gain The number the error is multiplied by in a controller. Too small and the correction is sluggish; too big and…
heading The direction the robot faces, in degrees, measured clockwise from a fixed reference. Zero is the direction…
holonomic Able to move in every direction it has freedom in, independently and at once. A holonomic robot on a floor…
indentation The spaces at the start of a line. In Python indentation is not decoration: the lines indented under an if,…
index The position of an item in a list or a string, counting from 0: the first item is at index 0, the second at…
infinite loop A loop whose condition never becomes false, so it never ends. Sometimes wanted (a robot's main loop), often…
innovation In a filter, the measurement minus what the filter predicted it would be: the part of the measurement the…
instruction A single command in a program that tells the computer to do one thing, such as move, print or compare.
integer A whole number, positive, negative or zero, with no fractional part: 3, -12, 0. Python calls the type int.
integral windup When the actuator is saturated and cannot do what the controller asks, the integral term keeps growing, then…
intrinsics The numbers that describe what a camera does to a ray, independent of where the camera is: the focal lengths…
inverse kinematics Working out which commands produce a wanted motion: rotate the wanted world velocity into the body frame…
inverse sensor model The rule that turns one sensor reading into evidence about map cells. For a range reading, the cells the…
iteration Repeating a set of instructions, either a fixed number of times (count-controlled, a for loop) or while a…
JSON JavaScript Object Notation: a text format for lists, dictionaries, numbers and strings that almost every…
Kalman filter An estimator that repeats two steps, predict with a model and correct towards a measurement, and works out…
Kalman gain The fraction k, between 0 and 1, by which a Kalman filter moves its estimate towards a new measurement. For…
label The right answer attached to an example in training data, such as open or wall-ahead for a depth reading.
log odds The natural logarithm of p / (1 - p), used to store a probability p. Zero means 50 percent, positive means…
logic error The program runs without complaint but does the wrong thing, because the instructions do not say what the…
logical operator An operator that combines Booleans: and (both must be true), or (at least one true) and not (the opposite).
look-ahead distance In pure pursuit, how far along the path the aim point is placed. A longer look-ahead gives smoother, gentler…
loop A set of instructions that is repeated. A for loop repeats a known number of times; a while loop repeats as…
low pass filter A filter that lets slow changes in a signal through and smooths away fast ones, such as sensor noise. The…
machine learning Making a computer work out a rule from examples instead of being given the rule. The examples are the data;…
measurement noise The random error on each sensor reading. In a Kalman filter its variance is R, which can be measured by…
median filter A filter that keeps the last few readings and outputs the middle one after sorting. Unlike an average, it…
model What a learning method produces: the rule, as data, that turns new inputs into answers. It can be saved as…
Monte Carlo localisation A particle filter used to find a robot's pose on a known map: move the particles by odometry, weight them by…
neural network A model made of layers of weighted sums and simple squashing functions. Training adjusts the weights so the…
non-blocking A function that returns at once and lets the effect carry on in the background, such as forward with no…
occupancy grid A map that divides the world into equal cells and stores for each one how likely it is to be occupied.…
odometry Working out where the robot is by adding up its own measured movements. Drift builds up: odometry is an…
open loop Control without measurement: send a command and hope. Drive for two seconds and assume you went forty…
outlier A reading that is not a noisy measurement of the quantity at all but of something else, such as a depth beam…
overfitting When a model fits its training data too closely, noise included, so that it does well on the data it was…
overshoot Going past the target before settling, because the correction was too strong or arrived too late.
parameter A name inside a function definition for a value the caller will pass in. The actual value passed when the…
particle deprivation The failure of a particle filter in which repeated resampling leaves every particle a copy of a few…
particle filter An estimator that represents its belief as a cloud of guesses, called particles. Each tick it moves every…
polling Checking repeatedly, in a loop, whether something has happened, such as whether a message has arrived,…
potential field A way of steering without planning a route: the goal pulls the robot towards it, every obstacle pushes it…
process noise In a Kalman filter, Q: the variance of the ways the state can change between ticks that the prediction does…
program A sequence of instructions that a computer can execute to carry out a task. The instructions run in the…
proportional control Correcting by an amount proportional to the error: command = gain times error. Big error, big correction;…
protocol An agreed set of rules for how messages are formed and exchanged, so that two programs understand each other.
pure pursuit A path-following method that steers towards a point a fixed look-ahead distance further along the path,…
ray casting Stepping along a line through a map from a given pose until it reaches an occupied cell, to predict what a…
reality gap The difference between the world a controller or policy was tuned in, such as a simulator or one robot on…
reinforcement learning Learning from rewards rather than labels: the program tries actions, is told how well each went, and shifts…
resampling Drawing a fresh set of particles from the old set, each chosen with probability in proportion to its weight,…
return value The value a function sends back to the code that called it, with the return statement. A function with no…
reward hacking When a learning system scores well on its reward function while doing the wrong thing, because the reward…
rotation matrix A 2 by 2 matrix of the sine and cosine of the heading that turns a vector in the body frame into the same…
RRT Rapidly-exploring Random Tree: a sampling planner that grows a tree from the start by repeatedly picking a…
runtime error An error that happens while the program is running, such as dividing by zero or using a name that has no…
selection Choosing which instructions to run depending on whether a condition is true or false: an if statement. One…
sensor A device that measures something about the world and turns it into a value the program can read: distance,…
sequence Instructions carried out one after another, in the order they are written. One of the three basic…
simulation A model of the real thing, run on a computer, that behaves closely enough to test programs on. The mat and…
state What a program is currently doing, held in a variable so that each tick of the loop can behave differently:…
state machine A program organised as a set of states and the transitions between them: in each state it does one thing,…
steady state error An error that is still there after the system has settled and stopped changing. A proportional controller…
step response How a system's output changes over time after its input jumps suddenly from one value to another, such as…
string A sequence of characters, such as a word or a sentence, written between quotes. Strings can be joined,…
syntax The rules that say how a language's instructions must be written: the spelling, the brackets, the colons,…
time constant How long a first order system takes to cover 63 percent of the way to its final value after a step, written…
timeout A limit on how long to wait for something before giving up and doing something else, so a program is never…
training data The examples a model learns from: each one is some input (the features) together with the right answer (the…
variable A named location in memory that holds a value which can change while the program runs. Assigning gives it a…
velocity profile Speed written as a function of time along a path, designed so that the machine can actually follow it within…
visual servoing Controlling a robot directly from what its camera sees, in a closed loop. Image based visual servoing keeps…
world frame The coordinate frame fixed to the mat, which stays put however the robot moves. Its x axis runs to the right…