KNOWLEDGE · ROBOTICS AND AUTOMATION

Robot types: what do you move, and in what pose?

Robot types are divided by how the joints are connected. The structure sets where the hand can reach (the workspace) and the poses the hand can take (the degrees of freedom). So in the lab, choosing a robot comes down to one question: what do you move, in what pose, from where to where?

The number of values a robot’s hand (end effector) can change independently is called its degrees of freedom. An object in space has three of position (X, Y, Z) and three of orientation (tilt forward and back, tilt sideways, and turn), six in all. The International Federation of Robotics (IFR) divides industrial robots by how the joints are connected, and this page looks at four of them: the Cartesian with three straight axes, the SCARA that bends horizontally, the articulated robot like a human arm, and the delta, whose three arms hold an end plate from above. Their relatives, the parallel-link arm and the Stewart platform, come along.

The lab has one condition: an open plate with liquid in it must not be tilted. Then moving a plate takes four degrees of freedom: three of position and one turn about the vertical axis. The plate’s footprint is also fixed at 127.76 × 85.48 mm, so the robot only has to pick it up in one pose and set it down in the same pose.

Sources: classification by structure: IFR World Robotics 2016 §1.2.2 · an object’s six degrees of freedom: Lynch & Park 2017 §2.1 · plate size: ANSI SLAS 1-2004 §4.1.1

01 Cartesian robot

The hand inside the instrument, coming down from above

Its three straight axes line up exactly with X, Y and Z. One axis is one coordinate, so it is easy to handle, and it is built to the size of the job by joining standard linear actuators. Hung from a frame that spans two rails, it is called a gantry.

So most motion inside instruments is built this way: the deck of a liquid handler, a microscope’s motorized stage (XY) and focus (Z), and the stage for automated microinjection of zebrafish embryos.

Its weakness is the direction the hand goes in. The Z rod comes down from above, so it cannot reach inside things or around obstacles. The frame surrounds the workspace, so the robot is bigger than the space it works in. The column type (the lift in an automated store), which stands its vertical axis on a column at the side and pushes out a horizontal fork, also goes into shelves sideways.

In numbers

The Tecan Fluent carries up to three of an 8-channel pipetting arm, a 96- or 384-channel pipetting arm and a gripper arm on one deck, and moves them separately and at the same time. Wang et al. 2007, who injected zebrafish embryos automatically, used two three-axis microrobots (25 mm of travel per axis, 0.04 µm resolution): one moved the embryo holder, the other the needle.

The review by Holland & Davies 2020 finds that lab automation leans heavily on multi-axis arms that mimic human movement, but that these are not always the most efficient, and for many labs they do not fit the space or the budget.

Sources: definition: IFR 2016 §1.2.2 · gantry, strengths and weaknesses: NIST 2019 · the Fluent’s arms: Tecan product page · microinjection: Wang et al. 2007 · automation in research labs: Holland & Davies 2020 · the column type goes in sideways: this page’s computation

02 SCARA

Moves the plate level

Two joints that turn about vertical axes sit side by side, and with up-down motion and a turn of the hand it has four axes. The hand always stays level and turns only about the vertical axis. That is exactly the four degrees of freedom needed to move a plate.

In a lab plate handler the up-down axis is a column, so the whole arm rides up and down the column. The Brooks PreciseFlex 400 has four axes, a horizontal reach of 433 mm (588 mm extended) and a 400–1,160 mm column, and grips SBS plates in landscape or portrait. Its thin arm goes in at plate height, so it reaches into shelves closed above (hotels). But it cannot tilt the hand.

Seen from above: planar robots

Take the up-down motion and the hand’s turn away from a SCARA and you have a planar two-link arm. The same planar work can also be done by a five-bar linkage, in which two arms hold one point together. On the left is serial (joints in a row), on the right parallel (several arms share the end). Inside the dashed half-circles the hand cannot reach.

Name and history

SCARA is an acronym for Selective Compliance Assembly Robot Arm. It gives a little horizontally and is stiff vertically, so when it pushes a round pin into a round hole, it shifts sideways to fit and goes in. Hiroshi Makino of Yamanashi University built the first prototype in 1978, and from 1981 the companies that had researched it with him sold it.

Sources: definition: IFR 2016 §1.2.2 · name and history: Makino 2014 · selective compliance: Wikipedia “SCARA” · PF400: PreciseFlex product page · five-bar linkage: Wikipedia “Parallel manipulator” · the unreachable inner area: this page’s computation

03 Articulated robot

Tilts, and goes in from the side

An arm with three or more rotary joints; the most common has six axes. The waist, shoulder and elbow place the wrist, and the three wrist joints set the hand’s orientation. A pose in space has six degrees of freedom, so with six axes the hand can take any pose almost anywhere it reaches. Five axes cannot set one orientation, and seven axes use the spare one to move the elbow around obstacles.

So it suits tilting tubes, loading through an instrument’s side door, and using tools made for people as they are. The UR5e collaborative robot has six joints, a 5 kg payload, 850 mm reach and ±0.03 mm repeatability. One system places people’s lab equipment, unchanged, around a two-armed robot (Maholo LabDroid), and an arm on a mobile base once moved between instruments and ran 688 experiments in 8 days.

There is a price. It is expensive and less suited to very fast work. One pose has several joint solutions (8 when computed with the UR5e’s dimensions), and in singular configurations the hand cannot move in some direction, so the path needs care. A multi-axis arm with a large range also needs a large safety fence.

Relative: the parallel-link arm

Parallelogram links in an articulated body hold the hand level at all times. It has four degrees of freedom, like the SCARA, so it suits moving plates, but it cannot tilt. Palletizing robots and inexpensive desktop robots (Dobot) are built this way.

Sources: definition: IFR 2016 §1.2.2 · strengths and weaknesses: NIST 2019 · UR5e: UR5e datasheet · Maholo LabDroid: JST research highlight · the mobile robotic chemist: Burger et al. 2020 · fences and research labs: Holland & Davies 2020 · solutions and singular configurations: Lynch & Park 2017 §5.3, ch. 6, and this page’s computation with the official UR5e DH parameters · parallel link: Sanjuan et al. 2020 §1

04 Delta robot

From above, the fastest

Delta is the original name. In the early 1980s Reymond Clavel at EPFL had the idea of building a parallel robot with parallelograms, and called that robot the Delta robot. It began with monotonous manual work seen in a chocolate factory. No source was found that says why it is called Delta.

Three motors fixed above swing three upper arms, and parallelogram rods keep the end plate level at all times. So the end plate moves only in X, Y and Z, and the basic delta has three axes. There are also four-axis models, which add a turn with a center axis, and six-axis ones.

With the motors above and light arms, it is the fastest. The ABB IRB 360 carries 0.1 kg through a 25/305/25 mm round trip in 0.30 s (about 200 a minute). In return, its workspace is a flat cylinder below the robot (usually about 1 m across and 0.2 m high), it is mounted upside down from above, its payload is small (1–8 kg), and it cannot reach sideways into an instrument.

It is widely used on packaging, food and pharmaceutical lines, but no case of its use in lab automation was found. That seems to be because in the lab, pose and precision come before speed (hypothesis). Its names also differ from company to company, which makes it unfamiliar: ABB calls it the FlexPicker, FANUC a parallel link robot, and its nickname is the spider robot.

Relative: the Stewart platform

Six legs that change length move the top plate in six degrees of freedom. It is often called a hexapod. If the delta is the ‘fast three-axis’ parallel robot, the Stewart platform is the ‘stiff, precise six-axis’ one. A small hexapod has a repeatability of ±0.15 µm horizontally and ±0.06 µm vertically, but its range is about X ±17 mm and ±10° of tilt, so in the lab it suits aligning optics or samples precisely rather than moving samples (hypothesis).

The name has a story too. The first to build one was Eric Gough, who ran a tire-testing machine in 1954; the platform Stewart published in 1965 was built differently.

Counting the degrees of freedom

A mechanism’s degrees of freedom are found by counting its links and joints (Gruebler’s formula). Counting the delta as it is gives 9, because each of the six parallelogram rods can spin idly about its own axis. Subtract those 6 and you get 3. The Stewart platform, counted as it is, gives 6.

Sources: name, history, structure: Bonev 2001, EPFL obituary 2025 · IRB 360: ABB datasheet (2022) · names that differ by company: ABB and FANUC product information, NIST 2019 · hexapod: Wikipedia “Stewart platform”, PI product information · the hexapod’s history: Bonev 2003 · counting degrees of freedom: this page’s computation

Summary

Structure sets the pose, and the pose sets the use

Work that comes down from above inside an instrument goes to the Cartesian: liquid handler decks, microscope stages, microinjection stages.
Moving plates level between instruments and shelves goes to the SCARA. It matches the four degrees of freedom needed exactly.
Tilting or loading through a side door takes six axes; moving light things very fast, the delta; aligning within a small range very precisely, the Stewart platform.

At a glance

	Cartesian	SCARA	Articulated (six-axis)	Delta
Basic number of axes	3	4	6 (also 5 and 7)	3 (also 4 and 6)
Hand pose	Fixed orientation	Always level, turns about the vertical axis only	Any pose	Always level (+ turn)
Workspace	A box inside the frame	Part of a cylinder around the column	A sphere around the body	A flat cylinder below
Approach	From above (gantry), from the side (column)	From above, or from the side at the same height	Any direction	From above
Place in the lab	Liquid handlers, microscope stages	Plate handlers	Collaborative robots, two-armed robots	No cases found
Relatives		Seen from above, a planar two-link arm	Parallel-link arm	Stewart platform

The five jobs in Try it

Job	Cartesian	SCARA	Six-axis	Delta	Parallel link	Stewart
Dispensing	OK	OK	OK	OK	OK	Range
Into a shelf	Hits	OK	OK	Hits	Hits	Range
Angled into a side door	Pose	Pose	OK	Pose	Pose	Pose
Fast sorting	OK	OK	OK	OK	OK	Range
Precise optical alignment	Pose	Pose	Precision	Pose	Pose	OK

"Pose": the hand pose cannot be made; "Hits": the body hits the upper shelf; "Range": out of reach; "Precision": not precise enough. Five robots can do fast sorting, but the fastest is the delta. The column-type Cartesian can do only "Into a shelf".

Three layers of the lab

Inside instruments it is the Cartesian; between instruments, the SCARA or a six-axis arm; between benches, an arm on a mobile base. Each layer asks for a different pose and precision, so the types split (hypothesis: a frame built from a few public cases). Transport between instruments is sometimes done, instead of by a robot arm, by maglev planar transport, which floats movers above a flat surface.

MythA robot with more axes is a better robot.

In factAs many axes as the pose needs will do. A four-axis SCARA is enough to move plates level, and a six-axis arm costs more and needs more care with paths and singular configurations.

MythThe delta, the fastest, is also the best in the lab.

In factThe delta reaches in only from above, and its workspace is flat. In the lab, pose and precision usually come before speed (hypothesis).

Confirmed · Hypothesis · Unknown

ConfirmedThe classes and definitions, the histories of the delta, SCARA and hexapod, and the manufacturers’ figures were checked against the originals. The verdicts on the five jobs, the UR5e’s 8 solutions, the delta’s degree-of-freedom count and the example of accuracy and repeatability were computed with a separately built model (most of the model’s dimensions are assumptions).
HypothesisWhy the delta is rare in labs, the three layers of the lab, and that the hexapod suits precise alignment.
UnknownWhy it is named Delta. Whether the SCARA and the six-axis arm also fit with real shelf gaps and gripper thicknesses (it differs by product). Speed was not computed.

Sources: axes and cost, paths: NIST 2019 · maglev planar transport: Beckhoff’s Biosero case story (2026) · the verdicts on the five jobs: this page’s computation

Questions

Frequently asked questions

What types of robots are there?

The International Federation of Robotics (IFR) divides industrial robots by mechanical structure: Cartesian (linear or gantry), SCARA, articulated, parallel (delta) and cylindrical (IFR World Robotics 2016 §1.2.2). The structure sets where the hand can reach and the poses the hand can take.

Is “delta robot” the original name?

Yes. In the early 1980s Reymond Clavel at EPFL called the parallel robot he built with parallelograms the Delta robot (Bonev 2001). No source was found that says why it is called Delta. ABB sells it as the FlexPicker, and FANUC as a parallel link robot.

What does SCARA stand for?

Selective Compliance Assembly Robot Arm. It gives a little horizontally and is stiff vertically, which suits assembly such as pushing pins into holes. Hiroshi Makino of Yamanashi University built the first prototype in 1978 (Makino 2014).

How does a five-axis robot differ from a six-axis robot?

An object’s pose in space has three of position and three of orientation, six degrees of freedom (Lynch and Park 2017 §2.1). With six axes the hand can take any pose almost anywhere it reaches; with five, one orientation cannot be set. Five axes are enough if the hand is symmetric about its own axis or one orientation is always fixed.

What robot moves microplates in a lab?

A plate must not be tilted, so it needs four degrees of freedom: three of position and one turn about the vertical axis. That is why SCARA-type plate handlers, with horizontal joints and a tall up-down column, suit the job (for example the Brooks PreciseFlex 400, four axes). When the plate must be tilted or loaded through an instrument’s side door, a six-axis collaborative robot is used.

What is a Stewart platform (hexapod)?

A parallel robot that moves its top plate in six degrees of freedom with six legs that change length. It is stiff and precise, but its range is small (PI’s small hexapod: repeatability ±0.15 µm horizontally and ±0.06 µm vertically, X range ±17 mm). The first to build one was Eric Gough, who ran a tire-testing machine in 1954 (Bonev 2003).

How does a robot’s repeatability differ from its accuracy?

Repeatability is the scatter when the robot returns to the same point; accuracy is how closely it goes where a coordinate tells it to (ISO 9283 measures the two separately). In a model built from the UR5e’s official dimensions, errors of 0.2–0.3 mm in link lengths and 0.02° in joint zeros put the hand off by up to 0.86 mm in 95% of cases, about 28 times the ±0.03 mm repeatability (a computation with assumed errors). That is why sending a robot to coordinates found by a camera needs calibration.

Are Cartesian robots and gantry robots the same?

A Cartesian robot is one whose three straight axes coincide with X, Y and Z (IFR 2016 §1.2.2). Hung from a tall frame that spans two rails, it is called a gantry robot (NIST 2019). Liquid handlers and microscope stages are built this way.