Actuators & Control · Study deck

Choosing the Right Actuator

An actuator is a part that turns a control signal into a physical change.

Motor Max is your guide for this deck.

classifications

Motor Max, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Explain: A relay may only prove that the controller energized a coil; it does not prove the contact did not weld, the strike received power, the door moved, or the door is now secure.
Explain: The physical boundary determines what must be checked: driver state, load current, mechanical movement, final position, thermal behavior, isolation, manual override, and whether the default state is safe when power or communication fails.
Explain: If the controller also drives an 80 mA relay coil, the coil still needs a transistor driver and flyback path because the microcontroller pin is not the load supply.

Soft fabric robotic gripper holding a strawberry

Major section

Start With the Story · Overview: Classify Actuators By The Job They Must Do

Those actions differ in motion, force, speed, position, power, feedback, and harm if the part sticks.

A sent command is not proof of movement; use an independent sign of the physical result where harm matters.
The final mechanism, mounting, driver, duty, and environment can change the result.

Classifying the output type narrows the actuator choice before comparing drivers, protection, evidence, and safe-state behavior.

Major section

Practitioner: Compare Types With A Selection Record

A useful actuator comparison record is a small design argument.

It states the required output, lists realistic candidate types, rejects poor fits with reasons, and records the driver, protection, feedback, and failure behavior that must be tested.
A remote irrigation valve could use a solenoid valve, motorized ball valve, or relay-controlled pump.

Two bonded SMA strips bend and release in turn, creeping the mechanism along a ratcheted track one tooth at a time -- the piezo-motor-using-SMA design named above.

Major section

Practitioner: Compare Types With A Selection Record (continued)

The right answer depends on whether the system needs normally-closed behavior, low standby power, partial positioning, serviceability, and direct evidence of flow.

A selection record makes those tradeoffs explicit before hardware is ordered.
For mobile robots, the comparison also has to name the controllable degrees of freedom.
That classification changes the evidence.

Industrial hydraulic cylinders with attached valve and manifold blocks

Major section

Practitioner: Compare Types With A Selection Record (continued)

A wheel motor command is not proof of a pose update; wheel slip, backlash, floor friction, battery voltage, and sensor delay can all disturb the motion model.

A differential-drive robot may have planar state (x, y, heading) but only two direct wheel-speed commands, so it is non-holonomic: it cannot move sideways without changing heading over time.
A holonomic platform has enough independently controlled motion axes to command every relevant degree of freedom directly.
Shape-memory and magnetic actuators deserve their own line in a selection record because "shape-memory" is not one material.

Labeled cutaway of a hydraulic cylinder: pressurized fluid at the cap-end port extends the rod, while fluid at the rod-end port retracts it.

Major section

Under The Hood: Classification Also Defines Failure Boundaries

A motor may stall, overshoot, or draw more current under load.

A relay may wear contacts or weld shut.
A solenoid may heat if energized too long.
An LED or buzzer may be safe electrically but still fail the human-factor job if it is ambiguous or missed.
Thermal behavior changes the classification too.

Major section

Under The Hood: Classification Also Defines Failure Boundaries (continued)

The selection is not only about opening a door.

A software command such as "open" or "turn on" is not enough evidence.
The physical boundary determines what must be checked: driver state, load current, mechanical movement, final position, thermal behavior, isolation, manual override, and whether the default state is safe when power or communication fails.
If one row is unclear, the actuator choice is not ready.

A piston, connecting rod, and crank pin turn reciprocating motion into continuous shaft rotation.

Major section

Under The Hood: Classification Also Defines Failure Boundaries (continued)

A relay may only prove that the controller energized a coil; it does not prove the contact did not weld, the strike received power, the door moved, or the door is now secure.

If the strike draws 1.5 A at 12 V, the relay contact, wiring, fuse, and supply must be rated for that DC load and inrush.
If the controller also drives an 80 mA relay coil, the coil still needs a transistor driver and flyback path because the microcontroller pin is not the load supply.
Actuator categories matter because they fail differently.

A solenoid-based electric bell: the coil's own current path pulls the armature and breaks its own contact, so the buzz cycle repeats for as long as the circuit is closed -- grounding the relay/solenoid row above.

Major section

Under The Hood: Classification Also Defines Failure Boundaries (continued)

Safe stop: that jump may need a latching mechanism or reduced hold current, not just a bigger coil.

If it pulses for 10 s every 60 s, the simple duty fraction is 10 / 60 = 0.167, so the average coil heat from the pulse pattern is roughly 1 W before enclosure effects.
If the same solenoid is held continuously, the heat case is six times larger and may require a latching mechanism, reduced hold current, or a different actuator family.
A building access controller might compare a relay-driven strike, a solenoid lock, and a motorized latch.

A rotating pinion gear meshes with a linear rack, converting rotary input into linear output -- the mechanism behind pneumatic and electric rack-and-pinion valve actuators.

Deck summary

Key takeaways

Those actions differ in motion, force, speed, position, power, feedback, and harm if the part sticks.

A useful actuator comparison record is a small design argument.
The right answer depends on whether the system needs normally-closed behavior, low standby power, partial positioning, serviceability, and direct evidence of flow.
A wheel motor command is not proof of a pose update; wheel slip, backlash, floor friction, battery voltage, and sensor delay can all disturb the motion model.
A motor may stall, overshoot, or draw more current under load.

Retrieval practice

Recall check 1 of 4

Motor Max says: answer from memory, then check your reasoning.

Q1What is the strongest first question when comparing actuator types for an IoT design?

AChoose the actuator type used by comparable demonstration projects and validate fit during the build.

BWhich output job, driver boundary, and motion evidence does the design actually need?

CChoose an actuator connected directly to GPIO without a separate load driver.

DChoose the actuator with the broadest listed specifications and treat those ratings as proof of fit.

Show answer

Answer: B Selection starts from the physical job, power boundary, and verification evidence.

Retrieval practice

Recall check 2 of 4

Motor Max says: answer from memory, then check your reasoning.

Q2A stepper motor is commanded to move a vent, but wind load can cause missed steps. What should the selection record add before release?

ADeclare that commanded steps establish true vent position under wind load.

BA decision to remove driver protection so the stepper can move faster against wind.

CA dashboard colour change in place of position sensing or a measured torque margin.

DA position reference or confirmation, such as homing or feedback, plus a wind-load torque margin.

Show answer

Answer: D Open-loop steps are not enough when load can make the mechanism miss movement.

Retrieval practice

Recall check 3 of 4

Motor Max says: answer from memory, then check your reasoning.

Q3Why is command acknowledgement not the same as actuator success?

ALogging delays can make a valid actuation look late even when it succeeds.

BDriver coil-current reports confirm that the actuator reached its intended position.

CThe command can succeed while the physical action fails.

DThe actuator's mechanical output is independent of command acknowledgement.

Show answer

Answer: C IoT actuation needs evidence across the electrical, mechanical, and state-confirmation boundaries.

Retrieval practice

Recall check 4 of 4

Motor Max says: answer from memory, then check your reasoning.

Q4A vent and a door lock both receive an off command. Why must their failure defaults be reviewed separately?

ABecause one driver type is enough to define both failure states without physical-output checks.

BBecause each device needs its own safe or secure resting state and proof of physical output.

CBecause an off command guarantees the same physical result for a vent and a door lock.

DBecause a logged off command proves that both mechanisms reached their intended states.

Show answer

Answer: B The physical job and failure consequence define the default, and feedback must prove it.

Print reference

Answers

Answer key.

B · Selection starts from the physical job, power boundary, and verification evidence.
D · Open-loop steps are not enough when load can make the mechanism miss movement.
C · IoT actuation needs evidence across the electrical, mechanical, and state-confirmation boundaries.
B · The physical job and failure consequence define the default, and feedback must prove it.