Actuators and Drive Selection
An actuator converts electrical, pneumatic, hydraulic, or thermal energy into motion or force. A drive converts a control signal into actuator voltage, current, pressure, or flow. The actuator, transmission, driver, power supply, feedback, and protection must be selected as one system.
Learning Objectives
By the end of this lesson, you should be able to:
- compare DC motors, steppers, servos, solenoids, linear actuators, pneumatic actuators, and hydraulic actuators;
- match actuator type to force, torque, speed, precision, stroke, environment, and duty cycle;
- choose driver voltage, current, thermal, braking, and protection ratings;
- identify feedback, limit, and emergency-stop requirements;
- avoid unsafe assumptions about open-loop motion and inductive loads.
Start from the Load
Do not choose the actuator first. Start with the motion requirement:
- required force or torque;
- travel distance or rotation angle;
- maximum speed and acceleration;
- duty cycle and expected lifetime;
- holding requirement when stopped or unpowered;
- environment, contamination, noise, and safety hazards;
- required positioning accuracy and repeatability.
Actuator Types
| Actuator | Strength | Limitation | Typical use |
|---|---|---|---|
| Brushed DC motor | simple speed control, good power density | brushes wear, needs feedback for position | pumps, wheels, small mechanisms |
| Brushless DC motor | efficient, long life, high speed | electronic commutation required | drones, fans, precision drives |
| Stepper motor | simple position commands at low speed | can miss steps, loses torque at speed | 3D printers, light positioning axes |
| Servo motor | closed-loop torque, speed, or position | cost and tuning effort | robots, CNC, industrial axes |
| Solenoid | fast short-stroke motion | heat, high inrush, limited stroke | locks, valves, latches |
| Linear actuator | integrated linear motion | slower, internal limits vary | lifts, doors, positioning fixtures |
| Pneumatic cylinder | high force and simple mechanics | compressed air, compliance, less precise | clamps, pick-and-place |
| Hydraulic actuator | very high force | leaks, safety risk, maintenance | heavy machinery |
Driver and Supply Ratings
For electric actuators, check:
- continuous current and peak current;
- driver voltage range and transient tolerance;
- thermal resistance, heatsink, airflow, and enclosure temperature;
- switching frequency, current regulation, and control interface;
- flyback, braking, regenerative energy, and clamp path;
- undervoltage, overcurrent, overtemperature, and short-circuit protection.
Motor torque is usually linked to current. Motor speed is linked to voltage headroom because inductance and back EMF limit current at speed:
$$
V_\text{supply} > V_\text{back EMF} + I R + L\frac{dI}{dt}
$$
This expression is simplified, but it explains why a higher supply voltage can improve high-speed torque when the driver and motor ratings allow it.
Protection and Safe States
Actuators move energy into the real world. A safe design defines what happens at startup, shutdown, communication loss, sensor fault, and emergency stop.
Important protections include:
- flyback diode, TVS, snubber, or active clamp for inductive loads;
- fuse, current limit, or electronic protection sized for the wiring;
- limit switches or soft limits for travel;
- brake, counterbalance, or self-locking mechanism for vertical axes;
- thermal monitoring for motors, drivers, and solenoids;
- safe startup state with outputs disabled until configuration is valid.
Feedback Choices
Open-loop drive may be acceptable for simple fans, pumps, or lightly loaded steppers. Closed-loop feedback is needed when missed motion creates process errors or hazards.
| Need | Feedback option |
|---|---|
| speed regulation | tachometer, encoder speed, back-EMF estimate |
| position control | encoder, potentiometer, resolver, load-side sensor |
| force or torque limit | current sensor, load cell, pressure sensor |
| end-of-travel detection | limit switch, home sensor, current rise, encoder limit |
Worked Selection Example
A small sliding gate needs 60 N peak force at 0.20 m/s with intermittent operation.
Mechanical load power:
$$
P = Fv = 60 \times 0.20 = 12\ W
$$
With 50% total mechanical efficiency:
$$
P_\text{motor} = 12 / 0.50 = 24\ W
$$
A practical design might choose a geared DC motor or linear actuator above this power level, then verify stall current, duty cycle, limit switches, manual release, and obstruction detection. A bare motor sized only for 24 W is not enough information; the drive must survive start, jam, and repeated cycling.
Common Mistakes
- Choosing an actuator before calculating load torque or force.
- Treating peak current or stall torque as continuous capability.
- Driving inductive loads without flyback or clamp paths.
- Assuming open-loop steppers never lose position.
- Forgetting braking energy and vertical-axis drop risk.
- Omitting limit switches because software has position variables.
Summary
Actuator selection starts with the load and motion profile, not the controller. Choose the actuator, transmission, driver, supply, protection, and feedback together. Verify continuous ratings, peak conditions, thermal behavior, safe states, and fault handling before relying on the system.
Further Reading
- Pololu, Maxon, and Oriental Motor motor and driver application guides.
- Texas Instruments and STMicroelectronics motor-driver selection resources.
- IEC 60204-1 machinery electrical safety concepts.