| name | actuator-selection |
| description | Actuator selection — electric (servo/stepper/linear), pneumatic, hydraulic, piezoelectric, SMA; force/speed/stroke requirements, backdrivability, duty cycle, power density comparison, IEC 61800. |
| metadata | {"priority":7,"promptSignals":{"phrases":["actuator selection","actuator sizing","electric actuator","linear actuator","servo actuator","actuator comparison"],"minScore":3}} |
Actuator Selection — Complete Skill
Selection Criteria Framework
Primary requirements:
- Force/torque [N or N·m]
- Stroke/angle [mm or °]
- Speed [mm/s or RPM]
- Duty cycle [%]
- Positioning accuracy [μm or arc-min]
- Power supply (electric/pneumatic/hydraulic)
- Environment (IP rating, temperature, ATEX)
Secondary:
- Backdrivability (critical for cobots, safety)
- Bandwidth (Hz; how fast can force/position track?)
- Stiffness (N/mm; position under load)
- Efficiency [%]
- Size and weight constraints
- Cost and maintenance
Electric Actuators
Servo Motor + Ballscrew/Gearbox
Force equation:
F = T_motor × η_lead / p [N; T_motor = motor torque [N·m]; p = lead [m/rev]; η_lead = efficiency]
Ballscrew efficiency η ≈ 0.90–0.95
Gearbox ratio i: T_out = T_motor × i × η_gear; speed_out = speed_motor / i
Motor sizing:
T_rms = √(Σ T_i² × t_i / Σ t_i) [RMS torque from duty cycle; must be ≤ T_continuous]
T_peak ≤ T_stall (peak torque capacity of motor)
Bandwidth:
f_-3dB ≈ f_n / 3 [target; f_n = mechanical natural frequency of load + drive]
f_n = (1/2π) × √(k_drive / J_total) [rad/s; k_drive = drive stiffness; J_total = reflected inertia]
BLDC servo (common):
Speed: 1000–6000 RPM standard; 20,000 RPM high-speed
Torque: 0.1 N·m (miniature) to 3000 N·m (large DD servo)
Accuracy: ±0.01° (encoder resolution dependent); 19–23 bit encoders = ±0.001°
Stepper Motor
Step resolution:
θ_step = 360° / (N_teeth × steps_per_rev) [°/step; N_teeth = rotor teeth]
Full step: 1.8°/step (200 steps/rev); Microstepping: 1/256 → 0.007°/step
Torque-speed:
T ≈ T_hold × [1 - (f/f_cut)^n] [decreases with speed; f_cut = pull-out frequency]
No encoder needed for open loop; add encoder for closed loop (step motor servo)
Use when: accurate positioning + low speed + no speed feedback required
Linear Motor (Ironless/Iron-core)
Force equation:
F = K_f × I [N; K_f = force constant [N/A]; I = current [A]]
K_f = BLN/2 [B = air gap flux density; L = coil length; N = turns]
Zero backlash; very high bandwidth: f_-3dB = 100–1000 Hz for ironless linear motor
Peak force: 50–10,000 N (product dependent)
Use when: high bandwidth, zero backlash, very high positioning accuracy required
Linear Actuator (Motor + Gearbox + Screw)
Commercially available: e.g., Thomson, Exlar, Linak
Typical specs: force 100–50,000 N; stroke 50–600 mm; speed 1–300 mm/s
Self-locking: Acme thread (η < 50%) → not backdrivable; Ballscrew (η = 90%) → backdrivable
Pneumatic Actuators
Force:
F = P × A_piston - P_back × A_rod_side - F_friction [N]
Extend: F = P × (π/4 × D_bore²) - F_friction (rod area negligible at extend)
Retract: F = P × (π/4 × (D_bore² - D_rod²)) - F_friction
Compressible flow → not suitable for precise velocity control or force control
Use for: fast binary motion (pneumatic cylinder); 100–500 mm/s typical speed
Positioning with pneumatics: requires servo valve + encoder → expensive
Advantages: very high power-to-weight in compact size; clean; simple
Disadvantages: compressible → poor stiffness; needs compressor; noisy
Hydraulic Actuators
Force:
F = ΔP × A_piston [N; ΔP in Pa]
Linear: 100 kN – 10 MN possible in compact package
Power density: 1–10 kW/kg (best of any actuator type)
Stiffness: very high (incompressible fluid); good for force control
Bandwidth: 10–100 Hz (servo valve bandwidth limited)
Use when: very high force density, heavy loads, high duty cycle
Disadvantages: leak risk; fire hazard; maintenance; pump + reservoir
Piezoelectric Actuators
Displacement:
δ = d₃₃ × V × n [μm; d₃₃ = piezo coefficient [pm/V]; n = stack layers]
Typical: 10–200 μm at 0–1000 V
Blocking force:
F_block = k_pzt × δ_free [N; k_pzt = axial stiffness of stack]
k_pzt = E_pzt × A / (n × t_layer)
Bandwidth: 1 kHz – 100 kHz (very high; limited by mechanical resonance)
Use for: nanometer precision, very high frequency actuation, small displacement
Limitations: very small stroke; high voltage; not backdrivable
Shape Memory Alloy (SMA)
Recovery strain: ε_r = 4–8% (NiTi)
Recovery force: σ_r = 100–400 MPa → F = σ_r × A_wire
Actuation speed: 1–10 mm/s (limited by heat transfer)
Use when: very compact space; low-cycle actuation; biomedical; aerospace morphing
Actuator Comparison Table
| Actuator | Force density | Bandwidth | Stroke | Accuracy | Backdrivable |
|---|
| Servo + ballscrew | High | Med (10–100 Hz) | Unlimited | ±1 μm | Yes |
| Hydraulic linear | Very high | High (50 Hz) | Medium | ±10 μm | Yes |
| Pneumatic cylinder | High | Low (binary) | Long | ±0.5 mm | No |
| Linear motor | Medium | Very high (1 kHz) | Medium | ±0.1 μm | Yes |
| Piezoelectric | High | Very high (kHz) | Very short | ±1 nm | No |
| SMA wire | Low | Very low | Short | Low | No |
Standards
| Standard | Scope |
|---|
| IEC 61800-5-1 | Servo drive safety |
| ISO 15552 | Pneumatic cylinders; dimensions |
| ISO 8643 | Hydraulic linear actuators |
| ISO 13849 | Safety functions (PL rating for actuator in safety chain) |
Output
Provide: actuator type recommendation, force [N], stroke [mm], speed [mm/s or RPM], duty cycle [%], continuous and peak torque/force [N·m or N], positioning accuracy [μm or arc-min], bandwidth [Hz], power supply requirement [V, A], stiffness [N/mm], backdrivability (yes/no), efficiency [%], IP rating, safety function (if required, PL category per ISO 13849), and applicable standard.
