actuator-selection

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:

1. Force/torque [N or N·m]
2. Stroke/angle [mm or °]
3. Speed [mm/s or RPM]
4. Duty cycle [%]
5. Positioning accuracy [μm or arc-min]
6. Power supply (electric/pneumatic/hydraulic)
7. 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.