How to select a voice coil or piezo motor

In short:Choose a voice coil when you need fast, smooth, cog-free motion over a few millimetres to a few tens of millimetres, or direct force control, because force is proportional to current. Choose a piezo motor when you need nanometre resolution, a position held with the power off and no heat at rest, or a non-magnetic or vacuum-compatible drive, and can accept lower force and speed.

Where these two technologies fit

Voice coils and piezo motors both sit at the precision, short-stroke end of linear motion, below linear motors and screw actuators. They solve different problems:

• Voice coil actuators are about dynamics and force: fast, smooth, cog-free motion and force set directly by current.
• Piezo motors are about resolution and stability: nanometre steps, positions held with the power off, no heat at rest and no magnetic field.

If the stroke is more than a few tens of millimetres, or the force is in the hundreds of newtons continuously, look at a linear motor or a screw actuator instead.

The selection sequence

1. What matters most: speed and force control, or resolution and holding?
2. Stroke and whether it is linear, rotary or limited-angle.
3. Force: peak, continuous, and holding at rest.
4. Resolution, repeatability and settling time.
5. Environment: vacuum, cleanroom, magnetic fields, heat at the payload.
6. Guidance and feedback: bearings or flexures, encoder or load cell.
7. Drive and supply voltage.

Voice coil actuators

A voice coil is a coil in a permanent-magnet field. The Lorentz force is proportional to current and reverses with it, force is fairly constant along the stroke with minor drops at the ends, and there is no cogging, no commutation and very low hysteresis (Laser Focus World). Laser Focus World’s survey lists strokes up to about 134 mm and peak forces up to about 7 kN for moving-coil types, though most are much smaller.

Force and stroke

• Force: F = Kf × I, where Kf is the force constant in N/A. Check continuous force against the RMS of the cycle and peak force against the hardest acceleration or press, exactly as for any servo.
• Stroke: quoted stroke is usually the range over which force stays within a stated percentage of nominal. Leave margin at both ends.
• Heat: coil loss is approximately (F ÷ Km)², with Km the motor constant in N/√W (Machine Design). Holding a force continuously, even at zero speed, costs heat.

Moving coil vs moving magnet

Magnetic Innovations sets out the trade-offs (Magnetic Innovations):

In their example comparison of similar-stroke units, the moving-magnet unit had a thermal resistance of 2.0°C/W against 7.1°C/W for the moving-coil unit, so it could sustain more continuous force; treat that as one maker’s example, not a rule.

Flat (planar) voice coils are also made for integration into low-profile linear stages.

Guidance and feedback

A bare voice coil has no bearing. You need guidance: integral shaft bearings (common on moving-magnet units), linear guides, air bearings or flexures. PI offers both mechanical and wear-free flexure guiding on its voice coil stages (PI). Flexures remove friction and are ideal for very short, high-cycle strokes.

For position control, add a linear encoder. For force control, current alone often suffices because force is proportional to current; add a load cell where the tolerance is tight.

Piezo motors

A piezo motor uses piezoceramic elements that vibrate or step to drive a runner, rod or rotor by friction. Unlike a simple piezo stack, which moves only tens to hundreds of micrometres, a piezo motor gives long travel, limited by the runner. PI groups them into three principles (PI):

Resolution and holding

Piezo motors resolve to the nanometre, and PI states its stepping drives hold position to nanometre level in power-off mode, with no current and no heat at the target (PI brochure). That combination, a position held indefinitely without power, heat or servo dither, is what voice coils and linear motors cannot offer. Holding force is finite: PI’s compact ultrasonic motors list a few newtons to around 15 N, and its larger stepping drives up to hundreds of newtons.

Voltage and drive

Drive voltages vary by principle. PI’s ultrasonic motors run at about 120 to 200 V peak-to-peak from dedicated controllers, its stick-slip drives at under 50 V, and its stepping drives at 45 V and below (PI brochure). Some compact rotary piezo motors in our catalogue data run from 12 V. Piezo drivers generate specific waveforms, so treat the motor and driver as a matched pair.

Environment

PI describes its ultrasonic and stepping drives as non-magnetic and vacuum-compatible, producing no magnetic field and unaffected by one. That makes piezo motors a natural fit for electron microscopes, MRI-adjacent equipment, ultra-high vacuum and beam lines, where a magnet-based motor would disturb the process.

Limits

• Lower force and speed than electromagnetic motors of similar size
• Friction drive: performance depends on preload and contact surfaces; check life in your duty cycle
• Usually need an encoder for closed-loop positioning to the nanometre
• Small holding force compared with a brake on a screw axis

Voice coil vs piezo: decision guide

Worked example: a force-controlled test probe

Application: a vertical probe presses on a pressure-sensitive device under test. It moves down 5 mm in 20 ms, presses with 5 N for 100 ms, returns in 20 ms and waits 60 ms. Cycle 200 ms (5 per second). Moving mass (coil, shaft, probe) 50 g.

Technology choice: programmed force and fast cycling point to a voice coil. A piezo motor could manage the force but would need a separate load cell to control it, and friction drive wear at 10 moves a second would need checking.

Move profile (1/3 trapezoid):

• v = 1.5 × 0.005 ÷ 0.020 = 0.375 m/s
• a = 0.375 ÷ 0.00667 ≈ 56 m/s²
• Acceleration force: 0.05 × 56 ≈ 2.8 N; gravity: 0.05 × 9.81 ≈ 0.49 N

Motor forces (magnitude; gravity helps going down and opposes going up):

RMS ≈ 3.4 N; peak ≈ 4.5 N (during the press, not the moves).

Selection: continuous force of at least about 4.2 N (25% margin on RMS, as a rule of thumb), peak comfortably above 5 N, and a stroke of 8 to 10 mm so the 5 mm move stays in the flat part of the force curve. That is a small cylindrical voice coil. With an assumed Km of 2 N/√W (use the datasheet value), coil loss is about (3.4 ÷ 2)² ≈ 2.9 W.

Coil or magnet moving? The probe touches the device, so heat into the probe matters; a moving-magnet unit keeps the coil heat in the housing. If the device were not heat-sensitive, a moving-coil unit would give lower moving mass.

Feedback and control: a linear encoder for the approach, then switch the drive to current (force) control for the press. Add a load cell if the force tolerance is tighter than the force constant’s variation over the stroke.

Guidance: a flexure or precision shaft bearing; friction directly offsets the applied force.

What changes the answer: if the probe instead had to hold a fixed height to 10 nm for hours with no heat, a piezo stepping or stick-slip motor with an encoder would be the choice.

Products in our range to consider

• Akribis AVM Standard: cylindrical moving-coil voice coils in 12–90 mm frames, up to 95.6 N continuous and 40 mm stroke.
• Akribis AVM High-Force: cylindrical voice coils from 14 to 250 mm, over 1.1 kN continuous and 4.7 kN peak.
• Akribis AVA: flat, low-profile voice coils with 20 mm stroke for integration into linear stages.
• Piezo Motion RBS Rotary Piezo Motor: compact 12 V rotary piezo motor with self-braking holding torque, up to 0.03 Nm and 100 rpm.
• Piezo Motion RAS Rotary Piezo Motor: rotary piezo motor from the same family, self-braking at rest; figures on request.
• Leadshine ELD3 miniature DC servo drives: 24–70 V DC drives supporting voice-coil, linear and brushed motors, with a second encoder input.
• Novanta Capitan servo drives: 18 g micro servo drive for 8–60 V DC, 10 A continuous and 20 A peak.

For linear piezo motors, tell us the stroke, force and resolution and our supply network will propose options.

What else the build needs

• Drive: a servo drive that supports single-phase voice coil motors and current-mode force control, or the matched piezo driver; see servo drives.
• Feedback: a linear or rotary encoder at the resolution you need; see encoders.
• Guidance: flexures, shaft bearings, crossed-roller or air bearings.
• Load cell for tight force control.
• Controller: for force-position switching or synchronised scanning; see motion controllers.
• Cabling: high-flex leads on moving-coil units; vacuum-rated cabling where needed.
• Power supply: low-voltage DC for most voice coil drives; the piezo driver’s own supply.
• For the wider family, see the linear actuators hub.

Things that catch engineers out

• Forgetting gravity on vertical voice coils, which must carry the moving mass continuously.
• Using the full catalogue stroke, where force has already dropped at the ends.
• Heat into the payload from moving-coil designs.
• Guidance friction spoiling force control.
• Expecting a piezo motor to hold a heavy load; holding forces are modest.
• Mixing piezo motors and drivers from different designs.
• Running a piezo friction drive at a high duty without checking its life.
• Confusing resolution with accuracy: both technologies need an encoder and a stable structure to deliver nanometre accuracy.

Checklist

Send these to our engineers, or enter them in the spec builder:

• Linear, rotary or limited-angle, and orientation
• Stroke or angle
• Moving mass and payload
• Move profile: distance, time, dwell, cycle rate
• Peak, continuous and holding force (or torque)
• Force control required, and tolerance
• Resolution, repeatability and settling time
• Power-off holding requirement
• Environment: vacuum, cleanroom, magnetic field, temperature, heat limit at payload
• Supply voltage, drive and interface preference