Autofocus actuator

01

Match the actuator's resonance and drive current

ROHM's BU64981AGWZ data sheet (2017) describes a linear constant-current VCM driver for autofocus in mobile camera modules, with a 2.3–4.8 V supply, a 10-bit DAC giving 98 µA per code step up to a 100 mA maximum output current (95–105 mA), 2.5 Ω typical output resistance, 70 µA standby current, a 2 MHz master clock, an I²C Fast-mode Plus interface and −25 to +85 °C operation, in a 0.72 × 1.13 mm package. [1]

It says each VCM has a resonant frequency, provided by the actuator maker or measured, that determines how much the lens rings after a move; to measure it, drive the lens to about half of the actuator's full stroke without hitting the mechanical end, observe the ringing period T and take f₀ = 1/T. The driver's resonant-frequency register covers 50 Hz to 152 Hz, and it notes that actuator samples vary so tolerance should be allowed for. [1]

Editorial note: the actuator's stroke and sensitivity (lens travel per milliamp) set how much of the driver's current range is used; ask the actuator supplier for stroke, sensitivity, resonant frequency and its tolerance, and the current needed at full stroke.

Give the supplier the lens mass and size, required stroke, sensitivity, resonant frequency and tolerance, drive current at full stroke, coil resistance, driver IC or interface, supply voltage, settling-time target, orientation range, operating temperature, module footprint and height, life-cycle test needs and quantities.

02

Control ringing and settling time

The data sheet describes a direct mode, in which the lens is driven straight to the target and ringing and settling time are at their maximum, and an intelligent slew-rate control (ISRC) mode with four settings of (1/f₀) × 0.48, 0.72, 0.92 and 1.2 that shape the drive to keep ringing at a minimum. For a 100 Hz actuator it gives DAC code change times of 7, 8.7, 12 and 18 ms for the four settings, scaling as those values × (100 / f₀) ms for other resonant frequencies, with ±3% variation from the oscillator. [1]

It adds that camera module orientation can affect settling time because of gravity acting on the lens. [1]

Editorial calculation: for an actuator with f₀ = 80 Hz, the fastest ISRC setting gives 7 × (100 / 80) = 8.75 ms per move and the slowest 18 × (100 / 80) = 22.5 ms; ask the module maker which setting meets the focus-speed target without visible ringing in your orientations. [1]

03

Match actuator stroke, coil and driver limits

Commonlands describes its CLA321 voice coil motor for M12 lenses as rated for lenses up to 7 g with a full stroke of 400 µm, and more than 600 µm at 100 mA with a 5 g lens in vertical orientation; it lists rated current 120 mA, absolute maximum 150 mA, a starting current of 45–65 mA for 10 µm displacement, sensitivity of 5 to 40 µm/mA depending on lens weight and orientation, terminal resistance 37 Ω ±5 Ω at 23 °C, insulation resistance above 1 MΩ at 50 V DC, and a life test of 500,000 cycles at 23 °C (0.5 s on, 0.5 s off, 0–90–0 mA). [3]

TI's worked example uses a 100 Hz actuator measuring 8.5 × 8.5 × 3.4 mm with an M6 lens, 75 mg, 15 Ω and 100 µH: a 50 µm step settles into a ±5 µm window within 10 ms with 1/f compensation when f₀ stays within ±10%, and within 20 ms with 2/f compensation when it can vary ±30%. The driver also detects open and short circuits on the VCM pins. [2]

Editorial calculation: at 100 mA a 37 Ω coil needs 0.1 × 37 = 3.7 V across it and dissipates 0.1² × 37 = 0.37 W; that coil resistance is above the 11–22 Ω range in the DRV201 table and the 120 mA rated current exceeds its 102.3 mA DAC maximum, so check driver compatibility with both data sheets before fixing the pair. At f₀ = 100 Hz, 1/f is 10 ms and 2/f is 20 ms, the same order as the 7–18 ms ROHM figures. [3] [2] [1]

A university prototype shows the force and timing scale of a phone-camera VCM. Liu, Chang and Li at National Chung Cheng University designed an open-loop autofocus VCM with no spring plates, using a magnetic restoring force in place of the springs; their abstract reports a size of 8.5 mm × 8.5 mm × 7.5 mm, a maximum input current of 40 mA, a maximum response time of 10 ms, and an average measured Lorentz force of 2,200 mgw at 40 mA over the whole 0.4 mm stroke. [4] Editorial calculation: 2,200 mgw is about 2.2 gf, or roughly 0.022 N, and 0.4 mm equals the 400 µm full stroke Commonlands lists for its M12 VCM, which is rated at 120 mA. [4] [3] Editorial note: spring-less designs change how the lens holds position when the module is tilted, so ask for settling and hold data in each orientation.

Give the actuator supplier lens mass and thread, orientation, required stroke, and ask for sensitivity, starting current, coil resistance and inductance, resonant frequency with tolerance, and life-test conditions; confirm the driver's current and resistance ranges cover them.

04

Find autofocus actuator manufacturers

The companies in the manufacturer list below show this product on their own websites. Confirm the legal seller, factory and scope for your order.

05

Trace the actuator components

This is a functional reference built from the sources cited; it is not one maker's design.

06

Fill the specification fields with values for your module

An autofocus actuator quotation needs values for your camera module. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the lens, motion and driver data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Lens: [mass; size]
Motion: [stroke; sensitivity; f₀]
Drive: [current; coil resistance; driver IC]
Timing: [settling target; orientation]
Module: [footprint; height; temperature]
Supply: [life test; quantity]

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