Coin Vibrating Motors :: Design Considerations

General layout and operation

Brushed coin vibration motors are constructed from a flat PCB on which the 3-pole commutation circuit is laid out around an internal shaft in the centre. The vibration motor rotor consists of two 'voice coils' and a small mass, that are integrated into a flat plastic disc with a bearing in the middle, which sits on a shaft. Two brushes on the underside of the plastic disc make contact to the PCB commutation pads, to provide power to the voice coils which generate a magnetic field. This field interacts with the flux generated by a disc magnet that is attached to the motor chassis.

The commutation circuit alternates the direction of the field through the voice coils, and this interacts with the N-S pole pairs that are built into the neodymium magnet. The disc rotates, and due to the built in off-centred eccentric mass, the motor vibrates!

Equivalent Circuitry, Commutation, and Terminal Resistance

The commutator is formed by 6 segments connected to two coils. The equivalent circuit is shown right. The coils can be magnetized in 6 different ways, effectively making this a 6 pole machine. However, a peculiarity of this commutation design is that during one rotation the resistance through the brushes is not constant.

For a third of the revolution the brushes “see” the two coils in series instead of only one; which is why in some orientations the resistance seen by a circuit will be double and therefore the start current half the rated value.

The current figures presented in Conformity Limits Specifications sections of datasheets represent the worst case current draw; i.e. where the brushes see only one coil.

Start Voltages and Drive Signals

The full term 'Certified Start Voltage' is the lowest voltage that you can apply to the motor, and still be sure that it will start.

Coin vibration motors have a relatively high start voltage (compared to cylinder pager vibration motors

) which must be considered in designs. Typically this is around 2.3v (all coin vibrating motors have a nominal voltage of 3v), and failure to respect this could result in motors not starting when the application is lying in certain orientations. This problem arises because in the vertical orientation, the coin vibrating motor must force the eccentric mass over the top of the shaft on the initial cycle.

Due to the start voltage issue, we recommend that coin type vibrating motors are switched hard on and off at a voltage above the guaranteed start voltage, unless a well tested haptics driver is used.

Mounting

Coin vibration motors are designed to be easy to mount. They come with either spring PCB connectors or a high-strength long life self-adhesive backing sheet that is pre-attached to the underside of the chassis. The adhesive allows for a secure mounting of the vibration motor to a wide range of surfaces such as PCBs or flat internal surfaces of the enclosure, and makes manufacturing installation fast and clean.

Three brands of adhesive are typically used on our coin vibrator motors depending on availability (they all have very similar specifications).

These are:

• 3M VHB 9448
• Sony 4000T
• Nitto 5000NS

These 0.16mm thick adhesive tapes typically offer a 180 deg peeling strength of 15N/20mm, and a tensile strength of around 20N/10mm. The acrylic adhesive is considered to be resistant to most solvents, UV light, moisture and temperature extremes.

Like the use of any adhesives, the final bond strength is dependent on the cleanliness of the mating surface, and it is recommended that this mating surface is clean, dry, and offers a good unified fit to the motor backing-plate (on which the self-adhesive pad is stuck).

Spring PCB vibrator motors have spring-loaded fingers on the motor which mat with pads on the PCB. This makes assembly easier for applications where it’s desired to have the motor mounted to the enclosure. Also higher frequency harmonics are absorbed and reduced by a rubber ‘boots’ that enclose these kinds of motors motor.

If extra security is required, consider the moulding securing walls within the enclosure body. This technique is commonly used in mobile phones to ensure that the maximum amount of vibration is transmitted through the case.

Leads & Connectors

Our stocked range of pancake motors come with flying leads, with the exception of the 310-105 vibration motor which comes with sprung gold terminals that sit on top of the motor (shown right). This pancake vibrating motor is designed to be fitted to the enclosure and then the springs mate with pads on the motherboard PCB which makes the connection; a neat way to avoid routing flying leads around the case if there is PCB real-estate available. You can read more on spring & pad vibration motors as part of our PCB mounted vibration motor pages.

As standard all of our motors in the Pico Haptic™ range come with AWG 32, 45mm UL 1571 flying leads. Like all our motors, these pancake vibrating motors are available to purchase via our online store in 1+ quantities.

If you need guidance on how best to mount or connect any of our coin vibrator motors within your application, please don't hesitate to contact our engineering support team, who will be happy to help.

Summary

We have covered how coin vibration motors have different design considerations, despite sharing the same fundamental ERM characteristics as other vibrating motors. Importantly there are differences in the equivalent circuit and electrical characteristics of coin motors vs pager motors.

Precision Microdrives’ coin motors can be mounted onto PCBs or housing using self adhesive backing, or have spring pad terminals for connection. The flexibility in mounting, available lead varieties, and small form factor make them ideal for vibration alerting and haptic feedback applications.