Vibration motors are used in a variety of applications including mobile phone handsets, game joysticks, handheld video games, pagers, toothbrushes, and razors, to name just a few. Of particular interest is the mobile handset market – with global production volume expected to be more than 1 billion units in 2007.
The handset market has driven innovation in the design and manufacturing techniques of miniature vibration motors. Smaller handsets have smaller PCB area and therefore require a thin motor design. Motor features for caller ID vibration tones and gaming applications are also being added to handsets. Most vibration motors consist of a small electrical motor that drives an unbalanced weight, as shown in Figure 1. The motors are direct current (DC) brush or brushless motors, and are configured in two basic varieties: coin (or flat) and cylinder (or bar).
Cylinder-type motors are simple brush motors with a traditional axial design, as shown in Figure 2. They are employed in a variety of applications but are undesirable in mobile handset applications due to their large size. Most cylinder-type motors feature the largest diameter space of all vibration motors.
All brush motors create sparks at the commutation points as the brushes switch the current in the motor coils. These sparks are excellent transmitters of broadband radio frequency interference (RFI). Brushes wear out and prove to be the major cause of motor failure.
The need for smaller, thinner designs led to the adaptation of brush motor technology into the coin-type motor (Figure 3). The commutator points that are in contact with the brushes energize the electrical coils in the rotor. Energizing the coils establishes a magnetic field strong enough to interact with the ring magnet integrated into the stator, causing rotation.
As shown in Figure 3, the commutation points are arranged in alternating polarity pairs. As the rotor moves, the coils are constantly reversing polarity as they pass over commutation points. In this way the motor continually rotates, and at a speed that is proportional to the applied voltage. The more complex brushes in coin designs are generally less reliable than their equivalent brush cylinder motors.
BRUSHLESS VIBRATION MOTORS
As discussed, brushless motors bring extended motor life and eliminate RFI by their lack of sparks. BLDC designs also feature the smallest diameter and are the thinnest coin-type motors in the industry. Figure 4 illustrates the basic construction of a brushless motor.
In this design, the rotor assembly includes the magnet as well as the weight that provides the vibration during rotation. The relatively bulky coils are moved to the stator, where they are connected to the controlling IC.
Digital commutation and linear soft-switching eliminates the sparks and therefore RFI interference. Allegro’s fully integrated A1442 Hall effect sensor and precision amplifier are coupled to an internal full bridge output through comparator circuitry that determines the proper commutation points. The third wire shown in the motor of Figure 4 is optional. This wire connects to an enable pin on the A1442 that can be used to control the active braking and sleep functions. It can be eliminated by tying the IC pin to the VCC on the PCB.
The A1442 is the only IC necessary to drive the motor. The functional block diagram in Figure 5 illustrates the device operation and advanced features. Notice in Figures 4 and 5 that the integrated Hall element eliminates the need for an external Hall element, thereby reducing the motor PCB component count to just the A1442. The only external component required is a standard circuit feature – a 0.1µF bypass capacitor located on the application motor mount PCB, which is used to minimize voltage spikes that are generated when switching an inductive load on the supply.
SOFT-SWITCHING AND COMMUTATION
When the device powers-up, it senses the magnetic field and activates the bridge. The active transistors are set according to the magnetic pole. A south polarity magnetic field activates the output transistors Q1 and Q4, driving current from VOUT1 to VOUT2 . As a north polarity magnetic pole approaches (due to rotation), Q1 and Q4 are turned off while Q2 and Q3 are turned on. This drives current from VOUT2 to VOUT1, thereby reversing the direction of current flowing in the coils.
Motor designs vary, but maximum rpm ranges from 9,000rpm to 15,000rpm. Most designs employ a four-pole rotor magnet, while a few designs use six-pole magnets. Using these parameters, it is easy to calculate the commutation switching frequency (fCS , Hz) using the following formula:
fCS = RPM × PP / 60, where PP is the quantity of pole-pairs.
For a four-pole magnet at 9,000rpm, fCS is 300Hz. For a six-pole magnet at 15,000rpm, it is 750Hz. Thus, the commutation signal and coil current switching events occur in the audible frequency range. The soft-switching drive algorithm is optimum for minimizing the audible noise and EMI produced by switching the inductive motor coil load by gradually reducing and then reversing current in the coils. The timing diagram in Figure 6 illustrates the switching behavior.
ACTIVE BRAKING, SLEEP MODE, AND ANTI-STALL
The A1442 BLDC motor driver has an integrated active braking function that can be used for fast stop-start cycling. Fast stop-start cycles are useful in mobile handsets for vibration ring tones, caller ID when the phone is in silent mode, and for gaming applications. The braking function is activated using the sleep pin, shown in Figure 5.
When a low signal is applied to the sleep pin, the motor device initiates the active braking function by reversing the polarity of the output bridge and thereby the current direction through the motor coils. The effect of reversing the current, and therefore the field, is to apply force to rotate the motor in the reverse direction, which will quickly decelerate the motor. After braking, the motor driver enters sleep mode by shutting down the active circuitry of the IC.
During sleep mode, the current consumption of the IC is typically less than 1µA. The sleep can eliminate the FET transistor on the customer PCB that would otherwise be necessary to switch power to the motor on and off. As a result, the motor can be permanently connected to the battery.
If the motor stalls, the driver will initiate an anti-stall algorithm. When a stall event occurs, the outputs will be continually turned on and off to restart the motor. The on-off cycles generate torque cycles that shake the motor and improve the probability of a start. It also prevents continuous stall current that can damage the motor coils.
Reverse battery protection is incorporated onto the A1442 to protect the device in case the motor wires are inadvertently soldered backwards on the PCB, making rework possible. If the outputs of the coil are inadvertently shorted when the device is powered-on, the motor driver’s thermal shutdown protection will disable the outputs as the IC heats up. The reverse battery protection feature and thermal shutdown have proven to be very robust features for assembly plants and for rework at OEM board assembly.
With the advent of very thin designs, the thickness of the vibration motor has become an important selling feature. The trend in motor thickness is moving toward 1.5mm and below. Additional design flexibility is obtainable using the EW package – an MLP (DFN) with an overall package height of 0.4mm maximum, and length and width dimensions of only 1.5mm × 2mm.
Click here for Illustrations:
Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7
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