Haptic Energy Consumption
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1 Introduction
Haptics provide mechanical feedback through the use of vibrations to simulate specific events, surfaces, and effects. It can simulate different surfaces and effects by varying frequency, amplitude, duration, and direction of a vibration.
Figure 1-1 illustrates three types of actuators, Eccentric Rotating Mass (ERM), Linear Resonant Actuator (LRA), and Piezo Actuator.
Figure 1-1 Three Types of Actuators1.1 Eccentric Rotating Mass
An ERM is a DC motor with an off-center mass that spins to create vibrations. When the ERM rotates, the off-center mass results in a centripetal force; this kind of centripetal force causes displacement of the motor. People perceive this displacement as a vibration. The ERM vibrates because of rotation forces so there is acceleration on two axes (X, Y, or Z axis). This creates losses in unintentional axes in some applications.
Advantages:
- Easy to drive
- Low cost
- Flexible form factor (bar or coin)
Disadvantages:
- Slow response
- Acceleration is correlated with angular frequency(ERM)
- High energy consumption
1.2 Linear Resonant Actuator
An LRA is a spring-mass system that vibrates in a linear motion. Inside, there is a coil suspended by springs and when voltage is applied, the coil generates a magnetic field. The coil interacts with the magnet and mass, whereas the magnetic field varies with the applied drive signal, the magnet and mass move up and down creating force. This movement is perceived as a vibration.
Noticeable vibrations only occur at the resonant frequency due to the spring constant. Linear actuators must be driven within a narrow band (±2 Hz) around the resonant frequency; otherwise it results in a drop-off of acceleration (see Figure 1-2). Due to manufacturing tolerance, component aging, temperature and mechanical mounting, the LRA’s exact resonant frequency varies. The DRV260x and DRV262x Drivers with auto-resonance function can detect the resonant frequency, helping increase acceleration performance.
Figure 1-2 LRA Resonant Frequency
DriftThe LRA acceleration occurs in one axis because it vibrates in an up and down motion. This benefits an LRA to create more vibration strength and have more energy saving
Advantages:
- Faster response time than ERM
- Larger acceleration
- Higher efficiency
Disadvantages:
- Resonant frequency drift reduces acceleration
- Difficult to drive
1.3 Piezo Actuator
Piezoelectric materials are a type of materials that will deform (move) when a voltage is applied. Piezo haptic actuators enable precise actuation for high-definition haptics which corresponds to a faster start-up time, a higher bandwidth of drive voltage, lower audible noise, and stronger vibrations compared with ERMs and LRAs.
Piezo actuators come in two types: single-layer or multi-layer. The single-layer Piezo requires higher voltage to move some distance because Piezo actuators are capacitive loads. Multi-layer Piezo requires lower voltages but higher current. The DRV2667 can drive Piezo actuators up to 200 Vpp.
Piezo actuators vibrate when a voltage is applied, which causes a deformation in the Piezo material. The deformation in the material creates a flexing motion, which results in acceleration in one direction. Overall, Piezo actuators have stronger acceleration, faster response times, and lower energy consumption.
Advantages:
- Quick response time
- Wide frequency bandwidth
- Larger acceleration
- Higher efficiency
- Lower audible noise
- Available in a variety of shapes and sizes
- Flexible mounting options
Disadvantages:
- Needs higher voltages to drive
- More expensive compared to LRA and ERM
Table 1-1 shows a comparison of ERM, LRA and Piezo actuator.
| Attribute | ERM | LRA | Low Layer Count Piezo | High Layer Count Piezo |
|---|---|---|---|---|
| Performance | Good | Better | Best | Best |
| Acceleration (g) | ~1 | ~1-2 | ~3-5 | ~3-5 |
| Audible Noise | Very Noisy | Moderate Noise | Silent | Silent |
| Response Time | ~50 ms | ~30 ms | 0.5 ms | 0.5 ms |
| Energy Consumption | High | Low | Lower | Lower |
| High-Definition Haptics | No | No | Yes | Yes |
| Cost | $ | $$ | $$ | $$$ |
