Selecting the right piezo haptic solution can be tedious at times. The solution is composed of a piezo actuator and a piezo haptic driver (amplifier). Both are deeply connected and relies on each other to deliver the best tactile feedback. Therefore, selecting the right components is very important to achieve the best possible results.
In order to help you select the perfect combination, we will see what technical specifications are relevant to look for when you are selecting the components in your piezo haptic solution. This decision will involve many people from your team: product designers, mechanical engineers and electrical engineers will have to help you analyze the piezo haptic components. Let’s start with the first step, selecting the right piezo actuator for your application.
Selecting the Right Piezo Haptic Actuator
The product you are designing will have a major impact on which piezo haptic actuator and driver combination you will be able to use. You need to consider multiple factors, because integrating piezo haptics in a smartphone will be significantly different than adding haptics to an automotive display. You need to be able to answer four questions in order to select the best actuator:
- What’s the mass you are trying to move?
- How much volume or space is available?
- What is the power source?
- Do you need force sensing in your product?
The first thing to consider before selecting your piezo components is the force requirement. The feedback strength is based on Isaac Newton’s laws of motions. You can learn more on mechanical fundamentals of piezo haptic actuators here.
Newton’s second law of motion tells us the sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma. (It is assumed here that the mass m is constant)
The piezo actuator’s job is to generate acceleration. It’s the engine of your haptic solution. What you need to determine is the mass you are trying to move. If your mass is small, like replacing a button in a smartphone, you can achieve high acceleration with lower force (N). On the contrary, if you are trying to move a big and heavy automotive infotainment system, you’ll need higher force (N) actuator to achieve the same acceleration and tactile feedback.
When considering a specific actuator, moving a higher mass will limit the maximum achievable acceleration.
The second thing you need to determine before selecting your piezo actuator is the actual volume you have available to fit the component in your product design. Piezo actuators often have different shapes and can create force in different axes.
You need to be aware of how much space you can use to fit the actuator and determine in which axis you need it to create the desired vibration.
This will again depend on your application and the type of tactile effect you are trying to achieve. You may have to update your design according to the piezo actuator in order to integrate it, so it’s best to have a good idea from the start of what type of piezo actuator you will end up using.
3. Power Source
Another important aspect to consider before selecting your piezo haptics component is the power source of your product. Piezo haptics requires high voltage to work. This is the reason why you need a piezo driver; it not only controls the actuator, it also generates the high voltage required from the power source. Therefore, you need to be aware of your power source to be sure it will provide enough power to feed the piezo driver. A mobile device battery doesn’t offer the same levels of power as an automotive alternator, for example. Piezo drivers are designed within those parameters, they might not work on every power source. You need to select your components accordingly.
4. Force Sensing
Thanks to the reversible piezoelectric effect, piezo actuators can also be used as force sensors. Piezo material generates an electric charge when they are deformed. Applications where feedback is triggered by user input, like button replacement can make great use of this feature.
You now have all the relevant information to select the best piezo actuator! You can now select a suitable piezo driver to complete your haptic solution. Please keep in mind that it is important to have the actuator’s technical requirements not too far, they will help you select your driver.
Selecting the Right Piezo Haptic Driver
The piezo haptic driver selection is mainly based on your actuator selection. The two main tasks of your piezo driver are to amplify the voltage of your power source and control the actuator. Therefore, your actuator technical specifications are very important to select the best piezo driver. The relevant actuator technical specifications you need to select your piezo driver are the required output voltage, polarity, capacitive load and output bandwidth.
The piezoelectric effect is the interaction between the mechanical and electrical potential in crystalline materials. Applying voltage through a piezoelectric material changes the crystals electric polarization and creates mechanical deformation (the material can expand or shrink).
TDK PowerHapTM 0904H014V060 elongation measured between cymbal end-caps as a function of voltage. Source: 0904H014V060 Datasheet
The piezo actuator’s mechanical movement is the result of the applied electric field through the material. Therefore, it is important to respect the minimum voltage requirements for your piezo driver. Using less than the required voltage will result in less material movement and less feedback from the actuator.
TDK PowerHapTM 0904H014V060 force-stroke diagram with different load springs. Typical stiffness 150 N/mm. Source: 0904H014V060 Datasheet
Talking of the material polarity, you need to be careful when you select your piezo driver. Some actuator will be bipolar, meaning they can sustain waveforms with both positive and negative amplitudes and some are unipolar, meaning they can only work with positive voltage waveforms.
TDK PowerhapTM line-up for example is composed of unipolar piezo actuators. They only work with positive voltage. Our BOS1211 piezo driver is specifically designed for TDK 120 V PowerHapTM line-up and therefore, is also unipolar. The BOS1211 voltage range is 0 to +120 volts.
On the other hand, our BOS1901 piezo driver is bipolar, it can drive 190 volts peak-to-peak (+ 95 and – 95 V).
You need to be aware of your actuator polarity in order to select a piezo driver that can effectively work. Generally, a bipolar driver will be able to drive a unipolar piezo actuator, but the opposite is not true.
3. Capacitive Load
Piezo actuators have inherent capacitance, meaning they electrically behave like a capacitor. Normally ranging from a few nF to a few µF for haptic piezo actuators, the capacitance determines the amount of energy required to create a voltage that will induce enough strain in the piezoelectric material to create a vibration.
A higher capacitive load will required higher power to charge it quickly, so make sure your piezo driver supports the actuator capacitance.
4. Output Frequency Bandwidth
TDK PowerHapTM 0904H014V060 acceleration peak-peak as a function of frequency for different loads. Source: 0904H014V060 Datasheet
Frequency plays a significant role in haptics quality for three reasons:
- A higher frequency generally equals a higher acceleration and a stronger perceived effect for a given sense receptor.
- Our somatosensory system (touch sense) uses different receptors depending on the vibration’s frequency. Depending on the type of tactile effect you are trying to achieve, you need to be sure to use the appropriate frequency range for the appropriate touch receptor.
- The natural resonance frequency of the mass you are trying to move. The natural resonance is the frequency at which the mass resonates when it is vibrating, meaning the mass displacement is amplified to its maximum. Normally, a lower mass increases the natural frequency, while a higher mass reduces it. This concept is important for applications desiring to maximize the effect of continuous vibrations.
Piezo actuators have a wide frequency bandwidth, meaning they can vibrate over a continuous frequency range. You need a piezo driver that can provide the maximum frequency needed for the actuator capacitance and maximum voltage in order to achieve its highest acceleration performance.
- Eccentric Rotating Mass (ERM)
- Linear Resonant Actuator (LRA)
- Piezo - Competition
- Piezo - CapDriveTM
Source: Data extracted from Haptic Energy Consumption, Application Report SLOA194, Texas Instruments (TI), May 2014. Data point for Boréas was extrapolated from comparative measurements between the TI DRV8662 chip and Boréas’ driver.
While power consumption isn’t related to the actuator’s feedback quality, it is still a major factor when choosing a piezo driver. High-power consumption will drain your device battery and generate an excessive amount of heat that may limit the functionality or even be a hazard for the system.
Not all piezo drivers are power-efficient. Previous piezo driver technologies used inefficient architectures to amplify voltage and generate the high-voltage waveforms, and thus were power hogs. Boréas Technologies CapDriveTM technology architecture is different. It recovers the energy from the actuator internal capacitance and reuses it instead of draining all the battery power. This allows our drivers to use less than 10 times the power of other piezo drivers on the market and avoid any overheating issues.
Bonus: CapDriveTM Exclusive Force Sensing Feature
Another advantage of our CapDriveTM technology is the driver’s ability to sense pressure applied on the piezo actuator. Since the piezo effect is reversible, it means that the actuator generates an electric voltage when mechanical stress is applied to it. CapDriveTM drivers can detect this voltage change and allow a single IC to be used for both sensing and haptic feedback functions. This means, depending on your application and actuator integration, that you may be able to replace the complete sensing hardware with your haptic solution.