Touchscreen user interfaces have become ubiquitous due to their distinct advantages over mechanical interfaces. For consumers, the interface is smooth and intuitive, and for industrial users, the sealed display avoids the intrusion of dust and moisture. But there are downsides to both types of users. For example, consumers may lose the convenience and satisfaction of flicking a mechanical switch, while industrial users have difficulty confirming button presses if they operate a touchscreen while wearing gloves.
Haptic feedback, which can indicate button operation via vibration, has been used to overcome the lack of tactile sensation in touchscreens, but existing solutions tend to use bulky and complex mechanical systems. These include Eccentric Rotating Mass (ERM) vibration motors and Linear Resonant Actuators (LRAs). Piezoelectric haptic feedback offers a more compact and flexible alternative. But, until recently, its high operating voltage made it difficult to use in low-power applications—a key requirement for battery-powered end products.
However, advances in piezoelectric “high-definition” haptic solutions not only address power consumption issues, but also bring other advantages to touch interface designs, such as compact and thin form factors, strong feedback, and fast response times.
This article briefly discusses the advantages of new piezoelectric haptic actuators over ERMs and LRAs, and then introduces a new generation of devices based on multilayer structures and bipolar drive modes that combine efficient and dedicated drivers to address previous power consumption issues. Next, the article uses touch example devices from TDK and drivers from Boréas Technologies and Texas Instruments (TI) to show engineers how to utilize these actuators and drivers in their next touchscreen haptic feedback product.
Haptic feedback options
To add haptic feedback to a touchscreen interface, designers have three options: ERM, LRA, and piezoelectric actuators. All three follow the same basic working principle, with a microcontroller overseeing the operation and a driver controlling the actuator to generate vibration (Figure 1). The difference is in the way the vibrations are generated: ERMs use eccentric rotating masses, LRAs use magnet masses suspended by helical springs, and piezoelectric devices rely on the (inverse) piezoelectric effect, which causes a crystalline or ceramic object to A dimensional change occurs when subjected to an electric field.
Figure 1: A haptic touchscreen system consists of a microcontroller, drivers, and actuators. In this example, the actuator is a piezoelectric device, but ERM and LRA are common alternatives. (Image credit: Boréas Technologies)
ERMs and LRAs are popular in portable designs mainly because they offer low voltage (~3 volts) options and only require simple driver designs. In contrast, conventional piezoelectric devices require much higher voltages (up to 200 volts) to generate enough mechanical deformation to provide a good consumer experience. These high voltages are needed because conventional piezoelectric haptic driver technology is often based on downscaled audio amplifier technology, rather than designed with low power consumption in mind. Another design challenge is the lack of dedicated low-power haptic actuator drivers, forcing designers to switch to less efficient solutions.
The main advantage of piezoelectric solutions, though, is that they support high-definition haptics, a feedback experience that goes beyond monotonous vibrations. For example, piezoelectric actuators produce vibrations of variable frequency and amplitude that can be used to represent different outcomes when a single button is pressed (Figure 2).
Figure 2: Piezoelectric haptic actuators support a range of vibrations that can be used to cue different outcomes from activating a single button. (Image credit: Boréas Technologies)
In summary, a high-definition haptic feedback solution requires:
Wide bandwidth: supports multiple vibration frequencies and modes
High acceleration [g]: Provides greater feedback force
Large displacement: Improves haptic feedback sensitivity
Low Latency: Fast response time increases feedback range
Table 1 summarizes the performance of each haptic feedback solution and confirms that piezoelectric solutions are the only choice for high-definition applications because they provide the required combination of bandwidth, feedback force and sensitivity, and delay.
Table 1: Comparison of operating characteristics of haptic feedback solutions. Piezoelectric devices provide the good acceleration (measured in “g”, where 1 g is the acceleration due to gravity on the Earth’s surface (9.81 m/s2)), displacement and response time, and custom waveforms required for high-definition haptic feedback. (Image credit: Digi-Key, via author)
Note that drivers for piezoelectric actuators are more complex, in part because of the need for additional functionality to generate custom waveforms to provide definition and context for haptic feedback. ERM and LRA do not support custom waveforms, so the driver is simpler.
Advantages of new piezoelectric haptic feedback devices
Recent product launches for piezoelectric actuators and high-efficiency specialty drives make this technology a better solution for battery-operated products. For example, recently introduced piezoelectric haptic products, such as TDK’s PowerHap B54102H1020A001 (12.7 mm2) and B54101H1020A001 (26 mm2), are less than 2.5 mm high and use a multi-layer construction rather than those used in devices based on audio amplifier technology single-layer structure. This multilayer structure relieves the driver requirements somewhat by lowering the drive voltage (to between 60 and 120 volts).
Each layer of TDK’s multilayer products expands only a small amount in the “z” direction due to the reverse piezoelectric effect. However, since the piezoelectric device must maintain a constant volume, the layers shrink simultaneously along the “x” and “y” dimensions.
TDK products use a pair of cymbals on either end of a piezoelectric device to mechanically amplify this contraction to increase z-axis motion by a factor of 15, allowing displacements of 35 to 65 micrometers (µm), depending on the model (Figure 3). Under a load of 100 grams (g) (unipolar operation, single-pulse sine wave, 200 Hz), the larger TDK actuator can reach a peak-to-peak acceleration of 30 g after only 1 millisecond (ms). The frequency range of 1 to 1000 hertz (Hz) allows designers to custom develop high-definition haptic feedback characteristics.
Figure 3: The TDK Piezo Haptic Actuator uses a multilayer structure and cymbals to amplify z-axis motion. (Image credit: TDK)
TDK piezoelectric haptic actuators can work in unipolar or bipolar mode. Unipolar operation applies a positive voltage to the actuator, while bipolar operation changes the voltage between positive and negative peaks. The advantage of bipolar operation is that a larger displacement can be achieved with the same peak-to-peak voltage, or the same displacement can be achieved with a lower peak-to-peak voltage. The disadvantage is that bipolar operation increases the mechanical and electrical load on the actuator (Figure 4).
Figure 4: Bipolar operation (right) provides the same mechanical displacement as unipolar, but uses a lower peak-to-peak voltage. (Image credit: TDK)
Chip suppliers have also recently introduced driver chips designed for haptic feedback applications. These enhanced designs are able to generate a range of vibration modes in a good frequency range and provide unipolar or bipolar drive characteristics while operating more efficiently than previous designs. Specific examples include the BOS1901CQT piezoelectric haptic driver from Boréas Technologies and the DRV2667 motor power driver from Texas Instruments.
The IC from Boréas Technologies is a single-chip piezoelectric actuator driver that uses energy recovery technology and is capable of generating multiple vibration signals. The chip can drive actuators up to 190 volts peak-to-peak from 3 to 5.5 volts. The BOS1901 uses a high-speed Serial Peripheral Interface (SPI) and all settings are adjustable through the digital front end. Typical startup time is less than 300 microseconds (μs), which reduces latency.
TI’s chip is a piezoelectric haptic driver that integrates a 105-volt boost switch and digital front end capable of driving both high- and low-voltage actuators. The digital front end relieves the microprocessor of generating pulse-width modulation (PWM) and frees the host system from the need for additional analog channels. The chip includes dedicated memory for storing and recalling waveforms, as well as an advanced waveform synthesizer. Typical 2 ms start-up time limits delays, and thermal overload protection prevents damage from overdrive.
Piezoelectric Haptic System Design
Both Boréas and TI chips can be used to run in touch systems that already include an application processor. The processor triggers the execution of the haptic feedback via the SPI. Additionally, designers can use analog inputs to trigger haptic effects (Figure 5).
Figure 5: An application circuit showing the TI DRV2667 motor power driver. Haptic events are triggered by the touchscreen application processor, which in turn drives the piezoelectric haptic device by the TI chip. (Image credit: Texas Instruments)
The design of piezoelectric haptic feedback touchscreen systems has become easier due to the level of integration of the latest drivers, but the choice of certain components is important to optimize design performance. For example, the boost voltage (BST) should be 5 volts greater than the peak voltage the piezo actuator is to withstand. This allows some amplifier overhead and is set up using the resistor divider network R1/R2 shown in Figure 5.
The resistor value is calculated as: V(BST) = V(FB) x (1 + R1/R2), where V(FB) = 1.32 volts.
So, for example, to achieve the TI driver’s maximum V(BST) capability of 105 volts, the R1 and R2 values should be 768 kiloohms (kΩ) and 9.76 kΩ, respectively.
The peak boost current is drawn from the power supply through inductor L1. This current is determined by R(EXT), but care must be taken to choose an inductor that can handle the programmed current limit (ILIM). The relationship between R(EXT) and ILIM is determined by the following formula:
where K = 10, 500, VREF = 1.35 volts, RINT (internal resistance of the driver) = 60 Ω, ILIM = peak current limit of L1.
Inductor selection is critical to ensuring optimum driver performance. For TI chips, the recommended inductance range is 3.3 to 22 microhenries (μH). The trade-off is whether to choose a larger inductor to reduce switching losses in the boost converter, or choose a smaller inductor to maximize its output current.
From a driver perspective, the key electrical specifications for piezoelectric haptic actuators are the voltage rating and capacitance. For example, at TI’s driver’s maximum frequency of 500 Hz, the device is optimized to drive up to 50 nanofarads (nF) at 200 volts peak-to-peak (the driver’s highest voltage swing capability). If the programmed boost voltage is lowered and/or the user limits the input frequency range to 300 Hz, etc., the chip can drive larger capacitors.
Another important component choice is the boost capacitor (C(BST)). Such capacitors must be rated at least as high as the boost voltage, preferably higher. For example, an X5R or X7R type 100 nF capacitor rated at 250 volts is recommended when running at the TI chip’s maximum boost voltage of 105 volts. C(BST) must have a minimum operating capacitance of at least 50 nF. For V(BST) of 30 to 80 volts, a 100 nF capacitor rated at 100 volts is acceptable; for V(BST) less than 30 volts, a 0.22 μF capacitor of 50 volts is recommended.
Due to switch pin current requirements, it is recommended to use a bulk capacitor (CBULK) next to the inductor. It is recommended to use an X5R or X7R type ceramic capacitor with a capacitance of at least 1 μF.
For engineers who want to experiment with the capabilities of TDK’s piezoelectric haptic actuators before committing to hardware, the company offers the single-channel Z63000Z2910Z 1Z 1 PowerHap evaluation kit. The kit includes a baseboard, a boost converter, an output driver board, and a microcontroller board.
The kit comes with configuration software that runs on a PC using Windows 7 (or later). After the software is loaded, the kit is connected to a PC via a USB cable and powered by a 12-volt (DC) power supply. The software then provides an interface for configuring the haptic response when the actuator is pressed. This interface allows configuration of the following signal parameters (Figure 6):
Amplitude – 5% to 100% (115V)
Frequency – 20 to 300 Hz
Waveform (Trapezoid, Sine Square, Sawtooth)
Trapezoidal Duty Cycle – 35% to 75%
Number of pulses – 1 to 1000
Trigger Level – 0 to 12 Volts (the lower the trigger level, the harder it takes to press the actuator to activate the signal)
Delay time (during which the actuator does not detect force)
Figure 6: The TDK PowerHap evaluation kit software provides a signal configuration interface. Once setup is complete, the configuration can be sent to the evaluation board processor via the “Transmit configuration” button. (Image credit: TDK)
In addition, the software enables engineers to create custom waveforms. Once the software is configured, the relevant information is sent to the kit’s processor via the USB cable.
The second evaluation kit, the PowerHap Z63000Z2910Z1Z44, is designed for engineers using the Boréas BOS1901CQT piezoelectric haptic driver. The kit includes a baseboard, a boost converter, two drivers, and a microcontroller. The basic kit comes with a TDK Piezo Haptic Actuator.
The evaluation board connects to a PC via a USB cable, uses the standard USB audio protocol, and outputs to any computer as normal audio. Waveform prototyping (up to 190 volts peak-to-peak) and playback can be performed using the USB audio protocol to prototype haptic effects in software such as MATLAB, Python, and Audacity.
Haptic feedback using piezoelectric, ERM, and LRA actuators enhances touchscreen control in consumer and industrial applications. However, with the development of low-voltage compact piezoelectric haptic actuators, the benefits of high-definition haptic feedback have extended to battery-operated devices.
At the same time, the design of piezoelectric haptic systems is simplified with the introduction of dedicated drivers that interface with popular application processors and provide various waveform support. Vendors such as TDK have provided evaluation kits for these devices, allowing experimentation and prototyping prior to hardware design.