Designing Haptic Feedback into Your ElectronicsBy ZM Peterson • Feb 1, 2020
From medical devices to augmented reality, haptic feedback is critical for making a digital experience more immersive and responsive. Haptic feedback might seem to be an opaque part of the electronics landscape, but it is quite easy to bring haptic feedback and vibration into your next system.
Bringing haptic functionality into your new product is as simple as placing transducers at strategic spots in your board. However, the electronics required for driving a transducer and the algorithm required for effective feedback need to be designed correctly for your new product to provide precise haptic feedback to the user. With the right tactile sensations built into a new product, users can eventually learn to respond automatically to a transducer vibration and create a more satisfying experience.
Designing with Haptic Feedback
Haptic feedback refers to tactile feedback technology. This technology uses vibration, force, or other physical phenomenon to provide information to the user of a device. The most common method is with vibration stimulus, where a vibratory signal is applied to the skin. If you’ve played on a VR system recently, then you’ve probably felt tactile sensations on these systems to make the gaming environment seem more realistic.
Haptic feedback enjoys plenty of applications outside of gaming and VR. The medical field, specifically virtual surgery and replacement limbs, makes use of vibratory haptic feedback to provide tactile sensations to the human body, giving the sense that the user is interacting with a real environment. Haptic feedback also provides a means for a user to control their motion in an artificial environment, or with an artificial limb. The use of haptic feedback with artificial limbs is still an ongoing area of research, both in terms of human perception of tactile feedback and algorithms for controlling haptic feedback.
The important points involved in designing products with haptic feedback are the following:
- Tactile feedback mechanism (varying frequency, varying amplitude, or both)
- Component selection for providing the desired feedback
- Developing a control algorithm for haptic feedback
- Placing everything on a PCB
This PCB for a position sensing contains all the components needed to support a vibration motor for haptic feedback.
The last point is arguably the easiest. Haptic vibration motors run at approximately 150 to 180 Hz as this is the frequency to which human skin is quite sensitive, and the frequency is low enough that it only appears as a semi-audible vibration. These vibration motors do not generate the same level of electrical noise as a DC or AC motor. They consume low enough power that they can be continuously operated with a large coin cell battery for almost 24 hours, and they are small enough that they can be connected to a board with a pair of wires. Some haptic vibration motors can be surface-mounted to a board without taking up much space. This is one reason haptic feedback functions are so easy to place in mobile/IoT devices. In total, the PCB design portion of a system with haptic feedback is no more difficult than any other system for driving a small transducer.
Driving a Haptic Vibration Motor
Motors or vibratory elements can be placed as piezo elements or as mechanical motors. Although frequency control is possible with a number of motors, humans have much greater sensitivity to changes in the vibration amplitude. The motor you choose should be controllable for variable frequency, variable amplitude or both. This area has become more heavily commercialized recently, and there are a number of components that can be easily brought into a PCB for haptic feedback in a new product.
The key to properly driving a haptic vibration motor is simply one of choosing the right motor. Some common types of motors include coin-style motors, linear motors, and linear resonant actuators (LRA). Most motors are voltage-controlled, meaning the applied voltage level will determine the amplitude and frequency of the motor. This is similar to a DC motor; increasing the DC voltage level will increase the motor frequency. This will also change the motor’s amplitude. Voltage-controlled motors can also be driven with an AC signal, which allows the frequency and the amplitude to be controlled simultaneously. Finally, an LRA is unique in that it has a narrow band near ~160 Hz where the motor resonates. This type of motor can be driven with an oscillating waveform (sawtooth, sinusoidal, etc.) with a fixed frequency, and varying the signal amplitude will vary the vibration amplitude.
To properly drive these motors, impedance bridging or impedance matching may need to be used. You should check your driver datasheets and your component datasheets to determine whether maximum power transfer, maximum voltage transfer, or maximum current is required. Note that the relevant frequencies in these systems are sub-khz, so there will not be any transmission line effects. Whether power, voltage, or current needs to be maximized at the haptic motor depends on how the driver works. I’ll get into this more on an upcoming article about impedance bridging.
Haptic Feedback Algorithms
Haptic feedback algorithms are effectively negative feedback control algorithms, which resemble proportional, PI, PD, or PID control algorithms in process engineering. The idea behind negative feedback is that the output signal from the control system will build up and eventually saturate at some desired signal level.
Simple yet effective open loop haptic feedback techniques can be implemented at the hardware level by simply reading sensor values through an ADC. The input sensor level is translated into an output voltage level, which triggers the haptic transducer. The voltage level simply modulates the vibration amplitude (for a linear resonant actuator) or the vibration frequency.
More advanced haptic feedback strategies require a closed-loop control technique, where the output from the haptic system is measured and fed back into the system input. Haptic feedback algorithms need to be designed for a specific system and need to meet particular requirements. These algorithms are still an active area of research, and there is plenty of innovation to be seen in this area.
Simplified block diagram for a closed-loop haptic feedback algorithm.
Haptic feedback algorithms are typically fast enough that they can be embedded in a standard MCU or small FPGA. Taking a modular approach is a great way to bring hapic capabilities into your next system as haptic motors do not require special signaling standards or layout practices. They won’t generate significant electrical noise, so most equipment will be robust enough to withstand any electrical noise.
If you need a PCB design firm that can help you design haptic feedback into your new wearable product, we’ll develop the hardware and firmware your system needs. We’re also a digital marketing firm, and we’ll develop a comprehensive strategy to market your new technology and engage with your target audience. Contact NWES today for a consultation.