There are numerous core technologies to consider when designing-in haptics. Certain approaches allow smaller solution size, lower BoM, reduced power consumption and increased responsiveness. By Steve Sheard, Senior Applications & Marketing Engineer, ON Semiconductor.

There is no escaping the fact that touch interaction within user interfaces (UIs) has grown in popularity immensely over the last decade, replacing many different forms of mechanically-based UIs. Engagement in this way is seen as more intuitive and less likely to lead to user frustration. What haptic technology does is bring a whole new dimension to touch control – allowing the user to receive feedback on operations performed via mechanical vibration or, in other cases, some kind of resistant force. Through this, it is possible to ascertain whether or not the intended operation has been completed successfully.

There are various everyday examples of haptics in action. Some examples include vibrations on a game’s console when a steering manoeuvre is made in a racing game, or a ‘shoot em up’ could imitate the shockwave from an explosion or the recoil from a gun being fired, making these games feel more realistic. Alternatively, it could be the vibrating of a smartphone handset to signify the receipt of a text message or an incoming call. The vibration of handheld devices provided to customers while waiting to be seated in a restaurant to let them know that the staff are ready to take them to their table. Haptics are now even being incorporated into childcare products such as teething rings.

Haptics deliver a degree of sensory stimulation to an individual operating a UI - but how does this actually happen? The basic procedure is as follows. As the user places their finger onto a button displayed on a touch screen, the touch controller passes corresponding touchpoint data over to the system processor to deal with. At the same time, the processor activates the haptic. This, in turn, initiates the motor, which then generates a vibration. The motor which generates the vibration needs to be driven by a suitable haptic driver IC.

Categorisation of Haptic Motors

There are several different types of haptic motor actuator systems currently used in UI designs.

The eccentric rotating mass (ERM) type - is where an off-balance mass is caused to rotate. The mass produces an asymmetric centripetal force, leading to the motor’s displacement.

The cylindrical type - is similar to ERM but is comparatively large and, because of this, has a slower response to a vibration request. Neither the ERM type nor the cylindrical type are particularly durable.

The linear resonance actuator (LRA) type relies on a magnet attached to the case by a spring. The magnetic field from the coil causes vibration to be initiated (similar to how vibrational movement is produced in an audio speaker so that sound can be created). The vibration is at a single frequency.

LRA haptics technology offers several key benefits - including more compact packages, quicker response times and greater operational robustness than alternative cylindrical or ERM haptic solutions. As a result of these characteristics, LRA is particularly well aligned with modern portable applications - covering everything from smartphones and tablet computers to wearable electronics. An LRA approach needs substantially less peripheral circuitry to accompany it.

Figure 1. The basic structure of a linear resonance actuator (LRA).

Haptic Driver Solutions

In the past, driving system haptics was usually accomplished via the implementation of discrete solutions. These would generally consist of a clock generator with two buffer amplifiers or a sine wave generator with an audio amplifier.

Figure 2. A discrete haptic driver based on a clock generator.

Figure 3. A discrete haptic driver based on a sine wave generator.

For clock generator haptic drivers, the buffer amplifiers work together with the clock generator to increase the amplitude of the output. They also help to smooth off the sharp edges of the applied voltage profile that would come from the square wave output of a clock generator. This type of driver also has a relatively high current consumption and requires the specific of several additional external components. These factors hamper this solution’s suitability for space-constrained and power limited applications like handheld electronic products.

With sine wave generator haptic drivers, the sharp edges that characterise the clock generator driver are no longer an issue. This approach offers a much smoother response, and the moving mass vibrates without the risk of it hitting the sides. Once again, however, the number of external components involved means that valuable board real estate has to be allocated.

Though traditional haptic driver solutions have had drawbacks, in more recent times, dedicated, highly integrated haptic motor driver ICs have started to be introduced. These ICs can surpass the performance of the discrete sine wave generator and clock generator solutions because of certain key features - such as their ability to alter their driver frequency (which, as we will see, is highly advantageous).

Driving the LRA at a frequency that matches the resonant frequency of the motor (FR) will maximise the response of the haptic element of the UI. It must be noted that FR can actually alter by as much as 1.0 Hz, depending on its orientation, the ambient temperature, or the material the LRA is lying on. For example, the FR will be different if the product containing the LRA is held in the user’s hand, is resting on a hard surface, is hanging from a strap, or is located in the user’s pocket.

Suppose the conditions are such that insufficient vibration is actuated. In that case, the vibration force can either be increased by raising the driving force or, alternatively, by adjusting the drive frequency to match the new resonant frequency. Clearly, it is more efficient if FR can be tracked and the drive frequency adapted accordingly to match it. Conversely, if the motor driver cannot harmonise itself with the motor’s FR, the resulting vibration strength will be diminished, and extra power will need to be expended to make up the shortfall.

Figure 4. A high-efficiency haptic driver based on the LC89830x from ON Semiconductor

Recognising that the ability to tune the frequency is of great value in haptic implementation, ON Semiconductor has developed a highly efficient LRA driver series, the members of which can be controlled by a single enable pin. Thanks to their auto-tune feature, the LC898300, LC898301 and LC898302, can automatically adjust their drive frequency to mirror the changes in the motor’s drive frequency. This can increase the force of the perceived vibration by over 20%, making it far more efficient than conventional haptic driving solutions.

These devices can produce the same level of vibration force as the sine wave drive method but use 20% less current. They do this by cutting in half the on/off periods in the drive current. By rounding off the corners on the output, there is no audible noise to contend with. In addition, through the brake function, the vibration can be turned off much more rapidly. 

This means it can be used to deliver a broader spectrum of haptic effects - opening up new possibilities for UI designers. Furthermore, they only require a solitary external bypass, thereby reducing the use of board real estate and lowering the overall bill of materials costs - both of which are highly important in space-constrained cost-sensitive consumer electronics designs. These drivers are initially configured using an I2C interface, and once complete, a single wire can be used to turn them on and off.

Figure 5. A comparison between square wave and LC89830x drive profiles

Haptics effectively make touch interaction a two-way thing and provide feedback to those operating electronic equipment to ensure that the system has received the desired input and safeguard against potential errors. Alternatively, they can mimic specific actions to enable markedly better user experiences to be derived. Through their incorporation, supported by an optimised driving technology, more advanced touch-based UI solutions can be created, delivering a higher level of engagement and differentiating themselves from standard touch-enabled UI’s.