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Texas Instruments LRA Actuators: How to Move Them? Application notes
Application Report
SLOA209 – November 2014
LRA Actuators: How to Move Them?
Mandy Barsilai ............................................................................................................ Haptic Products
ABSTRACT
This document describes from a high-level perspective the theory of operation for driving LRA actuators
and compares typical open-loop and closed-loop solutions.
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Contents
Background ...................................................................................................................
Moving an LRA ...............................................................................................................
Driving an LRA in Open-Loop ..............................................................................................
Driving an LRA in Closed-Loop ............................................................................................
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2
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List of Figures
1
Typical LRA Acceleration Versus Frequency Response ............................................................... 2
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Acceleration at Resonance ................................................................................................. 2
3
Acceleration Off-Resonance
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Pulsing Waveform in Open-Loop With Brake Problems ................................................................ 3
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Pulsing Waveform in Closed-Loop ........................................................................................ 3
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Double Click Waveform in Open-Loop With Brake Problems
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Double Click Waveform in Closed-Loop ..................................................................................
Buzz Waveform in Open-Loop With Over-Overdrive Problems .......................................................
Buzz Waveform in Closed-Loop ...........................................................................................
Closed-Loop Driving Signal With BEMF Feedback Signal .............................................................
Closed-Loop Driving Signal With BEMF Feedback Signal and Re-Synchronisation Feature .....................
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Background
Linear resonant actuators (LRAs) are resonant systems that will produce vibration when exercised at or
near its resonance frequency. All else being equal, an LRA will maximize the vibration strength when
driven at its resonance frequency (see Figure 1). However, the resonance frequency may vary due to
different factors, such as temperature, aging, the mass of the product to which the LRA is mounted, and in
the case of a portable product, the manner in which the product is held. LRAs tend to have high-Q
frequency response. Therefore, driving them with small offsets from the resonance frequency (typically 3 5 Hz) may result in a significant drop in vibration strength. Figure 2 and Figure 3 show an example that
compares acceleration at resonance versus 4 Hz away.
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Moving an LRA
Acceleration (g)
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Frequency (Hz)
¦(Resonance)
Figure 1. Typical LRA Acceleration Versus Frequency Response
Figure 2. Acceleration at Resonance
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Figure 3. Acceleration Off-Resonance
Moving an LRA
In the simplest of terms, moving an LRA consists of:
• Injecting enough energy at or near the LRA's resonance frequency to overcome the static friction to get
the mass moving
• Injecting the appropriate level of energy at or near the LRA's resonance frequency that results in the
desired vibration level
• Injecting appropriate levels of energy at or near the LRA's resonance frequency with 180° phase shift
in order to "brake" the actuator
Additionally, the actuator can be overdriven for a short period of time during the start or brake section of
the waveform to reduce the time it takes to go from a rest position to the desired steady state and vice
versa. Reducing the start-time and brake-time results in sharper, crisper effects. Refer to the data sheet of
the actuator for safe and reliable overdrive voltage and duration.
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Driving an LRA in Open-Loop
If the resonance frequency of an LRA is known, all that is needed to get it moving is to drive it at that
frequency and with an amplitude high enough to overcome its static friction. Actuator manufacturers
usually provide the typical resonance frequency of its actuators so this can be used as the starting point.
The resonance frequency of a particular LRA model will usually have small part-to-part variations due to
process and manufacturing variability; the more the part-to-part variations, the lower the part-to-part
consistency of the vibration strength.
2
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Driving an LRA in Open-Loop
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If braking is desired, the LRA can be driven at or near its resonance frequency with 180° phase shift in
order to stop it. Careful manual tuning needs to be performed for each actuator and each amplitude level
such that the LRA brakes optimally. If the braking signal is not strong enough or for long enough time, the
LRA will under-brake, which translates to longer brake times. If the braking signal is too strong or for too
long, the LRA will over-brake, which translates to the LRA moving in the opposite direction.
Figure 5 and Figure 7 show the advantages of having clean, sharp braking in a system. Figure 4 and
Figure 6 show the effect of poor braking in the acceleration waveform.
Clean Braking
Poor Braking
Figure 4. Pulsing Waveform in Open-Loop With Brake
Problems
Poor Braking
Figure 5. Pulsing Waveform in Closed-Loop
Clean Braking
Figure 6. Double Click Waveform in Open-Loop With Brake
Problems
Figure 7. Double Click Waveform in Closed-Loop
If overdriving is desired, careful manual tuning must be performed for each amplitude level such that the
LRA overdrives optimally. If the overdrive signal is not strong enough or for long enough time, the LRA will
under-overdrive, which translates to longer start-times and brake-times. If the overdrive signal is too strong
or for too long, the LRA will over-overdrive, which translates to the LRA moving beyond its desired level of
strength, potentially causing a "bump" in the haptic feel. The optimal overdrive signal can be defined as
the one that minimizes start-time and brake-time but is not perceptible in the acceleration waveform.
Figure 8 shows a buzz waveform with an open-loop driver and no braking. This waveform highlights the
effect that over-overdriving will have in acceleration. For comparison purposes, Figure 9 shows a buzz
waveform with a closed-loop driver and no braking.
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Driving an LRA in Closed-Loop
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Note that if overdrive and braking are used in an open-loop system, the manual tuning information must
be contained in the LRA's driving signal, which means that for different actuators, different driving signals
must be used.
Over-Overdrive
Overdrive
Overdrive
Over
Figure 8. Buzz Waveform in Open-Loop With OverOverdrive Problems
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Figure 9. Buzz Waveform in Closed-Loop
Driving an LRA in Closed-Loop
There are many types of closed-loop systems that can be implemented to control an LRA. A resonance
tracking closed-loop system for LRA (auto-resonance), constantly monitors the resonance frequency of the
LRA and adjusts the driving frequency to track it, maximizing the vibration strength for a given driving
voltage. Such an engine is present in devices such as the DRV2603, DRV2605, and DRV2605L. A level
tracking closed-loop system, once calibrated, regulates the output vibration strength to the desired level,
irrespective of variations of the resonance frequency. Such an engine is present in devices such as the
DRV2605 and DRV2605L.
A closed loop system that implements resonance tracking and level tracking loops have the advantage of
automatically driving at the resonance frequency, and automatically performing overdrive and braking at
optimum levels, removing the need for tedious manual tuning. An example of a closed-loop driving signal
and the feedback signal is shown in Figure 10.
Driving Signal
BEMF Signal
Figure 10. Closed-Loop Driving Signal With BEMF Feedback Signal
4
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Driving an LRA in Closed-Loop
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A weakness present in some closed-loop systems arises when the actuator exhibits too high of a static
friction for the desired levels of energy transfer, causing the actuator to not move. Since a closed loop
system relies on monitoring the actuator's movement in order to synchronize the driving signal's
frequency, a non-moving actuator can cause such a system to malfunction (since the feedback signal is
not valid). An example of such a scenario is when an LRA gets frozen. Advanced closed-loop drivers such
as the DRV2605L employ proprietary algorithms (1) to get around this limitation by driving a signal at a
default frequency if the driver fails to synchronize, and then automatically synchronize once the actuator
starts moving, such that features like auto-resonance and automatic overdrive and braking are available.
An example of a closed-loop driving signal with the feedback signal implementing the re-synchronisation
feature is shown in Figure 11.
Driving Signal
BEMF Signal
Re-synchronize
Figure 11. Closed-Loop Driving Signal With BEMF Feedback Signal and Re-Synchronisation Feature
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(1)
Patent pending control algorithm
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