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[FAQ] DRV2624: Closed and Open Loop Descriptions

Part Number: DRV2624


One of the most common questions when using haptics drivers is related to the different modes and configurations that our devices support. In general, all our haptics drivers solutions support open and closed loop modes. Depending of the actuator load and the end application, the user must select one or another. Here there are some considerations and advantages when using closed or open loop.

  • Closed-loop.

In some of the haptics devices, it is also named as smart-loop due to the advanced algorithms and configurations that involve the closed-loop system. A closed-loop mode consists in a device configuration based on the connected actuator. The internal algorithm optimizes the device parameters for a resonance tracking (in case of an LRA load), automatic level calibration, overdrive and braking levels, diagnostics routines, etc.

This closed loop mode is normally suggested for LRA loads. It is important to remember that the LRA requires of a resonance frequency to work. This frequency allows to reach the optimal value. Any variation of around 5Hz from the resonance frequency will cause a rapid drop in vibration performance. The smart-loop architecture allows to maintain the optimal frequency value even if there are tolerance values in the LRA specifications.

In both, the LRA and ERM loads, the overdrive and breaking algorithms allow to have the best actuator performance. The closed-loop feedback determines how high or how low must be the output voltage to accelerate or decelerate the actuator.

  • Open-loop.

Contrary to the closed-loop system, the open-loop mode doesn't have a direct feedback from the driver output. This mode is not useful for cases where the user requires of a diagnostic or an automatic calibration of the output level based on the actuator characteristics. However, there are some cases where the user does't require to perform an automatic calibration (which usually takes some seconds) in the application. So, the user may define the device parameters (rated voltage, brake and overdrive time, LRA period, etc) based on the experience and testing of many LRA and ERM loads.

When the auto-calibration is not required in production, the fixed settings procedure is suggested to perform with an open-loop mode. This procedure is applied before the manufacturing stage. It consists in taking many LRA or ERM loads and configure them with auto-calibration. After a minimum of 10 units tested, the user can take an average value and fix it for production purposes. In that way, the application/product won't expend some time to calibrate the actuator when it is powered on.