Feature: Embedded
Figure 3: SCH16T’s interpolation output
Figure 4: SCH16T measurement parameters
internal mass. For further stability, the area of the sensor unit was turned downwards; see Figure 1. T e inverted connection pins give the sensor housing a “more neutral” behaviour. A further innovation of the redesigned ASIC is that the sensor
is connected to a processor unit. Eight diff erent fi lter settings are available – from 13Hz to 370Hz cutoff . T e dynamic ranges for gyro (1.5-8g) and acceleration (52-300°/s) can be selected via registers. Previously, there were two individual ASICs, each with its own
SPI; now, there is a single unit. T e interface originated from the automotive sector, so SafeSPI v2.0 can be used with 20- or 16-bit words with a simultaneous CRC checks. For applications where multiple sensor units are connected via a
bus, reading the data without using an individual MISO line can take longer than the internal registers’ update time. T is can result in a signifi cant discrepancy due to sampling of various time points of the individual sensors; see Figure 2. A SYNC pin was integrated for such instances, allowing multiple sensors to be addressable. If the MCU clock of the host system is used as a reference, the value
of the data counter can be combined with the value of the frequency counter, to calculate the precise time stamp of a measurement. T is combination is used if integration processes for sensor data are performed and the time uncertainty, or “data jitter”, of the interpolated data does not meet the system’s accuracy requirements; see Figure 3. T e SCH16T off ers various data output options for the sensor data.
Figure 5: Shaker Platine PCB
To improve data jitter, the gyro and acceleration data can be output in interpolated form. If the resulting quantity of data needs to be reduced, the “decimation” option helps. For dynamic systems, it is important that all available data is
ready to be recorded simultaneously from all axes. However, since the output data rate is at 11.8kHz, this can result in overloading the MCU. T e purpose of the “decimation” function therefore helps to reduce the internal update rate, to give the host processor enough time to read each individual sample. A third option is interpolated data output, which is well suited for
short-term dynamic ranges of up to 25g. Various registers perform the tasks of a self-test function, by conducting a continuous self-test of the gyro, accelerometer and start-up routines with writing a “0”. T e start-up time is max. 250ms, with a stable power supply.
PCBs T ere are two diff erent PCBs for this sensor: One has just the sensor on a PCB (SCH16T-PCB) for a shaker, the second (APNM-SCH16T) can also be used for a shaker and is suitable for surface mounting; see Figure 5. In addition to the sensor, the second PCB includes a powerful 2xFPU 32-bit processor, which, combined with licensable fi rmware (algorithms, CAN) from Nordic Inertial, serves as an evaluation kit and acts as an open IMU.
www.electronicsworld.co.uk May 2024 21
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