Introduction

Time of Flight, ToF sensor used in Nordic ID FR22 extension BFA, beam forming antenna can be used to measure the distance to an object. The sensor shoots 16 beam grid (4x4) of infrared light and detects the reflected light and can then calculate the distance for each cell.

The sensor is typically used to detect movement under a ceiling/high-mounted device as part of an EAS, electronic article surveillance system. Because of the grid of 16 cells, the direction of movement can also be determined. This is not the only application; an SW solution can be created for other use cases.

Operation

The sensor is based on 16 separate units that shoot invisible infrared light laser beams into the human eye and detect the reflected light. Typical mounting height in ceiling or frame is about three meters. The sampling frequency is up to 60 Hz.

IoTEdgeClient

The easiest way to utilize ToF sensor for EAS is to use the IoTEdgeClient application for FR22 reader. Application can be installed using FR22 Software/App Center utility.

The application has a convenient web UI for defining various parameters.

The key parameters regarding ToF sensor

ToF Mode

Off – ToF usage is disabled
Presence – Something is detected in ToF grid
Direction – Movement direction in the grid is detected

ToF Alert Event – the event that triggers ToF alert

InToOutEarly
1. Movement from inside outwards is detected, and the alarm is triggered early.
InToOut
1. Movement from inside outwards is detected.
OutToInEarly
1. Movement from outside inwards is detected and alarm is triggered early.
OutToIn
1. Movement from outside inwards is detected.

Adaptive Distance Calibration

If enabled, the sensor is automatically calibrated to determine the distance to the floor.

Distance to Floor

If the distance to the floor is known, it can be inserted here. Note that Adaptive distance calibration ignores this value.

Other parameters in IoTEdgeClient are not ToF related but are briefly explained here for completeness.

Tag detection

Tag detection parameters determine which tags will trigger an alarm. BFA has 28 software-controlled RF beams, of which one or more can be enabled.

Detection beam

which BFA antenna beam is used to detect tags. Only one beam is used.

Detection TX Level

The RFID TX power used in the detection beam

RSSI Filter

This filter can be used to exclude tags that have too low RSSI value

Movement sensitivity

How easily a tag is detected

EPC mask

One or more EPC mask values that indicate potentially alarming tag

Tag muting

Tag muting blocks alarming tags that are visible to the reader but indicate no movement. Parameters are:

Muting beams

The antenna beam(s) used in tag muting process

Muting TX level

The RFID power used in the muting process

Manual Muting EPC List

Tags can be explicitly excluded from the detection process

Unmute timeout

Time in minutes after which a muted tag will be removed from the muted list

Software API

If IoTEdgeClient is not suitable for a purpose, your own application can be created for a specific use. Nordic ID Accessory API is part of NURAPI control interface and is used to control and get data from the ToF sensor.

The sampling frequency of the sensor can be adjusted between 1 – 60 Hz. It is done using C# commands:

var sensorSettings = new NurApi.AccessorySensorSettings();
sensorSettings.frFba.freqHz = hz;

To receive the data the application shall subscribe to the raw data event:

NurApi.AccessorySensorToFFrBfaRawDataEvent += MyToFEventHandler;

The event handler will then receive data from the sensor according to the set sampling frequency. The content of the received data is a 16 sized array where each item contains information about one cell:

ushort distCm – distance to an object or floor in centimetres
byte status – the status of the cell

0   Ranging data are not updated
1   Signal rate too low on SPAD array
2   Target phase error
3   Sigma estimator too high
4   Target consistency failed (Wrap Around fail 1/2)
5   Range valid
6   Wrap around not performed (Typically the first range)
7   Rate consistency failed (Wrap Around fail 2/2)
8   Signal rate too low for the current target
9   Range valid with large pulse (may be due to a merged target)
10  Range valid, but no target detected at previous range
12  Target blurred by another one, due to sharpener
15  No target detected

If the status is valid the measured distance can be trusted.

The grid of 4x4 light beams orientation can be determined from the device LEDs position. The edge where the LEDs are is the “outward” direction. The grid indexing is the following:

LEDs