Ultra-Wideband RF Solutions to help locate, anything, pretty much anywhere?
The answer is Decawave. Jamie Pegg talks us through three solutions.
A brief overview of the theory:
The aim is to measure distance between objects, in order to determine their precise location. The time taken for signals to travel between radio transceivers allows for calculation of range.
Two Way Ranging (TWR) can be used to calculate distance from Time Of Flight Measurements (ToF).
DecaWave’s DW1000 device is a highly-integrated transceiver specifically designed for micro-location systems. The DW1000 uses an Impulse Radio to create very short bursts of RF energy, spread over a large bandwidth.
For ranging, this Ultra Wide Band approach has significant advantages over narrow band technologies such as Wi-Fi and BT
UWB impulse radio advantages:
- RF pulse has “clean” edges, allowing precise determination of arrival time and hence distance.
- Short UWB pulses avoid reflected signal (multi-path) interference – multi-path is a major limitation for WiFi and Bluetooth.
There are a few ways the DW1000 can determine location using Time-of-Flight measurements:
- Two Way Ranging (TWR)
- Time Difference of Arrival (TDOA)
- Phase Difference of Arrival (PDoA)
These techniques have different merits, choosing the correct one for your application is a key initial design consideration.
Let’s look at each technique in more detail:
Two Way Ranging (TWR)
- 3 fixed position Anchors
- Software Location Engine (LE)
- A mobile Tag
- An optional 4th Anchor for elevation measurement
How does this work?
- The Tag measures the ‘round-trip’ ToF between itself and the anchors
- The Tag passes range measurements to the Location Engine (LE)
- LE calculates Tag position on a 2D plane
- 4th Anchor can provide elevation, allowing 3D positioning
- The LE can be a PC, dedicated MCU, or the Tag’s embedded MCU
Time Difference of Arrival (TDoA)
Basic Network Structure
Wired LAN network
Multiple battery Tags
How does this work?
- The Tags regularly send simple “Blink” messages
- All Anchors receive these Blinks
- The Blink arrival time is sent to LE over the LAN connection
- The Anchors are synchronized using the UWB Clock Sync Packets
Phase Difference of Arrival (PDoA)
This allows a single Anchor with two antennae, to determine the range and bearing of a Tag.
How does this work?
- Range is determined using TWR
- Bearing determination also uses the Tag’s TWR messages
- Bearing information is extracted from signals arriving at the two antenna
- Bearing can be calculated using either Phase Difference or Time Difference
TWR example use case – Geo-Fencing in Manufacturing or Construction industry
Virtual barriers to protect workers from hazardous equipment:
- TWR tracks a workers location, and if they enter a danger zone –
- Alarms sound, worker’s receive a warning alert or the equipment is switched to safe mode.
System is a Fixed Anchor network
Workers equipped with Tags
High Tag update rate
Battery powered Tags, recharged regularly.
TDoA example use case – Sports tracking
Real-time precision tracking of ball and competitors location
Statistics collection for individual players, post training analysis etc:
- A fixed TDoA anchor network, and the players and the ball or all tagged
Phase Difference of Arrival (PDoA) example use case – Follow me applications… drones, robots, first responders
Both the tag and the anchor are mobile – maintaining separation. The PDoA anchor in the following device calculates the range/bearing to the users tag, allowing a fixed separation:
- A mobile anchor with two antennae is set up
- The user carries a small battery powered tag and their is no fixed infrastructure
What tools are available?
Decawave have a great module: DWM1001.
It includes the DW1000 UWB transceiver and is paired with a Nordic nRF52832 MCU with BTLE and a 3-axis accelerometer.