Beggers Low-End Tof Radar

[Mark Johnston]

I thought I would discuss something that I hope to show in our first challenge set in a week at the HBRC meeting.

There is really not enough time to get into stuff in those really short demos so here we go.

An ultra-low-cost Tof Sensor based 'Radar' subsystem is what I am adding now to an already existing little motor controller subsystem of mine.

The subsystem processor is my Mark-Toys.com Esp32 tiny processor Dev board I have mentioned and shown before in assorted HBRC demos (http://www.mark-toys.com/Esp32_Dev_Board.html)

Already shown at HBRC in 'DroidBot' a few months back was this board doing PID motor control combined with dual Encoder reading ports.   

Freshly added to this over the last week has been these two things:

- A PWM port drives a mini-servo and steps it through perhaps 7 to 11 locations.

- At each location the VL53L0X  Tof (Time of flight) ST sensor takes a reading and saves them locally with timestamp)

- At any time over a second serial port some host processor can query these ToF readings in a bunch and each reading has a 'time in ms since this query that the reading was taken'.

Now on to ROS...

Also on top of this my ros  'base_control' node that is a motor subsystem for a few of my bots (Fiddlerbot, RoboMadgeEllen and Droidbot) has had this added:

- Support periodic readback over serial link of the Tof Radar readings

- Publish each reading to /sensorX topics using Range.msg messages which include the ROS clock time MINUS the times in ms of each reading

So in this way we have a 'slow scan' partial circle 'lidar ish' scheme that costs $15 or so.   

Because this ToF sensor sort of 'wimps out' at 1-1.5 meters I hope to make this use the newer 7 or 8 meter version but had to back away from that for this rev as that is a lot of sotware I don't have yet BUT it is planned.

Now imagine that you place 3 or 4 of these on one 3D printed thing and then cover 360 degrees but with something like 15 degree accuracy.    Not a Lidar but not too shabby for crude but cheap room detection.

Oh and beside above 'Tof Radar' also added has been local sensors like edge sensors that can also be queried quickly by ROS main host and are put out on a topic for edge or other on/off sensors.

It is this bot that I hope to do the tabletop challange with and avoid 'phase 4' ...  falling off the table!

Mark

[Kirill Uvaev]

For this family of sensors datasheets mention FOV around 25-27 degrees:

https://www.st.com/resource/en/datasheet/vl53l0x.pdf - 25 degrees

https://www.st.com/resource/en/datasheet/vl53l1x.pdf - 27 degrees

Doe this mean that these sensors can't be really used for measurement of the distance for specific bearing?

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Kirill Uvaev

[Mark Johnston]

They are broad and more of value for local cost map or object detection.  At this time I have used about 20 degree angular spacing for a fair amount of overlap.  This is in no way able to replace a lidar with 1 deg angular spacing and near zero overlap.  That is why I am calling it a 'Beggers Low End' sort of radar.   This is meant as an alternative to the  sonar unit(s) and not have the longer delays for worse case sonar return times or echo issues.   Consider it just one more 'choice' with it's own tradeoffs is how I think about it.  As mentioned, I am to use this to 'look' for a coke can or so type object on a table and have a fair idea of how far away it is and so on.

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Kirill Uvaev

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[David Wyland]

You might also look at Sharp range sensors. Pololu carries them, as do Parallax and others.

https://www.pololu.com/category/79/sharp-distance-sensors

For example:  https://www.pololu.com/product/2474

"The GP2Y0A60SZ distance sensor from Sharp offers a wide detection range of 4″ to 60″ (10 cm to 150 cm) and a high update rate of 60 Hz. The distance is indicated by an analog voltage, so only a single analog input is required to interface with the module. The sensor ships installed on our compact carrier board, which makes it easy to integrate this great sensor into your project, and is configured for 5V mode."

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