I am trying to build devices to measure vibrations in the ground. They will be placed, under the ground, along a circumference at every 10m. They need to communicate and pass info to stations present at every 500m along that circumference. Also there is a power constraint since frequent extraction and burying of the devices is not feasible. Hence, the device should run for around a year on a button cell. The communication with the station will be infrequent(once a few days).

1. Which communication technology would be best suited for this(LoRa, zigbee, ble)?
2. Can antennas work at that depth(50cm) without spending too much energy?

Any insight or contribution is welcome as I am still in the brainstorming stage.

 

Welcome to AAC.

Unfortunately the combination of requirements this impossible. The attenuation of RF signals caused by burial would require a large amount of power to overcome. The distance to the receiving stations is too great to expect a small signal to work.

Let’s say you ran a feedline in from the hole and exposed a small antenna (just a floppy wire, it could be very low profile) then you might have a chance to get reliable data transmission, but the obvious technology—BLE—can’t be expected to make the 500m trip while maintaining the Low in the BLE.]

You said “button” cell but probably mean “coin” cell—the button cell is strictly impossible while even a coin cell, or two, would be depleted quickly. Other things like LoRa, with a very low data rate, could possibly live within your energy budget but would probably bust your financial budget.

You have to give up something. You could possibly wire the nodes together and get the low power goal, or you could increase the number of receiving stations. You might be able to make a mesh network, but it would be using power and your current scheme just doesn’t have enough.

If you can keep your device on the surface, or very close, and run wires to the sensor buried deeper and expose an antenna, and use one of the high capacity Li coin cells—maybe you could do something.

All this said, without knowing the problem this is supposed to solve there’s no real way to help well. If you can describe the problem you are trying to solve instead of the problem that is your naïve solution, we might be able to help solve that problem.

 

Welcome to AAC.

Unfortunately the combination of requirements this impossible. The attenuation of RF signals caused by burial would require a large amount of power to overcome. The distance to the receiving stations is too great to expect a small signal to work.

Let’s say you ran a feedline in from the hole and exposed a small antenna (just a floppy wire, it could be very low profile) then you might have a chance to get reliable data transmission, but the obvious technology—BLE—can’t be expected to make the 500m trip while maintaining the Low in the BLE.]

You said “button” cell but probably mean “coin” cell—the button cell is strictly impossible while even a coin cell, or two, would be depleted quickly. Other things like LoRa, with a very low data rate, could possibly live within your energy budget but would probably bust your financial budget.

You have to give up something. You could possibly wire the nodes together and get the low power goal, or you could increase the number of receiving stations. You might be able to make a mesh network, but it would be using power and your current scheme just doesn’t have enough.

If you can keep your device on the surface, or very close, and run wires to the sensor buried deeper and expose an antenna, and use one of the high capacity Li coin cells—maybe you could do something.

All this said, without knowing the problem this is supposed to solve there’s no real way to help well. If you can describe the problem you are trying to solve instead of the problem that is your naïve solution, we might be able to help solve that problem.

Thank you for your reply Ya'akov
Basically I am trying to make a warehouse surveillance system which can use vibrations to detect burglars moving around on the ground. Maybe 50cm is too deep. I am just taking into account the soil erosion because of wind and rainfall. We can reduce the depth.
Yes pardon for the wrong use of button cell...I meant a coin cell.

 

If you are burying these, why the constraint on battery size? What difference would it make if it was a few more cubic cm? Agree with @Ya’akov, an above ground antenna makes this so much easier.

 

Erosion? Wind and rainfall inside a warehouse?

 

If you are burying these, why the constraint on battery size? What difference would it make if it was a few more cubic cm? Agree with @Ya’akov, an above ground antenna makes this so much easier.

I want to keep the form factor as low as possible. But yeah if it is not practical then will definitely need to increase the battery size

 

Erosion? Wind and rainfall inside a warehouse?

It would be better to detect the burglar outside the warehouse than inside it. Along the perimeter. So these factors will come into play.

 

If you are intending to actually bury a device to detect vibrations, bury a single coaxial cable as well. Each device can be powered from the one cable, and each can have an identification number to send along with it's report. This is a common scheme and it works very well. An added advantage is that it is much more resistant to interference as well as jamming. With external power you will not need to be so very low powered, either.

 

I am not sure why you have settled on vibration sensing as your detection method but I think you might be batter off with something like FMCW RADAR presence detectors which can provide a lot more information and would be far more realiable.

 

Hi Avi,
Have you actually carried out a small controlled surface area test, of this proposed vibration detection method, using an actual rat as the vibration source?

In the real World, you will get ground vibrations due to natural and man-made sources.

E

 

^ as Eric says.
What software will you be using to distinguish footsteps from the background noise (which I suspect would be of similar or greater amplitude) ?

 

If you are intending to actually bury a device to detect vibrations, bury a single coaxial cable as well. Each device can be powered from the one cable, and each can have an identification number to send along with it's report. This is a common scheme and it works very well. An added advantage is that it is much more resistant to interference as well as jamming. With external power you will not need to be so very low powered, either.

Thank you for the suggestion. However, I am trying to create a solution which would require little to no installation effort. I know it is quite difficult to achieve what I want without wires and a coaxial cable will solve a lot off issues. I will have to fall back to it if I don't find a feasible wireless solution

 

I am not sure why you have settled on vibration sensing as your detection method but I think you might be batter off with something like FMCW RADAR presence detectors which can provide a lot more information and would be far more realiable.

The reason I chose vibrations is because I want to design a portable system. A system that can be installed anywhere without special needs of power outlets, wires etc. I am thinking about training the data over a period of time to be able to determine from the vibrations if an intruder has entered the zone and MCU will come out of the deep sleep only when a particular sensor threshold would be detected. Then the MCU can contact the main station. This way, I think, it would require less power and the system can stay totally wireless.
Thank you for suggesting FMCW RADAR. I am apprehensive about how much power it would require. Wouldn't it require constant power supply?

Is it possible to use the wall as the source of vibration? I am assuming that the intruder will have to climb the wall and thus will make contact with it. The device can be placed on the wall. It can maybe solve the signal power issue since it will be above the ground. The sensor can be placed in contact with the wall to detect the vibrations.

 

Hi Avi,
Have you actually carried out a small controlled surface area test, of this proposed vibration detection method, using an actual rat as the vibration source?

In the real World, you will get ground vibrations due to natural and man-made sources.

E

No I haven't. Yes there would be a good amount of natural and man-made vibrations. I might be unsuccessful but this is what I am planning to do:
1. Initially I'll collect large amounts of data.
2. Train the system to differentiate the vibration signature of humans from other sources.
4. Figure out the sensor threshold. This threshold will vary depending upon the environmental conditions(rain, wind).
5. When deployed the system should have the capability to set device threshold wirelessly so that it works even when there are changes in the ambient conditions.

 

^ as Eric says.
What software will you be using to distinguish footsteps from the background noise (which I suspect would be of similar or greater amplitude) ?

Not thought that far ahead. You are right about the noise. It is quite likely that it can be greater in amplitude than the vibrations I am trying to detect. Will need an appropriate vibration sensor(assuming one exists for my use case) to make it work. I am assuming a great deal of filtering and signal processing will be involved as well. I plan on extensively training my system and figuring out the vibrational signature I need. If successful I can maybe set the threshold of the MCU so that it wakes up anytime the sensor detects a similar signature. The device can then contact the station. I plan on adding the functionality to program the threshold dynamically according to ambient conditions.

 

Is it possible to use the wall as the source of vibration? I am assuming that the intruder will have to climb the wall and thus will make contact with it. The device can be placed on the wall. It can maybe solve the signal power issue since it will be above the ground. The sensor can be placed in contact with the wall to detect the vibrations.

 

For protecting a large area, such as a warehouse, or an outside storage area, either radar, as "Y" suggested, or video with either an IR camera, or IR motion sensors could cover a much larger area.
Ground vibration sensors were tried during the Vietnam war, and the results have not been shared, other than to observe that scheme is not used now. Cameras with motion sensing software can be quite sensitive, and offer the option of providing both an actual view and also real-time watching.

 

Most deep into the soil penetrates ultra-low frequencies 10....100 kHz - so deep as half meter or at larger power even one meter deep I suggess to pull out the small duck-neck antenna from the soil and that is enough to creqte the 1...3 km afar well radioficated zone. 0.5...1 km for 2.4 GHz Nordic NRF024L at price 0.99 USD per piece, up to 3 km distance for 868 MHz Lora TM at price about 10 USD but the speed is multiple tens fold slower (however Your data bitrate seems not be any high). Probably cheapest solution is NRF025L transltor every 500 meters making the radiorelay-line.

PS: This NRF looks great bargain for 1 USD per piece https://www.aliexpress.com/item/1005007149944834.html?spm=a2g0o.productlist.main.7.65bdV7FeV7Fe1c&algo_pvid=cabd9b76-da20-4853-8d20-442fc2a566bc&aem_p4p_detail=202409260532272161662007820400005031737&algo_exp_id=cabd9b76-da20-4853-8d20-442fc2a566bc-3&pdp_npi=4@dis!EUR!9.59!9.59!!!10.43!10.43!@210387dd17273539472802588eddba!12000039598233797!sea!LV!929901862!X&curPageLogUid=5ONtnbx40xJF&utparam-url=scene:search|query_from:&search_p4p_id=202409260532272161662007820400005031737_1
And for case of LORA - this is well far-beating (5 km) per 5 USD per piece https://www.aliexpress.com/item/1005002079395324.html?spm=a2g0o.productlist.main.1.6199XAsjXAsjlt&algo_pvid=c3317e95-4681-4cda-a046-8462ec9462f6&algo_exp_id=c3317e95-4681-4cda-a046-8462ec9462f6-0&pdp_npi=4@dis!EUR!6.81!5.79!!!7.41!6.30!@210386d117273540429692802ef75a!12000018670645284!sea!LV!929901862!X&curPageLogUid=Uo2k9pwejuCR&utparam-url=scene:search|query_from:

 

The reason I chose vibrations is because I want to design a portable system. A system that can be installed anywhere without special needs of power outlets, wires etc. I am thinking about training the data over a period of time to be able to determine from the vibrations if an intruder has entered the zone and MCU will come out of the deep sleep only when a particular sensor threshold would be detected. Then the MCU can contact the main station. This way, I think, it would require less power and the system can stay totally wireless.
Thank you for suggesting FMCW RADAR. I am apprehensive about how much power it would require. Wouldn't it require constant power supply?

Is it possible to use the wall as the source of vibration? I am assuming that the intruder will have to climb the wall and thus will make contact with it. The device can be placed on the wall. It can maybe solve the signal power issue since it will be above the ground. The sensor can be placed in contact with the wall to detect the vibrations.

Yes, it would require rethinking the method of deployment. I would be reduced to fewer, higher profile nodes that used much larger batteries and possibly solar charging. You would be able to cover relatively large areas with newer nodes. I assume, too, that vehicles would have to be involved and the currently available modules from Hi-Link during the 24GHz FMCW include ones that can discriminate multiple vehicles, their speeds and direction.

By providing two layers—an outer one looking for vehicles headed in your direction and a inner one that looks for people approaching entry points I would expect much better outcomes with more information about the actual threat.

 

I think you are getting ahead of yourself hear. A couple of people have pinted out, but Ill state the obvious
Prototype your vibration detector and see if it works. Just power it off a big battery. prototype your detection software and test. can it be built into a small enough processor and can it consume small amounts of power. If it consumes a lot of power then you have to figure out how to reduce that or whether a larger battery is required. Can you transmit the data? is it small packets (suitable for Lora) or is it more. Then can you transmit the data over RF? what frequency/aerial to use. Transmission of RF through ground is generally difficult.
Small steps