Passive RFID Temperature sensing

[Don M]

Does anyone here have any experience in this? My application does not allow me to drill holes into the cavity with which I am wanting to monitor the temperature of so I am looking for a wireless and possibly a batteryless solution. The distance would be several inches or less. Prefer not to require a battery in the remote portion although something with very low low power that would run for many months to a year would be acceptable.

Thanks.

Don

[Mike Green]

A battery powered sensor and some kind of wireless connection would certainly do. What kind of material is the cavity made of? How much room is there? What kind of temperature range? How often do you have to make a reading?

If you had a transparent window in the cavity, you could possibly use IR for communications using a battery. If there's an opaque, but non-conductive window, you could use low frequency electromagnetic signals like RFID uses with a small coil on both sides of the window. In this case, you could provide power as well. The simplest scheme would have a diode rectifying the received signal from the coil and storing power in a capacitor. The external controller would provide a signal for power for a period of time, then cut off the signal for a short time. The internal controller would listen for this and send either a short or a long pulse of energy at the same frequency to denote a bit value. This would be repeated enough times for all the bits and a checksum to be sent. A long period of power without a pause would reset the internal controller for the next sequence of bits.

[Don M]

Mike- Thanks for your thoughts. Think of a small chest freezer where you have a plastic inner liner and a metal outer cabinet with insulation and possibly a freon line in between.

As far as how often it might be read, probably not necessary to be much more than once a minute as the temperature would be slow to change. Maybe even longer but the point being it doesn't have to be very fast. The temperature range would be in the -15F to room temperature range.

I like your idea of RF energy being transmitted for a defined period of time to charge a capacitor then doing a quick read of the data. Do you know of any design examples using this?

I see from a Google search there are companies that already manufacture passive or semi-passive cards that are used in the food industry during transport and warehousing. Was thinking they may be more cost effective if the wheel doesn't need to be re-invented so to speak.

[Mike Green]

You're not going to be able to transmit either RF energy or IR energy through the metal outer cabinet. You'll do much much better just using conductive foil. I've seen stuff where there's a mylar outer layer on both sides, adhesive on one side, and two copper foil inner conductors sandwiched between the mylar layers. There's some mechanism for connecting to the foils at either end. It's sold for use in speaker wiring where you place it on a painted wall, connect to it, then paint over it or apply wallpaper over it so it's invisible. If you use a 1-wire sensor powered by the 1-wire line, you'll only need two conductors. The BS2p Stamps have 1-wire statements and the Propeller's Object Exchange has an object to handle 1-wire communications. This stuff is very thin and would, if carefully applied, seal very nicely against whatever gasket is normally present.

[ElectricAye]

Is your metal housing made of steel? or is it something non-ferrous like aluminum? Would it be possible to use an electromagnetic pickup coil to send a low frequency signal across the barrier?

[Don M]

@Mike- Using anything to drape around the edge of the opening unfortunately is not an option. Agreed it would make the project much simpler and believe me I looked at that already and wished it could be but without divulging the details of the project, there are factors that won't allow this.

@ElectricAye- The outer cabinet is steel the same as any major appliance you may have in your home.

[Mike Green]

Well, I guess you're stuck. The outer cabinet will shield the inside from any RF energy or magnetic field you'd use for communication and the insulation and both inner and outer shells would prevent light from getting through, similarly for sonic energy. You might be able to get some energy through the seal area, but that depends on the details of the dimensions and materials involved.

[kwinn]

If the outer cabinet is fairly thin sheet metal it may still be possible to use magnetic coupling to provide power to the sensor circuit and send temperature data back if the gap between the inner and outer cores do not have to be too far apart. Cosider it to be a split transformer with an air gap. Can the inner portion of the core be placed close to the outer sheet metal?

[Don M]

@kwinn- the closest it could be would be the thickness of the wall which includes foam insulation and liner. Approximately 1-2".

@Mike- Then I'll have to consider Plan B. The top of the cabinet utilizes glass sliding doors. And no I can't utilize anything optical as I cannot impede the access or looks of the sliding glass doors. I am thinking I should be able to use some sort of RF communication albeit won't be battery-less. X-Bee, Bluetooth, Cypress CyFi or something similar. It just needs to consume little power so as to preserve battery life.

[ElectricAye]

... I am thinking I should be able to use some sort of RF communication albeit won't be battery-less...

If this cabinet is going to be -15F, just be sure your batteries can provide useful current at that temperature.

[Kevin Brown]

hi Don,
this sounds like a commercial display unit. It is definitely a low-tech approach but have you considered the drain plug most units that I'm familiar with how small plastic drain plug at the bottom of the unit for cleaning you can easily pass the wires for temp sensor through that whole just make sure you plug it back up with something or the cold will drain out the bottom.it deftly beats trying to drill a hole through the side and missing the refrigeration coils. And if it is that freezer unit batteries will be of little use inside.

Kevin
KI4SCC