Powering and communication with implants - build from scratch
It's pre-exam times and the perfect opportunity to find lame excuses not to study. Today: building circuits!
This time I decided to go the most basic way of doing things. With a focus on trying to build something that's robust, safe to use, doesn't require any fancy integrated circuits and still provides the most necessary functionality.
Sry bout the non-embedded images, I still need to find a better replacement for my old webhosting service than google.
So far I got Inductive charging done, with the circuit ready to transfer data,too.
Have a look at it in action here: https://drive.google.com/file/d/0B48XgkYBTqDlWGhiTk4wXzVSVUk/view?usp=sharing
In that picture the Implant is receiving about 25mW of power across about 1cm of airgap. The transmitter's power is limited to about 150mW maximum in this setup.
That's 150mW to cover all losses which includes the losses of the transmitter coil and capacitors, the power the implant draws (including all it's internal losses), eddy currents from metallic objects/shielding nearby etc.
So no matter what happens, short circuits, metal objects etc, the transmitter should never be able to cause serious damage.
Still it's enough to light up an LED and to charge the NiMH coin cell there. Max usable range was about 30mm and maximum usable output power (for 10mm) was about 50mW.
The circuit looks pretty much like this: https://drive.google.com/file/d/0B48XgkYBTqDlMWk0X0RmVG52cTg/view?usp=sharing
Most of the components used are available as rather small SMD packages so even if it looks like a lot of parts, it's not going to take up a lot of space. The circuit includes power transfer, rectifier, voltage doubler, battery charging, indicator led and a shunting mosfet to do loadmodulation (communication) but it can also be used to blink the LED if the charger is present.
The battery in the picture is a 40mAh NiMH type which can be safely trickle charged for indefinite time periods.
The aluminum bar in the pic was used to simulate the effects of the implant's metal shielding on the system. Turns out it consumes a bit of power but mostly shifts the resonating frequency. Compensating for this effect is reasonably easy and the power transfer continues to work. Even covering most of the coil with a thick aluminum plate will not entirely stop it from working. Adjusting the frequency and increasing the power output by a bit makes it work again.
I have not done any tests regarding data transfer speed but the simulation suggests that you would want to keep it somewhere below 1200 bit/s. So none of your favorite tv-show episodes on your implant, but plenty for debugging and setting parameters, and maybe a bit of poetry.
The circuit so far requires a total volume of about 11x13x30mm (or about 2/3 of a AA battery) when the parts would be assembled in a compact way (and that's with no SMD parts). Coil and battery take the biggest share. With a custom wound coil you can trim down the 11mm to 7mm.
The circuit can be build almost entirely from parts that are robust vs performance degradation over time. Except for the battery.
Next on my list is the actual power management. Means converting the low battery voltage to something more usable for microcontrollers and op-amps to work with and stuff like standby modes etc. After that it's wiring up the microcontroller and setting up the communication for data exchange. Followed by an electrode driver to allow the implant to send feedback to the body.
This time I decided to go the most basic way of doing things. With a focus on trying to build something that's robust, safe to use, doesn't require any fancy integrated circuits and still provides the most necessary functionality.
Sry bout the non-embedded images, I still need to find a better replacement for my old webhosting service than google.
So far I got Inductive charging done, with the circuit ready to transfer data,too.
Have a look at it in action here: https://drive.google.com/file/d/0B48XgkYBTqDlWGhiTk4wXzVSVUk/view?usp=sharing
In that picture the Implant is receiving about 25mW of power across about 1cm of airgap. The transmitter's power is limited to about 150mW maximum in this setup.
That's 150mW to cover all losses which includes the losses of the transmitter coil and capacitors, the power the implant draws (including all it's internal losses), eddy currents from metallic objects/shielding nearby etc.
So no matter what happens, short circuits, metal objects etc, the transmitter should never be able to cause serious damage.
Still it's enough to light up an LED and to charge the NiMH coin cell there. Max usable range was about 30mm and maximum usable output power (for 10mm) was about 50mW.
The circuit looks pretty much like this: https://drive.google.com/file/d/0B48XgkYBTqDlMWk0X0RmVG52cTg/view?usp=sharing
Most of the components used are available as rather small SMD packages so even if it looks like a lot of parts, it's not going to take up a lot of space. The circuit includes power transfer, rectifier, voltage doubler, battery charging, indicator led and a shunting mosfet to do loadmodulation (communication) but it can also be used to blink the LED if the charger is present.
The battery in the picture is a 40mAh NiMH type which can be safely trickle charged for indefinite time periods.
The aluminum bar in the pic was used to simulate the effects of the implant's metal shielding on the system. Turns out it consumes a bit of power but mostly shifts the resonating frequency. Compensating for this effect is reasonably easy and the power transfer continues to work. Even covering most of the coil with a thick aluminum plate will not entirely stop it from working. Adjusting the frequency and increasing the power output by a bit makes it work again.
I have not done any tests regarding data transfer speed but the simulation suggests that you would want to keep it somewhere below 1200 bit/s. So none of your favorite tv-show episodes on your implant, but plenty for debugging and setting parameters, and maybe a bit of poetry.
The circuit so far requires a total volume of about 11x13x30mm (or about 2/3 of a AA battery) when the parts would be assembled in a compact way (and that's with no SMD parts). Coil and battery take the biggest share. With a custom wound coil you can trim down the 11mm to 7mm.
The circuit can be build almost entirely from parts that are robust vs performance degradation over time. Except for the battery.
Next on my list is the actual power management. Means converting the low battery voltage to something more usable for microcontrollers and op-amps to work with and stuff like standby modes etc. After that it's wiring up the microcontroller and setting up the communication for data exchange. Followed by an electrode driver to allow the implant to send feedback to the body.
Comments
Results:
Pic4
Pic5 (ruler is in cm)
Even the smallest inductor i had around (2.5x2.5x1.6mm) was able to light up the LED with more than 16mm spacing between the coils.
Main observation was that bigger coils have less strict requirements on tuning the LC circuit to the right frequency. However, if you do properly adjust and tune (which can be automated on the transmitter side), you can transmit power even with pretty small off the shelf coils. For practical reasons 7mm diameter and up seems to be doing okay.
I also read a bit about "resonant induction charging", but again, no information on how to size the coils to make this happen. Or for RIC controlled by the charging circuitry...