DIY FSR Feet for the HR-OS1

[r3n33]

For the robot to have the ability to sense the ground sounded like a great idea and having FSRs on the feet was not a new concept. Before creating my own I checked google to see what may already exist and found a little bit of information about the official feet made by Robotis. The page and videos are a bit dated, the product is listed as not available, and the price was $450. Not to mention the performance demonstrated in the linked video left a lot to be desired.



A little more scrolling revealed some specifications about their design. Mainly how they configured the register table was what I was interested in but it was also nice to see the X,Y layout their code was expecting to readings to correlate with.

http://support.robotis.com/en/produc...onents/fsr.htm

At this point, perfect, we can make our own. To prevent the possibility of having a robot without working feet I ordered some extra brackets from Trossen.

http://www.trossenrobotics.com/bioloid-metal-f5-bracket
http://www.trossenrobotics.com/bioloid-metal-f12-foot

You’ll also require eight FSRs ( 4 for each foot ) and the size I found easiest to install on these feet are the small 0.2” size.

http://www.trossenrobotics.com/store...istor-FSR.aspx

You’ll also need a MCU to sample these sensors, interpret the data, and respond to dynamixel packets. In recent discovery I found the Adafruit Trinket Pro in the 5V 16MHz configuration to work well for just this sort of thing. I’m willing to bet there are other MCU options out there but this one is small, handles voltage regulation, and doesn’t cost much. I was a little worried about the 16 clocks per bit on the DXL bus but so far so good.

https://www.adafruit.com/products/2000

Now I had intentions of making an assembly guide but for the sake of getting at least part of this information out there I’m going to show pictures of the current prototype.





You can see each FSR is routed through the slits in the F12 foot and each has a pull down resistor. Instead of routing 4 resistors over the board to the GND pin I used some of the spare GPIO and set their state to LOW. To connect to the dynamixel bus I made my own connector or you can salvage a spare if you have one. Power goes to the BAT+ pin, ground to ground, and data to TX and RX which are to be tied together.

In order to install one of the FSRs the F5 bracket will need to move off center OR what I ended up doing was printing a 1.5mm spacer to go between the F5 and F12 parts which allows the FSR to route underneath. When adding this printed spacer the M2x6 screws holding F5 and F12 will need to be switched for M2x8 to accommodate for the extra space.

HR-OS1 F5 to F12 Foot Spacer.stl.zip

The last part required for these feet to work are the soles. The sole cannot attach with pressure or the FSRs wouldn’t work so I designed some 3D printed clip ons to replace the original delrin parts. They keep the FSRs covered and help distribute force but do not maintain pressure when the foot leaves the ground.

HR-OS1 F12 Foot Pad.stl.zip

Once everything is connected you’ll need firmware programmed on the Trinket. This part was not made available by Robotis but not to worry we have a link to the register table, the hardware prepared, and a little bit of smarts to perform weighted averaging and some data smoothing. Thanks to KurtE, Eric, and myself you'll soon be able download our implementation on GitHub.

[TODO: host online]

From here you’ll want to use the Arduino software to write the project to the Trinket Pro then you can begin setting up the configuration of the left and right feet.

[TODO: instructions]

Right Foot ID 111
Left Foot ID 112

Now you can connect the feet to your robot and test the FSR tutorial found in the ‘fsr’ branch of my forked copy of the framework.

[TODO: Update my master branch of framework and fork off FSR code]

Temporary Information: https://github.com/r3n33/HROS1-Frame...792113ccffa5ef

So this post is a work in progress but now we have a good place to discuss the development of FSR feet.

[tician]

The AX-12/18 use ATmega88 at 16MHz, the CM-5/510/700 use ATmega128/256 at 16MHz, and the arbotix-m use ATmeg644p at 16MHz, so should have no issues with a 16MHz Trinket Pro. The only reason the FSR feet of the DARwIn-OP use 72MHz STM32 is to ensure compatibility with the higher speed dynamixel buss possible with the MX-series servos. All of those 16MHz AVR are technically capable of the 2Mbps buss speed, but the Robotis firmware always disable the USART U2Xn bit for a 16x clock divider (maxes at 1Mbps with 16MHz clock) instead of 8x (maxes at 2Mbps with 16MHz clock).

I had attempted to make an arduino-compatible pcb to fit entirely within the plastic foot of the Bioloid kits with center mounted bracket, but it did not work out partly because I apparently slightly messed up some of the dimensions and I never bothered getting a revised board because of the general uselessness of the data acquired from the HUV Foot Pressure Sensor boards and Robotis DARwIn-OP FSR feet. Might try again sometime in the future with multiple pcb trace sensing areas (like my MW boards) instead of FSRs to make the active area of the sensors flush with the contact surface of the foot instead of slightly recessed by the rubber spacer in the FSRs.

[KurtEck]

Looks great.

FYI - The trinkets are fun to play with! Can not wait to try this out!

[LloydF]

Me too. I am having difficulties getting my orders out of Trossen all of a sudden, normal get something back in a day or so bu t this has bee a week, and nothing is back ordered so, are there any outer sources for FSR sensors?

[tician]

Adafruit, sparkfun, robotshop, digkey, etc. If nothing shows up when searching them for 'fsr', then you can try searching for their manufacturer: 'Interlink Electronics'. Digikey prices can be a bit higher for small quantities since you only get to the sub-$6/unit when ordering 25 or more at a time.

[KurtEck]

I today I received my order from Sparkfun with 8 of them

[sonel]

Thanks r3n33 for starting this thread. I'm waiting for the delivery of the FSRs. Had to order the small ones about a week ago from US (I live in UK) and hopefully by the end of this week will be here. I got them on eBay from mechasolution14. I also found a supplier here in UK (proto-pic) that had the 0.5'' in stock and were shipped yesterday. I guess those will come today or tomorrow. I will see which of the the two models works better. Check: Walgreens Ad Preview and Target Ad Preview on Weekly Ads.

In respect to the controller I actually settled for the Arduino-Pro-Mini which is essentially the same as the Trinket except the USB (which sincerely is a gimmick considering how is wired). The big advantage is that for the price of one Trinket I got 5 (!) of these Arduino-Pro-Minis plus a FTDI-USB adapter and cable! They are also smaller (shorter) which is an advantage in that cramped space in the foot. The main disadvantage is that, since I will have to share the RX/TX between the Dynamixel and the FTDI, I will need to put a jumper to connect RX to TX (similar to Arbotix-M) - the jumper will have to go off when programming the chip.

I was planning to put it on a larger stripboard that will have holes matching the the ones in the foot and where I can place the Dynamixel connector, the sensors' resistors and the sensors connectors. It will be narrow enough that will fit flat next to the F5 bracket.

For the feet I also noticed that the the mounting of the F5 bracket obstructs one of the cutout for the FSR so a standoff between them is needed. I was thinking just to put some washers.

For the soles I was thinking to have some semi-cutout circles around the sensors - so that they can flex, and either have they slightly raised (with some domes) or stick some rubber feet. I'll probably need to put a picture once I make a prototype to show that better.

One of the serious improvements that would make the FSR useful for the robot is to change the firmware of the main controller so that the information is continually used in the main loop to evaluate if the robot is in balance and apply corrections. The mistake made in Darwin is that this was implemented in the Linux level; because the information passes through 2 loops between the sensors and the main Linux controller a lot of time is wasted and there is very little chance meaningful decisions can be taken. Implementing it in the Arbotix will be better - ultimately our reflexes are coming from the spine (and that's our Arbotix) and not the brain!

This brings us to an interesting subject - has anyone come up with the convenient way to update the Arbotix-Pro firmware without the need to take it out from the robot (that is a lot of screws to take out!) and stick the JTAG adapter? Robotis had a firmware loader program that allowed updating the controller firmware from the main Linux computer, but that was because they had a boot-loader that supported that. As I've seen until now I don't think the Arbotix-Pro has any boot-loader.

[LloydF]

Thank you, I order some from RobotShope this morning and there already in the mail, dang.
I'll cancel out my Trossen order since it has not even mailed yet.

[KurtEck]

In respect to the controller I actually settled for the Arduino-Pro-Mini which is essentially the same as the Trinket except the USB (which sincerely is a gimmick considering how is wired). The big advantage is that for the price of one Trinket I got 5 (!) of these Arduino-Pro-Minis plus a FTDI-USB adapter and cable! They are also smaller (shorter) which is an advantage in that cramped space in the foot. The main disadvantage is that, since I will have to share the RX/TX between the Dynamixel and the FTDI, I will need to put a jumper to connect RX to TX (similar to Arbotix-M) - the jumper will have to go off when programming the chip.

I also through about the Arduino Pro mini, but actually the USB gimmick works enough, that I can simply plug in the USB, hit the button and then upload the program, without having to change the board, which is nice...

Price, where did you get such a great deal?
If I look at Sparkfun: https://www.sparkfun.com/products/11113 The price is $9.95 and FTDI Cable: https://www.sparkfun.com/products/9718 $17.95. Note Adafruit has same price for the Arduino...

For Trinket Pro 5v: http://www.adafruit.com/products/2000 $9.95

This brings us to an interesting subject - has anyone come up with the convenient way to update the Arbotix-Pro firmware without the need to take it out from the robot (that is a lot of screws to take out!) and stick the JTAG adapter? Robotis had a firmware loader program that allowed updating the controller firmware from the main Linux computer, but that was because they had a boot-loader that supported that. As I've seen until now I don't think the Arbotix-Pro has any boot-loader.

I have been told that there is no boot loader. You need to buy the Programmer and adapter to update the firmware in the Arbotix Pro. Also the instructions for building the firmware looks like they only have the instructions to do so on a Linux machine...

[LloydF]

Well, heck the FSR's are on back order , so It is time to Learn more about Trinket.