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I2C Smart Servos (Indiegogo)

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janoc:

--- Quote from: m98 on March 25, 2021, 12:45:41 pm ---Literally nobody is going to care about occasional bus upsets that might or might not happen in a toy robot.
And with properly dimensioned pull-ups or a current source, standard-rate I2C runs just fine over a few meters in a regular home/lab environment.

--- End quote ---

You think? When such "bus upset" freezes the robot/gadget because the hw I2C peripheral is now stuck waiting for that clock line to be released? Or worse, when it causes the motor to keep spinning until something breaks because the micro is unable to read some sensors?

Cheap DC motors like those in those servos are terrible when it comes to EMI. I had one of those cheap yellow plastic gear motors in a project and without a boatload of extra noise suppression components (two chokes, several capacitors, ferrite ring ...) there was absolutely NO chance  to run an I2C line to a sensor nearby. And we are talking about maybe 15 cm of wire, not a 50cm+ long antenna you may easily get in a small robot once you actually dress the wires properly.

That I2C runs "just fine over a few meters in a regular home/lab environment" means squat all about running in an environment where you have several brushed motors spewing EMI in one place, along with a wifi or some other radio and a bunch of sensors.

This is a terrible design decision even for toys and will give the user no end of grief. And the OP a lot of customer support/returns/bad reviews.

RS485 would cost literally peanuts to add and would work much better - all you need is a transciever (few cents) and an UART on both sides of the link. Oh and you can actually make a multidrop bus with RS485 too with some care (even though then CAN is probably better).


--- Quote from: m98 on March 25, 2021, 12:45:41 pm ---There is no RS-485 or CAN driver on any regular Arduino, NodeMCU or Blue Pill board.

--- End quote ---

That's wrong. Blue Pill certainly has CAN (but you can't use it with USB at the same time). Also the more recent Black Pill has CAN peripheral, I believe. You only need a transciever like MCP2551.



Arduino Due (ATSAM3X8E) has 2 CAN peripherals built-in, possibly others. There are also CAN shields available. You can also attach a dedicated CAN controller that talks to the host over SPI if the host doesn't support CAN natively - like MCP2515.

This is literally all you need to make CAN work even on a lowly Uno:

https://www.mikroe.com/can-spi-board

Two chips + a few passives, maybe $5 in parts.

RS-485 does not need any special hardware support, only UART which all of the above have - you just need to add a transciever IC. Those cost peanuts and are typically a simple 8 pin device. Heck, there is even an official Arduino shield with an RS485 transciever.

oPossum:

--- Quote from: m98 on March 25, 2021, 12:45:41 pm ---So you haven't got any qualified arguments?

--- End quote ---

I do, but I am going to bow out and let others with more clout continue with the arguments.

Doctorandus_P:
I also don't like I2C (with it's passive pullups) for such a noisy environment, although the way I2C adressing works would be a quite nice fit.

Regular RC servo's are horrible things. The PWM wastes 90% of the time, which makes resolution inherently low, and there is no way to set the speed of the servo, just position, which makes almost all movements inherently jerky. Dynamixel has some nice servo's with RS485 and HALL sensors for the position, but those are not in my price range. There are some Chinese which make similar servo's for smaller prices, but I have not looked at their prices.

I recently bought two S-bus Servo's Corona SB-3019 and these have an STM8 inside. Unfortunately the S-Bus protocol is also very symplistic (Just positions), no speed, no PID parameters, but with an STM8 inside you've a decent base to put anything in it you'd like.

So if RS-485 or CAN is "too much" (it needs an extra 80ct driver chip) then raw push-pull UART such as S-Bus would be my second choice. S-Bus is only unidirectional though and that's also quite a limitation.

Also:
You write in Indiegogo that you've also got plans for a smaller lower power version, Have you done any "market research" for that? I think I'd be more interested in more powerful versions with a higher current drive, and a version which works with a Hall sensor that can also be used for multiple rotations or continuous speed.

You also write that the 5V option for I2C would be more robust, so apparently you've already had stability problems with I2C @3V3?

You also write that you do not aim for "High precision industrial applications".
Why not? Now you put yourself into a limited toy corner. By just changing the interface to a reliable RS485 you keep the grow path open to something that can be high precision and of (near) industrial quality, depending on the hardware your customers add.
There sure is a market for small lightweight servo's, especially in those flying quadcopters, but the market for bigger higher power servo's such as the "dog-like" walking robots may be growing a lot quicker, and with more money to be made. Having a library for the servo's to integrate them in ROS (Robot Operating System) is probably also a big plus.

I like the Idea of selling small PCB's to upgrade existing servo's, but not I2C. At the moment the Servo's with an STM8 already built in and reprogramming that would be a more interesting option for me.




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