Servo Conversion - I2C Control
In various groups and blogs I peruse on here, there's been talk about I2C control of servos (as opposed to standard PWM). This certainly solves several problems, as it allows one I/O port (I2C) to service over a hundred servos with ease. With some built-in intelligence, the servo itself can take care of interpreting and following commands without the need for continual updates from the system (i.e., true asynchronous operation). In addition, servo power lines are kept separate from the autopilot board, thus eliminating the possibility of overheating board traces due to excessive current draw (a stack of 8 servos, especially digital ones, can easily exceed the amperage rating of the traces and wiring used in some autopilot boards). The one drawback to I2C control... if one servo's electronics fails and "latches" the bus, the entire network is toast. However, that possibility can be mitigated through an isolation network built onto the servo control board.
Coincidentally, today I read a message on the UAV Dev Group about an open source servo project called (not surprisingly) OpenServo*. Among other things, they offer complete ready-to-go boards that fit inside some standard servos and convert them to I2C. Here's a list of features, directly from their page:
- High performance AVR 8-bit microcontroller
- Compact H-Bridge with high performance MOSFETs
- Precision control over servo position and speed
- I2C/TWI based interface for control and feedback
- Feedback of position, speed, voltage and power
- Advanced curve based motion profile support
- EEPROM storage of servo configuration information
- Software written in C using free development tools
- I2C/TWI bootloader and GUI programmer
I thought I'd mention it here, since there seem to be people from several diverse groups interested in this (I2C servo control). Price is $14.95 from SparkFun Electronics.
*Credit to Peter Holands of UAV Dev Group for spotting this.
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Comment by mook on May 7, 2010 at 11:51pm -
@sandro
my mistake, I didn't realize not all the pins are exposed.
good insight. I guess I just meant any generic uC with an i2c controller could be used. you don't really need a PWM controller if that is it's main function, you can implement pwm on all IO pins with just an interrupt.
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Developer Comment by Sandro Benigno on May 8, 2010 at 9:53am -
Yes, you're right about this, mook. In fact, nothing is impossible. Sure you can controlling PWM by interrupt with overflow, using two timers, etc... but it tends to be very performance dependent or tricky. And, in mine opinion, it would be a waste of time, because you will need a lot of effort to achieve something similar to a fast and solid PWM signal that already comes functional on some chips. Resuming... like Obama said: "we can do it". But, in that case, with a lot of trouble to face.
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Developer Comment by Sandro Benigno on May 8, 2010 at 10:04am -
Ops... no need to stone me.. I'm correcting Obama's slogan: "Yes, we can". :)
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Comment by Lew Payne on May 10, 2010 at 2:45pm -
Auxiliary I2C servo controller boards (best of both worlds)...
20 Channel I2C to Servo Driver Chip (off the old block)
I2C Servo Interface Board (8 servos)
8 Channel Servo Controller Module
(I'm posting these for my own reference, to make them easier to find in the future)
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Comment by Sylvain on May 17, 2010 at 2:29am -
A digital servo on an I²C (or CAN) bus seems like a very good idea, because it can potentially offers features that a standard analog servo (and even some digital ones) can't offer:
- a very low latency for the commands
- a digital control loop, way more accurate than an analog one
- by measuring current and back-EMF, the servo µC can have more info (torque and speed)
- possibility of torque limiting, speed limiting and power limiting
- such servo can auto-test, and send an interrupt in case of motor or H-bridge failure
- the parameters of the control loop can be dynamically tuned
- and finally the servos can send some info (real position, torque) that can be fed back in the autopilot control loop
Overall, there's great potential :)
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