OpenServo hardware feature requests
As the OpenServo platform develops, we hope to bring more features for a more diverse application use. This is a partial list of features requested in the OpenServo forums.
Sensors:
Hall based current sensors: ACS706ELC-05C, LM3822MM-1.0 or CSA-1V
- Carefully Consider the use of the +5V programming pin.
- 3.3v MCU operation for running the system from +5V (Tested OK on current hardware. Replace regulator with 3.3v version)
Interfaces
Current (2.1) OpenServos use TWIProtocol, using TWI signaling ( also called two-wire interface, also called I2C). They can also be set to use standard ServoSignaling.
Other proposed signaling hardware include:
SPI (advantage: "daisy-chained SPI" allows any number of slave devices on a bus, without needing to set any "address" on any of them, and requiring 4 wires on the master -- 4 total, no matter how many slave devices )
- CAN bus ?
- RS-485: because it has 2 wire differential, it can signal at high speeds over longer distances than most protocols.
- something like the MIDI electrical spec: because it has 2 wire current loop and is fully opto-isolated, it can signal over long distances and is more immune to "motor noise" on the power lines.
CPU
Here are some of the CPUs we are considering for Open Servo 4:
Atmel ATtiny45 AVR with 4K of Flash in a 8 pin package. This was used in OpenServo 1.1, but the 4K of Flash doesn't seem to be enough room to do everything we want to do in an OpenServo.
Atmel ATMEGA168 AVR with 16K of flash in a 32-TQFP package, 9mm x 9mm including pins, 0.8 mm pitch (~1/32 inch pitch). This was used in OpenServoV2 and OpenServoV3.
- NXP LPC2103 ARM: It's a 32 bit CPU, with 32KB Flash and 8KB RAM, for under $5 qty 1. You might think that a LQFP48 package would be larger than a 32-TQFP package, but they are both 9mm x 9mm including pins. Alas, its pins are on a more difficult 0.5 mm (~1/51 inch) pitch -- is that a show-stopper?
- Cypress PowerPSoC CY8CLED04D02 -- integrates two 2 A 32 V nFETs and a voltage regulator and high side current sense amplifiers and a CPU in the same package. (OK, it's really four 1 A nFETs, which we can parallel to get 2 A). under $11 qty 1. It only comes in a 8mm x 8mm 56-QFN package, with pins on a 0.5 mm (~1/51 inch) pitch -- is that a show-stopper?
Atmel ATtiny85 AVR with 8K of Flash in a 8 pin package. This was used in Robot Room: Monkey Mints Robot by David Cook. We probably would have used this for OpenServoV2 and OpenServoV3 but it was not yet available then ("ATtiny45 vs. ATtiny85"; see also the "resurrected" comments at Hardware). Is there any reason not to use it for OpenServo V4? A 8mm x 5.25 mm 8SOIC package is smaller than a 32-TQFP package, which makes it easier to layout and/or leaves room for larger H bridge transistors or additional stuff on the PCB -- but it only has 8 pins, so it can't be connected to much additional stuff. A 8SOIC package has a wider 1/20 inch pitch that is easier to hand-assemble, compared to 32-TQFP.
Physical
Increased drive current capabilities, such that it can be used on larger motors. Perhaps a collection of these VNP20N07
The OpenServo V3 uses two IRF7307 chips: dual N and P channel MOSFET in 8SOIC package; rated at 5.7A, 20V (under completely unrealistic conditions)
- ROHM SP8M4FU6TB appears to be a pin-compatible drop-in replacement for the IRF7307 in the same 8SOIC package, but rated at 9A, 30V
- VISHAY SUD50NP04 is a slightly larger dual N and P channel MOSFET in a slightly larger surface-mount device, rated at 8A; 40V
- VNP20N07 is a N channel MOSFET in a through-hole TO-220 package; 70V 20A. The bigger motors that need this amount of current would hopefully have plenty of space for this larger transistor. Even bigger motors that need even more current would hopefully have room for a collection of these wired in parallel. There's a huge collection of other transistors with other current/voltage/cost tradeoffs that have the same pin-compatible package. under $3 each.
- FAIRCHILD FDD6685: a P channel MOSFET in surface-mount TO-252 package; rated at -40A; -30V; under $1 each.
- NTMFS4833NT3G: a N channel MOSFET in a DFN5 surface-mount package; rated at 191A; 30V; under $2 each. (over 100 A: wow).
see also
Hardware for the hardware we actually used on past Open Servos.