moti v0.3 hardware revisions

A project log for Moti, a smart servo

Moti is a smart servo that simplifies the design of intricate robots.

Nicholas StedmanNicholas Stedman 08/20/2014 at 19:070 Comments

We're currently working on v0.3 of Moti's hardware.

Here's the major changes. 

- Magnetic encoder replaces mechanical encoder. v0.2 used two continuous pots back to back to give 360º coverage. These pots were prone to misalignment, and difficult to compensate for in firmware. The magnetic encoder offers full rotation sensing out of the box without hacks. It is also contactless, so there is no mechanical wear and tear.

- Monolithic IC replaces Open Servo v2 H-Bridge. Things would occasionally get damaged on the Open Servo board (Sparkfun Derivative). Either the current sensing shunt resistor or mosfets would burn up, apparently due to excessive current draw. The VNH5180A automatically shuts down with excessive current draw, or over voltage. It also has built-in current sensing, can handle 8A continuous draw, is simpler to use, and is likely much better tested.

- RS485 replaces I2C bus. In practice, we found that using I2C to daisychain around 8 x Motis v0.2 over a distance of meter caused some nodes to disappear from the network. RS485 can theoretically span for 1200m.

- Added more A2D pins…3 in total. v0.2 only had one. Allowing sensors to be connected at each node is one of Moti's key features, so we better have more than one.

- Added mosfet to power line for reverse polarity protection

- FFC cable replaces rigid pin connectors to other boards. The encoder board needs to attach to top of the servo case, while the other boards attaches to the base. A Flat Flexible Cable allows that distance vary, which means the boards can be placed inside a greater variety of servo cases.

- Single header replaces three. v0.2 had a 2.54mm header for power, and two different 1.27mm data headers. In v0.3 all pins were grouped into one 22 pin 1.27mm header, that uses 4 pins each for power and ground to exceed the current ratings 2.54mm pins. Header breaks out SPI in case you want to overwrite the Arduino bootloader.

- All components are now surface mount. v0.2 had several through-hole pin headers. SMDs eliminate hand-assembly, and should thereby reduce the cost just a tad if Moti gets manufactured.

To summarize, if there’s a theme to v0.3 it is this…use pre-engineered solutions over home-brew hacks wherever possible.