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IoT and RESTful HardwareWhat We’ve Learned from Three Years Treating Motion as REST

Alden Hart - Synthetos

O’Reilly Solid Conference

May 21, 2014

TinyG Motion Controller

what’s TinyG?• multi-axis motion control system designed for desktop CNC, 3D printing, pick & place…• accepts Gcode from USB port and executes it locally on the board• behaves like a RESTful server

The Controller Problem & Design Goals

the problem• the motion control functions are all good – but people got stuck at controller implementation• specialized parsers and stateful clients got in the way

design goals• control interface that’s easy and intuitive

• simple• compact / lightweight• human readable / typeable / hackable• relatively ubiquitous / low learning curve• easily persisted and instantiated• object oriented

• don’t have to be an industrial automation engineer to use (1 : 1000)• does not require gateways or other additional hardware

not design goals• highly optimized transfer / storage• support bandwidth starved and flakey communications channels• support for hard, a-priori contracts• XML-isms like Xpath/Xquery, annotation, schemas

RESTful Hardware

what is RESTful?• state exists on the server and is transferred to the client• representation is flexible, independent • well-defined resource model

RESTful hardware• server is an embedded chip / may not have an OS• RESTful conventions make talking to the device very easy

JSON• JSON is the natural representation• meets most of our design goals

• simple• compact / lightweight• human readable / typeable / hackable• relatively ubiquitous / low learning curve• easily persisted and instantiated• object oriented

Putting JSON / REST on the Hardware

put it On.The.Hardware• code runs on the chip - not a gateway / translator / some-other-intermediary

• “Run JSON right over the traces”• simple JSON parser, serializer, dispatcher and serial IO in < 8K C/C++ code• bind names to dispatch tables to perform actions for the verbs• marshalling in and out of floating point

• strtof() < 70 uSec on a 16 MHz AVR atmega328P (Arduino Uno)• strtof() < 7uSec on a 84 MHz ARM Cortex M3 core

• all memory statically allocated – no mallocs• transport doesn’t matter

• USB, SPI, Serial, MQTT, CoAP, HTTP…

subset of JSON spec• 7 bit ASCII only (UTF-8 sub-set)• objects on a single line (no embedded line feeds)• objects limited to 512 characters (less on smaller CPUs)• decimal only, no hexadecimal• limited support for arrays

Implementation Considerations

well defined resource model• static model – e.g. X axis, motor 1, machine setup parameters…• dynamic model – e.g. position, velocity, coordinate system in use, tool in use…• favor flat namespaces – right level of object granularity

keeping it compact• short, mnemonic tokens for names – e.g. xvm instead of XaxisVelocityMaximum

• don’t need to be globally unique – objects define scope• many values are enumerated – e.g. 3 instead of GcodeProgramStop• simple verbs (get, post)

cheats & shortcuts • no request headers. Use null to mean get• no request ID – request / response is synchronous• can consume Gcode natively without a JSON wrapper

• so people don’t have to pre-process their Gcode files

Implementation Examples from TinyG (1 of 2)

get • {“xvm”:null}

• {“ct”:null}

• {xvm:n}

• {ct:n}

post (set) • {xvm:16000}

• {ct:0.01}

objects• {x:{vm:16000}}

• {r:{x:{am:1,vm:16000,fr:10000,tn:0,tm:100,jm:200,jh:10000,jd:0.0100,sn:0,sx:0,sv:3000,lv:100,lb:20.000,zb:3.000}},f:[1,0,6,5473]}

• {r:{sys:{fb:428.01,fv:0.970,hp:1,hv:8,id:"2X2660-EVD”…

• {gc:”N75 G3 X0.6785 Y2.2371 Z0.0014 I0.0003 J0.0594”}

footer revision status code flow control checksum

footers

• f:[1,0,6,5473]

Implementation Examples from TinyG (2 of 2)

status reports• event reporting for the dynamic model• timed and filtered

• timed reports during movement – e.g. every 250 milliseconds and at end-of-move• filtered so only changed values are reported

• setup status report• {sr:{posx:true, posy:true, posz:true, vel:true, momo:true, stat:true}}

• status reports for an X move from 0 to 100• {sr:{posx:0.000,vel:1.54,momo:0,stat:5}}• {sr:{posx:2.494,vel:1740.87}}• {sr:{posx:18.137,vel:5540.05}}• {sr:{posx:43.528,vel:6500.00}}• {sr:{posx:69.885,vel:6393.34}}• {sr:{posx:91.760,vel:3918.13}}• {sr:{posx:99.670,vel:479.38}}• {sr:{posx:100.000,vel:0.00,stat:3}}

exception reports• {er:{fb:428.01,st:29,msg:"Generic exception report"}}

Useful Conventions

We need some conventions for non-application specific functions

don’t innovate. copy.• $ root to discover objects (cribbed from JSONpath)

• {$:null} return top-level objects

• {r:{x:n,y:n,z:n,a:n,b:n,c:n,1:n,2:n,3:n,4:n,5:n,6:n,sys:n,gm:n,rm:n}},f:[1,0,6,6271]}

• @ prefix for addressing, binding and routing

• {@3F2504E0-4F89-41D3-9A0C-0305E82C3301:ex1} assign a short name (handle)

• {ex1:null} address device “ex1” and route object to that device

• # topic prefix

• {#time:”2013-03-28T15:52:49.680Z”} devices that want time will process this message

• ! priority. Jump the queue

• {!fh:true} stop movement now! (feedhold)

Questions