Renbotics Servo Shield Sequencer
by Adriaan | September 20, 2009While thinking of exciting things to do with the new Renbotics Servo Shield, I realized that most of them required some sort of sequencing. Google-ing did not result in much so I decided to create my own.
The first challenge was; how to store the sequence? I decided to use the on chip EEPROM and came up with this simple structure:
| Position | Description | Type |
| 0 | Sequence count | byte |
| 1 | Length of memory used | byte |
| n | Sequence Number | byte |
| n+1 | Sequence Length | byte |
| n+2 | Sequence Interval | int |
| n+4 | Step Number | byte |
| n+4 | Servo Number | byte |
| n+5 | Servo Positon | int |
The first two bytes are used as a sort of allocation table to allow the system to know how many sequences there are and where the next available EEPROM byte is.
The next 4 bytes form the header of the sequence containing the sequence number, to allow multiple sequences to be stored, the amount of steps in the sequence, and the interval at which the steps should be executed.
The last 4 bytes are the step information containing the step number, servo number and position of the servo in that step. This information is repeated for each step/servo combination in the sequence.
Next, a way to program the sequences were needed, so I quickly hacked together a simple protocol:
| Command | Description |
| NS | New Sequence |
| SI [interval in ms] | Set Sequence Interval |
| SS [step number] [servo number] [position] | Add a Sequence Step |
| CS | Clear Sequences |
| DS | Display Sequences |
| PS [sequence number] | Play Sequence |
| SP | Stop sequence Play |
| SB [servo number] [min pos] [max pos] | Set servo Bounds |
| SM [servo number] [position] | Move Servo |
Thus to program a simple sequence to pan two servos from 0deg to 180deg stopping at 90deg and back at a 1s per step interval (for total pan time 5s):
CS
NS
SI 1000
SS 1 0 1000
SS 1 1 1000
SS 2 0 1500
SS 2 1 1500
SS 3 0 2000
SS 3 1 2000
SS 4 0 1500
SS 4 1 1500
PS 1
To use the Servo Shield Sequencer import the EEPROM and Servo Shield libraries and add EEPROM.pde and Sequencer.pde to your sketch. In your sketch's setup method initialize the sequencer first and in your loop method simply poll the serial comms and the sequencer.
e.g.
Full code is available with the Renbotics Servo Shield library: servoshield.zip
Requirements:
FEED