Mini SSC II
Frequently Asked Questions (FAQ)
What is a servo?
What is a Mini SSC?
What range of positioning can I get with a Mini SSC and servo?
Is the Mini SSC II available in kit form?
Where can I get suitable servos for use with the Mini SSCs?
Where can I get an AC power supply for servos?
How do I send an instruction to a Mini SSC?
Where can I find program examples?
Can I move two or more servos simultaneously?
How do I make servos move slowly and smoothly?
What if I want to control more than eight servos?
What about even more servos?
How do I reduce electrical noise that's making my servos twitch?
How do I control the Mini SSC with a TicKit controller?
How do I control the Mini SSC with a 68HC11 Handy Board?
I don't want to write programs; do you have ready-to-go software for the Mini SSC?
"Servo" in this case refers to R/C or 'hobby' servos used in radio-controlled cars, boats and planes. These are small, self-contained positioners consisting of a motor, geartrain, and control electronics. As inputs, these servos need a source of power (4.8 to 6Vdc) and a control signal (variable-width pulses repeating at a 60-Hz rate). Servos' mechanical output is a shaft, disk, or lever whose angle is proportional to--controlled by--the width of the pulses at the input.
The Mini SSC is an electronic interface that allows a computer to control eight servos. The computer sends simple commands to the Mini SSC at 2400 or 9600 baud, and Mini SSC generates eight channels of precise, stable servo-control pulses.
Servos are primarily designed to drive bellcrank mechanisms, like those that control the angle of control surfaces in a plane, or steering in a car. These mechanisms have a limited range of motion, so a servo's basic design calls for operation over a 90-degree range. The Mini SSCs' control signals break this 90-degree range into 254 parts, for 0.36-degree resolution. Optionally, the Mini SSC II can be configured for coarser resolution (0.72 degrees) and wider range (up to 180 degrees). However, not all servos can cover a full 180 degrees because of their basic design.
No. It's a surface-mount design, making it more difficult to hand assemble. Even if that were not the case, surface-mount components are too tiny to economically kit up for hand assembly, and a kit would probably cost more than the assembled version (which is put together in the blink of any eye by automated pick-and-place technology).
Any hobby dealer can help. On line, check out www.towerhobbies.com or www.lynxmotion.com or www.robotstore.com .
Servos are designed to operate from four-cell battery packs supplying 4.8 to 6.0 Vdc. Under load, they can draw pretty heavy current (100s of mA). Being designed for battery power, they aren't very tolerant of power-supply noise. An ideal power supply is a linear, regulated 5Vdc supply capable of 1A or more output. Jameco carries quite a few suitable supplies--mostly OEM surplus from video game consoles. Here are some examples from their April 2000 catalog:
- Atari 5W supply, part number 116089: 5V at 1A
- Lexmark 7.5W supply, part number 160899: 5V at 1.5A
- Atari 6.1W supply, part number 147467: 5V at 0.72A and 12V at 0.21A
You'll have to cut off the connectors on these supplies' outputs and determine voltages and polarity with a meter, but you'll be getting a husky, reliable power supply for less than $10. The 12V output from the 6.1W supply can be used to power the Mini SSC electronics, replacing the 9V battery too.
Instructions have a simple format consisting of a sync byte (always ASCII 255), the servo number (0-254), and relative position (0-254, where 127 is centered). Send the appropriate three bytes (unsigned chars in C parlance) and the Mini SSC will send the specified servo control pulses that make it move to the commanded position. Servos are held in the last commanded position until instructed otherwise.
In the Mini SSC manuals, in BASIC for the Stamp 1, Stamp 2, and PCs. There's a great Windows programming tutorial for the Mini SSC (written in Visual BASIC 5 Pro) published by Reynolds Electronics (off site at www.rentron.com). We provide a free DLL for experienced Windows programmers; see our app note. For Linux users, check out the Perl/Tk GUI controller at www.howtoandroid.com.
Yes and no. Servo-control pulses repeat every 18 milliseconds or so. During that time, you could send as many as 6 instructions to the Mini SSC (at 9600 baud). It also takes servos a while to respond to new positioning instructions--typically 300 milliseconds to carry out a 60-degree change in position. So although you cannot send commands simultaneously, you can send them quickly enough to amount to the same thing.
By sending them a series of intermediate positions in connect-the-dots fashion. If a servo is at position 50 and you send the command for position 250, it will move as fast as possible to that new position. But if you break the move up into a series of smaller moves and send them with time in between, you can slow down and smooth out the move.
Two Mini SSCs can share the same serial line. The first Mini SSC would be set up for its default servo numbering of 0 to 7; the second would have an 'ID' jumper installed, giving its servos numbers 8 through 15.
Mini SSCs can be chained together to control up to 255 servos. To get servo numbers higher than 15, you must special-order the Mini SSC with the appropriate firmware; contact Scott Edwards Electronics, Inc. (info@seetron.com) for details. See our app note for information on connecting multiple SSCs.
The Mini SSC's output signals are rock steady, so a properly functioning servo should be steady too. However, if the electrical environment is exceptionally noisy (RF sources, relays, contactors, big motors, noisy fluorescent lights), or your servo wiring is more than a few feet long, the servos may pick up noise on their signal inputs and twitch or jitter. Your hobby shop can supply a glitch-filter circuit that will help. A customer-recommended model is the "Futaba P-AB0104 (Extension cord W/filter B3P)."
A user of the TicKit controllers contributed the following code fragments to set up serial output and send three bytes to the Mini SSC:
rs_param_set( rs_invert | rs_9600 | pin_d0 ) ; set up serial out on pin d0
; only needs done once
rs_send( 255b ) ; send the sync byte (255)
rs_send( 0b ) ; send a servo number (0)
rs_send( 200b ) ; send a position value (200)
The MIT-designed Handy Board is extremely popular in robotics applications, so it makes sense that many of its users want to team it up with the Mini SSC. The Handy Board Home page features a 9600-baud serial driver that's supposed to be perfect for Mini SSC apps.
Third parties have developed fantastic Windows-based software for the Mini SSCs:
- Mister Computer offers a complete animatronics package with an assembled Mini SSC II, servos, and software.
- Reynolds Electronics' Robo-Ware software offers sophisticated control, scripting, .WAV (audio) files, etc.
- Roscoe Robotics' ServoMotion software supports joysticks, .WAV (audio) files, and more. Also comes in special version for Lynxmotion robot arm.
- Brookshire Software's Visual Servo Automation offers .WAV sync--even 'lip sync' for animatronics--via graphical timeline.
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