Servo Controllers
Servo controllers serve as the interface between the PC and the servo motors. Here is a brief discussion of some of the available boards. All of the controllers noted here are supported by VSA.
This is one of the early servo controller boards, and probably the most popular. It is an 8-channel board with an RS-232 serial interface and currently sells for US$44. This board connects to the computer through a telephone-style connector (known as an RJ11), and comes with a cable that converts this to a standard 9-pin D connector. Since power is not directly available on the old style serial ports, the board sports a 9V battery clip and an onboard regulator which brings this down to the 5V needed to power the board. (This will eat through 9V batteries at a prodigious rate if you leave them connected!) There is also a pair of wires for providing servo power. Finally, there is series of pins in the top right hand corner which can be connected together using shorting jumpers to select different options.
As noted in the Raffles section, most modern PCs do not have an RS-232 serial port. Instead, they have USB ports. You will probably need a USB to serial convertor in order to use this board. The Cables Unlimited USB-2920 has worked well for us, and can be purchased for less than $20.
If you decide to power your servos at 5V, it is possible to modify the board to power both the board and the servos from a single, high current, 5V supply. This requires some soldering skills. It is not supported by the manufacturer. For those with the appropriate skills, the basic idea is to bypass the onboard regulator. This is not a recommend option for novices.
Unfortunately, connecting servo power is a little problematic. We tend to use desktop switching power supplies which come with a variety of power connectors. Your options include cutting off the connector for the power supply, and directly connecting the wires together or adding a mating connector to the Mini SSC II (which will typically involve soldering).
As for jumper settings, use the higher baud rate (9600 BAUD) by connecting the bottom two header pins as shown in the manual. The Mini SSC II allows you to limit the commanded motion range to ~90 degrees. Your initial reaction might be to always use the jumper (on the two middle pins) to get the maximum range. However, the tradeoff is that the you lose some resolution (there are only 255 possible positions for either range), and your motions might be slightly less fluid when using the 180 degree option. It is also possible to damage the servos by asking to go further than they physically can, so be very careful setting up the limits. We tend to use the 90 degree setting, just to play it safe.
Pololu makes a series of servo controller boards which have some significant advantages over the Mini SSC II. The Pololu controllers support faster serial connections (up to 38400 BAUD) and have much finer positioning resolution (4096 positions vs. 255). These boards also offer a Mini SSC II emulation mode, but there is no reason to use this since VSA supports the higher performance Pololu mode.
This board is comparable to the Mini SSC II in that it has 8 channels, and expects RS-232 style serial commands (although electrically inverted). It is physically smaller and significantly cheaper, selling for less than $20. However, connecting this board will offer some challenges. To connect it to a USB port we use an FTDI USB to Serial Convertor cable (TTL-232R) which provides board power and the properly inverted interface. (Unfortunately, the cable connector does not have the pins in the right order, so some rewiring is required.) Servo power is also problematic. We have found that it is possible to solder a standard barrel power connector into existing holes upside down on the board, but this is a fairly advanced operation.
Pololu also makes versions with a standard 9-pin D connector (both 8 and 16 channel) if you prefer using a standard USB to serial convertor.
This is the controller that does it all, and is the one we use in “Perspectives”. It includes the FTDI USB to serial convertor on the board, so it plugs directly into a USB port - no external convertor needed! It also sports a small terminal strip for easy connection of servo power that removes the need to solder. The only downsides of this board are the single mounting hole and the price - US$81. But when you consider that you get a 16 channel controller which doesn’t need a separate USB to serial convertor, the price seems fairly reasonable.
This is our new favorite controller. It does pretty much everything the Pololu USB controller does (USB, 16 channel, nice power connector), but at about half the price! (US$39.95). It’s a bit larger, but that extra size allows for four nice mounting holes. The Parallax protocol resolves 1000 separate positions, which may not be quite as good as the Pololu, but the difference is probably indistinguishable in most applications.
Copyright 2007, Paul H. Dietz