Servo - March 2011

Our resident expert on all things robotic is merely an email away.

roboto@servomagazine.com

Tap into the sum of allhumanknowledge and get your questions answered here! From software algorithms to material selection, Mr. Roboto strives to meet you where you are — and what more would you expect from a complex service droid?

by Dennis Clark

This month, there is a mix of questions from sensors to programming. I hope that you learn something new from Mr. Roboto’s tower-top lab. Let’s dig in!

Q. Mr. Roboto, I have an Arduino Pro 328 using 5V power from SparkFun and a SparkFun FTDI Basic board that I use (over USB) to program it. When I use my Arduino 017 setup, this works fine. But, when I try to program the board using avrdude, it fails saying that “The programmer is not responding.” What am I doing wrong?

avrdude -c arduino -p M328p -P
/dev/cu.usbserial-A700e5Jl -U flash:w:gen328.hex

After scratching my head, searching the Web, and finally doing some of my own experimentation, I discovered the problem. We (neither of us) didn’t specify the baud rate. The SparkFun FTDI Basic board is a USB-to-serial translator. The Arduino Pro (ATMEGA328p) expects data at 57600 baud. Try this invocation and your setup will work:

— David (via email)

A. David, You didn’t say how you were invoking avrdude, so I put together a sketch for the Arduino IDE and indeed it worked. I then used avr-gcc to create a simple LED blinker and tried to use avrdude to program my Arduino Pro board (see Figure 1 for my layout) and sure enough, it failed for me. I started with this invocation to program the board:

Avrdude -c arduino -p M328p -P
/dev/cu.usbserial-A700e5Jl -b 57600 -U
flash:w:gen328.hex

Figure 1. Arduino Pro with FTDI Basic USB card.

The secret is the –b 57600 which tells avrdude how to set the FTDI baud rate. Downloads pretty fast, too.

Q. I want to use simple radio control servos to run my robot. How fast do these servos go and can I use their own controllers to change their speed? I like using only one controller line to control the motor. Thanks.

— Tom, Council Bluffs, IA

A. Tom, I’m assuming that you want to know how fast your robot will go when using a hobby servo and not how fast the servo runs. Those two details are related, but not the same. There are a variety of hobby servos available and they have a variety of speeds and power. Table 1 shows a small selection of popular hobby servos and a few that are more expensive to give you a quick range.