Servo - January 2008

the 0xEB command, the target position can be changed, and when the servo has been re-enabled it will move to the new target position. Command 0xF6 will also do this.

Command 0xE5 will read the current position of the servo in a similar manner as command 0xE9. The difference here is that any servo’s position can be read with the 0xE5 command and the 0xE9 command reads the position of a specific servo ID number.

Command 0xE8 will read the current draw and the voltage draw of a servo. The value ranges from 0 to 255. When the servo is unloaded and not moving, the current draw value is typically 0 and the voltage value is 127. When I try to force the servo horn to move by hand, the current draw value increases and the voltage value decreases. I don’t have any information about correlating these values to actual current draw and voltage draw. At the very least, this information can be used to help determine optimal reaction load performance and to determine if the servos are being overloaded.

Command 0xEA can be used to change the proportional and derivative gains for the servo to make their motion “softer” or “stiffer.” I have not personally experimented with these numbers, but I have heard that changing them could make the servos respond faster to sudden direction changes, and it could help reduce servo jitter under certain loaded conditions.

An interesting command is 0xEB, the servo Stop/Start command. When the servo is commanded to stop (Param2 = 0), the servo will stop moving where it is. The rotor will be locked like it was commanded to move to this position, and will not let you forcibly change it. What is interesting is that once the servo is stopped, its current position can be read, the velocity can be changed, the current draw can be read, and other parameters can be changed.

When the servo is restarted (Param2 = 1), the servo will move from where it is currently located to its new commanded position. This

Header Command Param1

Param2 Checksum Return1

Return2

0x80

0xE1

0x29

0x00

0x76

0x00

0x03

0x80

0xE1

0x2C

0x00

0x73

0xB3

0x03

tmp = 0x0A - 0x00 = 0x0A (tmp = 10 - 0 = 10) checksum = 0xB3 - tmp = 0xB3 - 0x0A = 0xA3 (checksum = 179 - 10 = 169)

Table 3

command can be easily demonstrated when the servo velocity is set to near zero, and commanding the servo to move from one extreme to another. Shortly after the servo has started to move, you command the servo to stop, then change the end position or servo speed, then restart the servo, and you will notice that it will move to the new position at the new velocity.

The last motion command is 0xEF. This command will stop all motion control of the servo. Unlike the 0xEB command which locks the servo position, the 0xEF command will allow the servo to be moved by hand. Servo positions can still be read after the execution of this command. This feature enables the ability to manually move servos to specific positions, and then records these locations so that they can be played back later. This is a feature that will simplify the programming efforts for animatronic and walking robot routines.

In addition to the basic motion commands described previously, the servos can be reprogrammed with a microcontroller. The key is to know which memory locations contain which parameter. This is where the 0xE1, 0xE2, 0xE3, and 0xE4 commands come in handy. These commands can be used to read and write to different memory and EEPROM locations.

Unfortunately, the memory locations/addresses and their functions are not published. But has this ever stopped anyone from getting the information they wanted? With

the help of a serial port monitor (I like to use the Free Serial Port Monitor program from www.se rial-port-monitor.com) and the HMI Servo Programmer, I was able to determine several of the EEPROM addresses for several different parameters. With these two software packages, I can monitor the data transmitted to and from the servo, determine which memory locations/addresses are changed, and how they are changed when I change a parameter in the HMI Servo Programmer software package.

For example, I was able to determine that the servo ID is stored at the EEPROM address 0x29. This can be read using the 0xE1 command, and a new value can be used to change the ID number using the 0xE2 command. By making a set of systematic changes in the HMI Servo Programmer, I was able to determine that every single change to the EEPROM also changed the EEPROM address of 0x2C.

The value at 0x2C always seems to change by the same numerical value as I changed the other values. This gives an indication that this memory location is some sort of a Checksum. The best I can tell from monitoring the data using the servo reset function (covered next) is that the checksum is computed using the same method for individual command packets. Thus, the checksum is equal to 256 minus the mod 256 of the sum of all of the bytes written to EEPROM addresses from 0x00 to 0x2B (Param2 using the 0xE2 command or read by Return1 using the 0xE1 command).