Do I need to add in a for loop and use the iteration number, i as angle for the position? I have an error in this line:

                    for (i=0;i<=800;i++);
                    mx28.SetMovingSpeed(servo1, 100, false);
                    mx28.GetPresentPosition(servo1,i);
                    mx28.SetGoalPosition(servo1,i);

Error: Argument of type "int" is incompatible with parameter of type "std::uint16_t *" in "MX28/main.cpp", Line: 24, Col: 52

By using GetPresentPosition, will the motor only run when I push up the joystick and stop running once I stop pushing the joystick?

Thank you

The syntax for getPosition isn't the same as setGoalPosition. It returns a float of the current position. At least it does in the libraries I found, they don't seem to quite match the syntax you are using but since you didn't say which library you are using I had to guess.

You only want to set the goal position once, when the button is first released. Something along these lines,

bool moving = false;
while(1) {
  if (up) {
    moving = true;
    ...
  }  else if (down) {
    moving = true;
     ...
   } else if (moving) { // we were moving but button was released
    moving = false;
    mx28.SetGoalPosition(servo1,mx28.getPresentPosition(servo1)); // set the goal to the current location
  }
}

#include "mbed.h"
#include "MX28.h"
#define MX28_DEBUG

MX28 mx28 (p9, p10, 57142);
DigitalIn up(p15);
DigitalIn down(p12);
bool moving = false;
int main() {
 
    uint8_t servo1 = 0x01;
 
    while (1) {
            if (up == 1)
            {
                    mx28.SetMovingSpeed(servo1, 100, true);
                    moving = true;
            }
           else if (down == 1)
            {
                    mx28.SetMovingSpeed(servo1, 100, true);
                    moving = true;
            }
            else
            {
                    mx28.SetMovingSpeed(servo2, 100, false);
                    mx28.SetGoalPosition(servo1, mx28.GetPresentPosition(servo1));
            }
      }
}

These are my codes to run dynamixel RX-64. I am using MX28.h library.

I understand what are you trying to teach me about the getpresentposition but may I ask what '&presentPosition' should I set as complier prompts me this error.

Error: Too few arguments in function call in "MX28/main.cpp", Line: 39, Col: 80

Also, if I do not use mx28.SetGoalPosition for the if(up==1) and else if(down==1) statement, how can I make it rotate in the opposite direction for (down==1) ?

Sorry to trouble you. I have only learnt mbed few days ago :(

Thank you for your time! Eugene

Which MX28.h library? The search here is not great and many libraries have multiple versions. Can you post a link to the page please.

http://developer.mbed.org/users/GIPetrou/code/MX28/

I uses this version of mx28.h library.

OK I think this should be the correct syntax.

#include "mbed.h"
#include "MX28.h"
#define MX28_DEBUG
 
MX28 mx28 (p9, p10, 57142);
DigitalIn up(p15);
DigitalIn down(p12);
int main() {
 
    bool moving = false;
    uint8_t servo1 = 0x01;
 
    while (1) {
            if (up == 1)
            {
                    mx28.SetMovingSpeed(servo1, 100, true);
                    mx28.SetGoalPosition(servo1,800);
                    moving = true;
            }
           else if (down == 1)
            {
                    mx28.SetMovingSpeed(servo1, 100, true);
                    mx28.SetGoalPosition(servo1,100);
                    moving = true;
            }
            else if (moving)
            {
                    uint16_t currentPos;
                     mx28.GetPresentPosition(servo1,&currentPos); // read the position
                     moving = false;
                    mx28.SetMovingSpeed(servo1, 100, false);
                    mx28.SetGoalPosition(servo1, currentPos); // set the current position as the goal
            }
      }
}

Note the else if (moving) on the code for stationary. This means that that code is only run when the button is first released, after that no new commands are sent to the motor until either up or down are pressed. We've set the goal for the motor, I would hope it would try to hold that location until we tell it otherwise.

Since all the moving speeds are the same you could probably set that before the start of the while loop rather than before every command but it should't hurt anything doing it this way.

As for you're other questions earlier, the bits within the up/down code should be the same as before with the addition of setting moving to true, I skipped those lines since they were unchanged and I couldn't copy/paste them.

One other lazy trick in c, anything other than 0 is considered true. Since a digital input will either be 0 or 1. which means that

if (up) { do something }

will have exactly the same effect as

if (up == 1) { do something }

This is a personal coding style issue, I find the shorter version cleaner in this situation.

Thank you for your explanation and the codes ! It works perfectly just like what I want :)

Thank you !