digital servo motors, default speed

[tjump]

I'm testing a servo controller (SD20) and noticed something unexpected. My Hitec HSR-5980SG servos took four seconds to make a 180degree sweep. The spec sheet reports speed of servo at 7V+ to be 0.19sec/60 degrees. My calculations lead me to expect a full 180 in 0.6 sec.

As I started looking into this I found that the 5980 is programmable by what Hitec has dubbed HMI, and through a serial interface the servos can be programmed, ergo set speed (among other things). This looks like a really cool feature, but my current task is to confirm that the SD20 is functioning properly, and I suspect (hope) the default speed setting of the servo is what is causing my unexpected slow response speed.

If anyone can confirm this, or enlighten me as to what else may be causing this speed issue I would appreciate it.

[Admin]

Is the servo modified? Is the battery fully charged?

[tjump]

This was as I expected. The servo I was testing had the speed set low. Once I reset the servo to the default settings it worked fine.

There are two different programming interfaces for the Hitec robot servos. One connects to a computer via a serial comm. while the other is a rechargeable self-contained unit. The self-contained unit was very simple to use but I suspect the computer interface offers more fine control. However, I could not get the serial interface to work over a usb to serial converter and I have not taken the time to work this through. It was not my focus at the time so getting the servo reset to factory defaults was sufficient to test the SD20.

BTW, the SD20 v.3 does have a bug that has been confirmed by the supplier. A fix is in the works.

[Admin]

I own the servo programmer that you speak of, its pretty easy to use.

As for the usb to serial converter, is it TTL or RS232?

[tjump]

RS232

[Admin]

I'm fairly sure it requires TTL and not RS232. You can get adapters for under $15 at sparkfun.com.

[tjump]

With the included cable the RS232 to TTL conversion should be managed (See http://www.robotshop.us/PDF/hitec-hmi-servo-programmer-manual.pdf). The software saw the HMI interface over the COM port but just would not connect to the motor. It could be that without the external battery the motor can not be seen by the computer (I don't remember if I did this but I'm guessing not since I had the other programmer. I probably just defaulted to what worked rather than tracking down and charging a 7v battery pack).  I have also read where some of the programmable servos, if they are too early a production model, will not connect. Again, as I was not focused on the programming interface at the time but rather just resetting the servo I just did not spend any time on this.

Thanks for your help.

[Admin]

With the included cable the RS232 to TTL conversion should be managed

Not necessarily. I'd try to do a direct USB to TTL conversion.

It could be that without the external battery the motor can not be seen by the computer

Hmmmm the servo needs to be powered!

[tjump]

Okay, you got my curiosity up so I hooked it up again. The cable included with the HMI programmer does do the RS232 to TTL conversion and works fine with a standard USB to RS232 Serial adapter. External power must be supplied to the servo via the HMI cable (the HMI cable has three branches...one to the RS232 connector, one to the servo, one to V+ and Gnd.) in order for the programmer software to connect to the servo. Simply enough, just start the HMI programming app., select the proper COM port, select connect and away you go.

I should note, these Hitec digital robot servos are looking for 6 to 7 V+. Any dip below 6v and they will not run/connect to the programmer. They also draw more current so battery drain will be faster than with standard analog servos. But...they are very cool with all the added controls, torque and precision.

Cheers.

[Admin]

So, it works now?

[tjump]

Yes.