Hi,
I have to build a servo-actuated tilt axis for a laser pointing device to be used in a medical device. It has to project a laser line at a distance of one meter, with an accuracy of 1 to 2 millimiters. This means the servo should have an angular accuracy of about 0.1 degrees. I’m hoping to use a digital R/C servo, SES mounting brackets, and an SSC32 controller.
Does anyone know how accurate the angular positioning of Hitec HSR-5995TG or HSR-8498HB digital R/C servos is? The literature on these products says that they are more accurate than analog servos, but I can’t find any measured specs on their actual accuracy.
The SSC32 has positioning values from 500 to 2500, i.e. 2000 steps for 180 degrees, therefore the controller can handle steps of 180/2000=0.09 degrees. But how accurate is a digital servo in traveling to the requested position?
Cheers,
glp
I think this is a servo with the ssc-32 (servo controller).
The SSC-32 (serial servo controller) is a small preassembled servo controller with some big features. It has high resolution (1uS) for accurate positioning, and extremely smooth moves. The range is 0.50mS to 2.50mS for a range of about 180°. The motion control can be immediate response, speed controlled, timed motion, or a combination. A unique “Group Move” allows any combination of servos to begin and end motion at the same time, even if the servos have to move different distances. This is a very powerful feature for creating complex walking gaits for multi servo walking robots. The servo’s position or movement can be Queried to provide feedback to the host computer. There is even a 12 servo Hexapod sequencer built in. This allows complete control of all aspects of the alternating tripod gait simply by transferring a few values from the host controller. Any output can be used as a TTL level output. There are 4 digital inputs that are static or latched, so you don’t have to worry about missing a short event. They can also be used as analog inputs. There are three terminal blocks for powering options. The DB9 input has true RS-232 levels for use with a PC. There is a socket for a 24LC32P EEPROM, which will be used in future versions of the firmware.
Hey,
If your planning on using SES, you can’t use the HSR-8498HB servos. They are specially made for the robonova and will not fit into the SES brackets.
As for the HSR-5995TG’s accuracy. Who knows. If it doesn’t say in the manual for the servo, then I don’t think the number exists unless someone bought one and measured it themselves in which case it would be on google but i found nothing. I would say its pretty good.
-robodude666
Oops, forgot to mention that too!
I’ve tested the standard Tower Hobbies servos and the most descrete moves I could get in ~180 deg of rotation was ~425. that would be about .425 deg resolution at best. Don’t know how much better the digital ones would be. High resolution setups usually use geared stepper motors like found in printers and such.
plus u have to account for slack in the assembly itself or in the gear train of the servo (I am guessing on the second one) Slack in the assembly bettween the output shaft of the servo motor and the tip of the laser pointer is probably a bigger concern than the accuracy of the servo itself I think, but like i hav said many times before just my $.02
Thanks for the useful feedback!
This is a summary of what I gathered so far:
- The SSC-32 controller has the desired features and step resolution (Italian_guy299). I should try it.
- I should not use the HSR-8498HB with SES (robodude666). I will try HSR-5995TG instead.
- There is no official data on the accuracy of digital servos, but analog servos have an accuracy of about 0.425 degrees (zoomkat). That’s quite far from the desired accuracy of 0.1 degrees, and I have a feeling digital servos might not improve by a factor of 4. But nobody seem to know…
- The assembly itself could contribute inaccuracies in the angular position (Hydrohexapod). Good point, I shall use the SES just to anchor the servo to its base, but not to move the laser. I will fit the laser tightly right on the servo horn.
It looks like the only option I have is to test a digital servo and report my findings back to the Forum, unless anyone else owns a digital servo, a laser pointer and an SSC-32 and wants to do a quick experiment…
Cheers,
glp
