Hi Tec HS 311, HS 5485HB, HS 5645MG, and HS 7955TG Servo Comparison With Tapsterbot - jackskalitzky/tapsterbot GitHub Wiki
There are many different servos on the market. It's difficult to tell which ones will suit your needs exactly, and often the specs alone won't tell you everything you need to know. While we were trying to figure out which were the best servos to to use for Tapsterbot, we tried out four of Hi-Tec's servos: the analog HS-311 and the digital HS-5485HB, HS-5646MG, and HS-7955TG. Hopefully the results we found will be useful to someone, somewhere, and perhaps save a bit of money.
To test each servo, we calibrated the servos and then ran through most of the Tapsterbot's whiteboard functions. On paper, the highest tier of servos we tried, the HS-7955TG, should have been the best (and hopefully justified its price point) but instead we found that for our purposes the HS-5645MG servos were the best fit.
The analog HS-311 servos are the servos currently used in the Tapsterbot design. Their best feature is their price; for about $8 per servo you get surprisingly good accuracy from them.
The first test was drawSquare. The bot was told to draw a square, centered at (0,0) with a side length of 40mm.
Then we told it to draw a star.
Followed by a circle, centered at (0,0) with radius 40.
And finally, a spiral.
These shapes show the pros and cons of the HS-311. They are cheap, fast, and quiet, but at the expense of accuracy. It tends to draw crooked lines and has difficulty connecting points.
The bot was also instructed to write "Hello" in a variety of languages (because Tapsterbot is a polyglot). Here is an imgur album of its attempts.
Again, the same drawbacks are present. Crooked lines, unconnected points, and messed up letters abound. Although the words are mostly legible, there is very clearly a lot of room for improvement.
For their price, the HS-311 are good servos, but they're not exactly what Tapsterbot requires.
Onto the digital servos. These can be programmed using a separate device, allowing for greater accuracy and better results. The first tier, the HS-5485HB, cost about $25 each, and promise more torque, more speed, and smoother operation.
We ran it through the same tests as the HS-311 and found that these servos actually fare worse than the analog 311's.
As you can see from these images, the 5485HB's suffer from the same issues as the 311's, and more. The servos seem to pull up when they shouldn't be, bringing the marker off the whiteboard briefly. They do appear to be slightly better at connecting points and their lines are straighter, but the trade-off between speed and accuracy is still very large.
Here is the imgur album of the results when we used the 5485HB's to write 'Hello'.
The writing is much less smooth and much more difficult to read. There are several points where it failed to draw a straight line or connect two points properly, which really detracts from the legibility. There are also a few points where the marker was lifted up when it shouldn't have.
Overall, even with a digital servo programmer, the 5485HB's are more expensive and less reliable than the 311's. For Tapsterbot's purposes they are far from sufficient.
The HS-5645MG is the next tier of Hi-Tec digital servos, boasting more torque, speed, and precision, for around $47 each.
After doing some testing, we determined that these are the best servos for Tapsterbot to use. They offer the precision and speed required for a decent price. In future iterations of Tapsterbot it's possible that the analog 311's may be replaced by these.
The 5645MG's were much more accurate than the other two sets of servos. They drew much straighter lines, and did not have any issues with raising up. However, what the specs and images do not convey is the level of noise these servos made. Except for when in a few positions, these servos make a very loud buzzing sound, which gets annoying quickly. The sound was much more noticeable than the practically-silent sets before this one.
The highest tier of digital servos that we tested was the HS-7955TG. These servos, at $100 each, have titanium gears, incredible torque, high speeds, and very precise moement. They are extremely powerful and extremely accurate.
In the tests, they drew high-quality shapes, quickly.
As you can see, the 7955TG's are very, very accurate. The shapes they drew were very good looking and smooth. However, their power worked against them -- on multiple occasions, the servos would move so quickly and with so much force that they actually popped the arms off. They also made an unbearable amount of noise. It might not be an issue when used in a model airplane, but with Tapsterbot the noise was just too much.
Imgur album of the 7955TG's attempts at 'Hello'.
###Comparisons
Here are some side-by-side shots of the shapes the Tapsterbot drew with the various sets of servos.
###Final Thoughts
The HS-311 servos are good servos for just starting out. They are cheap and relatively accurate, but leave plenty of room for improvement.
The HS-5485 servos are the worst of the four. They cost significantly more than the HS-311 servos but performed worse. When upgrading your servos it may be worth skipping this tier and heading straight to the next one.
The HS-5645MG servos are the best of the four, for our purposes. They offered the greatest precision for the best price. Although the noise was an issue, these servos still easily outclass the two lower tiers.
The HS-7955TG servos are powerful, accurate, and loud. However, they offered only marginally better precision than the HS-5645MG servos for twice the price. This, combined with the amount of noise they make, renders them unsuitable for Tapsterbot, even though they are very good servos.