The first result is entire PDF though. I remember back in the old days being a poor student trying to save money spending hours trying to find scans of texts (or making scans in the library). Now it’s the first result in Google! Here’s hoping none of us need to make money through writing…
man back in my college days (2006-2012), it was a pain in the ass to have books (materials science) and find stuff online. anyway, i was known as "the king of pdfs". because i always knew how to search things. but now the game is in a whole another level and my fame would fall into oblivion!
With that said, I personally prefer buying an official book if available, so that the author can get their royalties. Many publishers even offer DRM-free ebooks.
When I can't find a pdf I like buying old editions on eBay for $10 or whatever. I find that having to browse through more info to actually find what I'm trying to reference leads to better ancillary knowledge and understanding than CTRL+Fing a pdf.
its funny how huge chunks of the upper class white collar work force spent their entire youth stealing thousands upon thousands of dollars of stuff with zero punishment.
> Here’s hoping none of us need to make money through writing…
i'd assume pretty soon hardly anybody can still make money through writing as it's easy to ask chatgpt "write me a book titled Motors for makers: A Guide to Steppers, Servos, and Other Electrical Machines".
or maybe i lack vision, maybe soon chatgpt can answer "should i use a stepper or a servo for my robot arm", or "how do i wire up stepper xyz on CNC machine ABC", or even "design a 3d printer with parts <$100 on ali express".
should i use a stepper or a servo for the robot arm i'm designing ?
Stepper motors are best for precise, open-loop control applications where position and speed can be accurately controlled. They are also a good choice for low-speed, high-torque applications. However, they tend to be less efficient than servo motors and can generate more heat.
Servo motors are best for closed-loop control applications where precise position, speed, and torque control are required. They are also more efficient than stepper motors and generate less heat. However, they tend to be more expensive and less precise than stepper motors.
Therefore, if precise control and speed of the arm are the most important factors, a stepper motor may be the better choice. But if overall efficiency and cost are more important, a servo motor may be a better option.
Robot arms should use harmonic drive gearboxes and servos, IMO. In practice, if the loads are not massively high, the important thing is closed loop steppers or servos, with accurate homing routines and good error handling.
It depends on the kind of load and the job. Harmonic drives tend to shear teeth if they get a shock load. It's good to have a back-driveable gear train, so that if the arm hits something or is overloaded, the forces push back to the motor, and the controller can see them and stop or deal with the problem. For something like a pick and place machine, you don't need that, but if you're trying to put screws into holes and might miss, you do.
3-phase servomotors (which used to be called "brushless DC servomotors") are much more available than they used to be. Drone motors are tiny 3-phase motors, and can be controlled as servomotors. The drone industry has done a lot to make such motors cheaper.
The controllers are much smaller and cheaper now, too. They used to be the size of a book or worse. But they're still more expensive than they should be. I was once at a trade show talking to a rep from a motor company I'd used, and noticed they now sold controllers, too. He said, yes, they had to get into the business because others were selling controllers for 10x the cost of the motor, and they cost about the same to make.
For a long time, you had to bolt the encoder on the back of the motor, and encoders cost way too much. Motors with built-in encoders have become more common.
"RC servos" from model aircraft are cheap, but crappy actuators. They're advancing from 1970s pulse width modulation control to 1980s serial, and for under $20 you can now get some force feedback.
When we build actuators in the Leg Lab, every actuator had some electrical feedback element. For example, leg length actuator had a resistive strip that was as long as the leg's travel. A fixed 'wiper' allowed us to therefore have a potentiometer, giving a leg-length-proportional voltage on the wiper when top and bottom were driven by a constant current source. This was used in a 500Hz (sometimes 1 kHz) servo loop for position control.
(somewhat related): We also had a velocity sensor on the legs -- a simple coil of wire and a magnet (but COTS). This gave us a Velocity signal directly -- rather than differentiating the position measurements. Velocity estimates made by differencing position measurements tend to be noisy, and filtering esp FIR filters introduces phase delay -- things you do not want. Lesson was: If at all possible, get a sensor that senses the quantity you want to sense, so you do not have to integrate or differentiate to get your desired quantity. It's the same reason we had a REAL gyroscope, and not a rate gyro (as in most MEMs IMUs -- in IMUs they call them 'gyroscopes' but they are actually 'rate gyroscopes' -- they only directly measure "theta-dot").
> Velocity estimates made by differencing position measurements tend to be noisy, and filtering esp FIR filters introduces phase delay
Yes. Which is why you see encoders for sale with a zillion points per rev. Not because the position is that precise, but so that you get enough events per second to calculate velocity. Actual tachometers, which are little DC generators, are sometimes used in servo systems, but not often. The last time I saw one, it was on a mainframe tape drive.
You do want velocity and force feedback if you're doing anything beyond pick and place in a totally controlled environment. Simple preprogrammed blind movement is most of industrial robotics, though. There are fancier systems, but most of them don't sell, because the engineering cost exceeds the value add. Sometimes you see a little force feedback to get something inserted properly, but more often you see some mechanical spring setup to get the same result.
I used to be into legged locomotion, but there's no profitable market. Many of the technical problems have been solved, though. The sensors and actuators are good enough. Electric motors finally are strong enough. (It's sad that Schaft went under. Google bought them and dumped them.)
The first result is entire PDF though. I remember back in the old days being a poor student trying to save money spending hours trying to find scans of texts (or making scans in the library). Now it’s the first result in Google! Here’s hoping none of us need to make money through writing…