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Ultra Servo

An ultra strong and fast servo that is reasonably priced. The goal is to generate 60ft*lbs (11 520oz*in) with 60 rpm no load rpm speed.

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Building anything that can interact with everyday life objects require high powered servos. This servo is intended to serve the maker ecosystem as to facilitate macro projects that interact with the world.

Specs:
60ft*lbs stall
60 rpm no load speed
Programmable angle range
6" x 5" x 3" overall dimentions
12V or 24V operational voltage
TTL, SPI and Standard RC communications

The servo will be have a custom motor controller that will be Arduino based so anyone that needs to modify the parameters or so simply add functionality. Not all the I/O will be used in this project so they will be brought to a header pinset so that the user can then add/program in new features as they see fit. The position sensor will be a magnetic hall effect sensor. This type of sensor will make the system much more versatile because it will allow the user to select the angle range of motion, continuous motion or what ever the programmer wants it to be.

The servo project has started in 2015 when I decided to build a robot arm. First and foremost is to find a drive system that is at the very least as strong as a human arm (I used my arm as a benchmark). After looking under every internet rock it seems like there is no real product that can do this under 3000$. There are some servos that do come close but there is no way to extract any information from them. Ex: Position, speed, torque applied etc....

All these problems ultimately made me want to make my own high powered servo. My benchmark for a servo will be as follows:

  1. 60 lbs*ft at stall
  2. 60 rpm free rotational speed
  3. Absolute positioning
  4. Programmed output angle range
  5. Arduino Based micro controller
  6. 30A continuous motor controller
  7. Current sensing
  8. On board temperature sensing
  9. Brushed DC motor
  10. Most compact design possible
  11. water cooled as option (Would increase the overall power rating by 15%-20%)

This servo will not be super duper accurate down to the 15 arc-min. This is meant to be used by the hobbyist, prototype validation or robotics that do not need accurate positioning (like prosthetic, remote control lawn mowers, remote control cars etc.). This limitation is due to the absolute positioning sensor (resolution of 0.1 degrees). When the servo is done, if someone wants to put incremental sensors onto the motor output shaft then I would think that an accuracy of 20 arc-min is achievable (limited by the backlash of the gearbox) .

I think that this servo will be an essential building block for any projects that want to interact with human scale objects or systems. Without this (relatively) inexpensive servo, it will greatly reduce the development time of large project. This will liberate some precious time to give the inventor to focus onto the larger problem that he/she it trying to solve. I think that many inventor and entrepreneurs are simply overwhelmed by the complexity of human scale projects because they will have to design E.V.E.R.Y.T.H.I.N.G. from scratch. These "Lego" pieces will help to alleviate the development process.

As of now, I have done the following:

  1. Tested 3 types of gearboxes to measure efficiency at high torques (planetary is king)
  2. Built Prototypes with worm gearbox(nmvr 40), bane bot gearbox and CNC planetary (PLE60)
  3. Programmed PID Loop, then added in a velocity error signal to tame overshoot. Now a PIV loop
  4. Sized all components: Motor, Gearbox, timing belt, timing pulleys, FET, shunt resistor and mounting hardware
  5. Cad all parts and completed 80% of the design

What is left to do:

  1. Finish the ESC design
  2. Build ESC board
  3. Program in the motor controller
  4. Order all aluminum parts to be laser cut
  5. Order remaining parts
  6. Bend aluminum parts
  7. Weld the main body of the servo to mounting plates
  8. Final assembly
  9. Lots and lots of testing

Once the 2nd prototype is completed I will compile the parts list and see where I sand for a price point. As of right now I am aiming at 750usd per unit.

Licenses:

  • Circuit Maker (open source)
  • Arduino Development platform (open source)
  • Autodesk Inventor (Yearly subscription)

Below is a link to all of my shared files on the projects so as the project moves along, the information will become more and more complete.

https://1drv.ms/f/s!Ai2uF3UuOcw7irUw5dLHe-PWO3gtZg

PLE60 Planetary Servo.pdf

Final dimensions of servo

Adobe Portable Document Format - 213.81 kB - 06/12/2017 at 03:15

Preview Download

Servo analysis.xlsx

A very comprehensive spreadsheet to compare current servos and my custom designs.

sheet - 36.29 kB - 04/30/2017 at 03:32

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  • 1 × TI DRV8702-Q1 Automotive H-Bridge Gate Driver
  • 1 × RS775 BDC Motor
  • 1 × PLE60 160:1 Planetary gearbox with 160:1 reduction
  • 4 × FDB7030BL Mosfet N-Channel 30 V 9 mohm Surface Mount Logic Level PowerTrench Mosfet TO-263AB
  • 1 × Atmel ATMEGA168-20AU ATmega Series 20 MHz 16 KB Flash 1 KB SRAM 8-Bit Microcontrolle

View all 6 components

  • Parts ordered and design completed

    patchartrand06/12/2017 at 03:34 0 comments

      Since I have created this project here, I have done some work to push the project from "pie in the sky" to reality.

      1. Built a ref flow oven to cook the custom boards
      2. Completed the design of the circuit board
      3. Completed the CAD design of the servo
      4. Ordered all the electronics parts from digikey
      5. Ordered all the mechanical parts
      6. Ordered the PCB board

      As of today I have the PCB and electronics parts here, ready to be installed. Last week I had to tweak the arduino controlled oven to follow the heating profile better. (picture below)

      If anyone wants me to show how I built the oven, please let me know I will make a write up about it.

      Below are some photos of the PCB and the stencil.

      I am hoping to put the circuit board together this coming week to I can start programming. It is my first pcb design and build so in all fairness I have no clue how this this going to work out. I triple checked my connections, diagram and data sheets. I am sure that I commited a rookie mistake somewhere that I will find myself quite dumb for doing. But hey, you need to start somewhere right?I am sure that at the end of this there will be a log on the trouble shooting that when on to make it work.

      What is left to do:

      1. Send the flat patterns out to the laser cutter
      2. bend all the pieces
      3. Populate the board
      4. Update my code to go onto the new board (that's the big one for sure)
      5. Build the servo

      I leave you with some glamour shots of the servo's last CAD design iteration.

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Discussions

BotLawson wrote 05/01/2017 at 03:17 point

Something that can help with overshoot is to zero-out the integrator/accumulator when the limits of the motor are exceeded by the given command or load.  (i.e. too much torque needed, too rapid an acceleration change, etc)  When your servo can't keep up with the command or load, what use is it to accumulate up errors?

Fyi, a "command filter" will let you tame over shoot even more.  Basically you feed input commands through a simple model of the servo
and limit the commands to what the servo can actually do.  (the model ideally has some idea of the load)

  Are you sure? yes | no

patchartrand wrote 05/01/2017 at 16:59 point

Thank you for the advice. I have not thought of a command filter but I can definitely see that it would be a good addition to the control loop. Would this be a limit on the feed forward signals or just a fixed point that would cap off the error?

I know what you mean by eliminating the integrator`s built up value. I am not sure when and how to "reset" its value..... I was thinking of using the instant acceleration and speed to "burn off" the integral error as it approaches the set point. This would leave the steady state error alone. Let me know what your thoughts are.

  Are you sure? yes | no

VijeMiller wrote 04/25/2017 at 03:45 point

Have spent a solid two years hating steppers for hit and miss torque -- constantly returning to servos. If you build this, they will come. *nudge* that's what she said.

  Are you sure? yes | no

patchartrand wrote 04/25/2017 at 12:11 point

Thank you for the nudge in the right direction! You have quite the project list btw. The Imgur mouse made for quite the chuckle.

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samuel_carriere wrote 04/21/2017 at 13:02 point

It's about time someone makes a decent servo!

  Are you sure? yes | no

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