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3D Printed Robot Actuator

A high speed and high torque robotic actuator using low-cost brushless motors, custom controller, 3D printed parts and bearings.

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I wanted to make a quadruped or biped with enough power to jump off the ground. Typical servo solutions were not good enough.

Design sections
Mechanical - Cycloidal gearbox with rolling elements, bearings and dual input and output shafts
Motor - Low-cost high torque Brushless outrunner using Multistar Elite Motors
Sensors - Dual absolute magnetic encoders for motor position and joint position
Electronics - Custom PCB including Mosfets with Mosfet driver DRV with Cypress PSOC4 SOC, sensor inputs and communications. Plus Magnetic Encoder PCBs
Software - Field Oriented Control (FOC) for position control. Mosfet Driver, Encoder and CAN communications.

The purpose of the Actuator is to create a design than I can easily make with a 3D printer using cheap and powerful Brushless motors from China. I have three prototype sizes of Cycloidal gearboxes 44mm, 60mm and 78mm diameter. The size of the motor, number of poles and their shafts are also adjustable.

WS3.cywrk.Archive02.zip

PSOC4 Project files Hardware and software (code) files More CAN comms and move instructions

x-zip-compressed - 1.76 MB - 10/23/2018 at 16:28

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tail.zip

Full Tail CAD Files Inventor Pack and Go

x-zip-compressed - 21.73 MB - 10/21/2018 at 17:40

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Leg.zip

Full Leg (with SAE) CAD files Inventor Pack and Go

x-zip-compressed - 28.52 MB - 10/21/2018 at 16:28

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6807_tail.stp

Full Tail Step Files

stp - 3.93 MB - 10/21/2018 at 16:28

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6908_LEG.stp

Full Leg (with SAE) Step files

stp - 5.83 MB - 10/21/2018 at 16:23

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View all 16 files

  • 1 × CY8C4247AZI_L485 PSOC 4 SOC uC with FPGA and CAN
  • 1 × DRV83055PHPR MOSFET Driver with SPI and 3x Current Amps
  • 2 × AS5147-HTSM 14-Bit Magnetic Absolute Encoder Sensor
  • 6 × SIR638ADP-T1 N-MOSFET 40V Ron 1mOhms (100A*)
  • 1 × MCP2557FD-H/SN CAN Transceiver

View all 11 components

  • Arduino Brushless FOC Controller - Development

    Paul Gould10/20/2018 at 08:08 0 comments

    The Cypress PSOC4 Controllers are not very common in the maker community. As I would like more makers to be able to use/modify/improve this Actuator, I am designing an Arduino version. It will be based around the MKR Zero with the SAM D21.

    Earlier I got the Centered Aligned PWM working for controlling the MOSFETs for driving the Brushless motor. Next was the SPI for reading the Magnetic Absolute encoders for the motor and joint positions. The SPI on the SAMD21 only has a single buffer which is not time efficient for this application. It does have a DMA on the SPI which is much more complex but the most efficient for operation. The problem is that it is not a standard feature for the Arduino system. 

    I started with the following information.

    https://forum.arduino.cc/index.php?topic=344029.0

    Now the Adafruit Zero DMA library makes it too easy

    https://github.com/adafruit/Adafruit_ZeroDMA

    and can now be installed by the standard Arduino IDE --> Sketch --> Include Library --> Manage Libraries

    It also support Interrupts "dma_callback" to tell when the transfer has completed.

  • Robot Control software (for initial testing)

    Paul Gould10/13/2018 at 16:09 3 comments

    Visual Studio and C# in .Net was used to write a simple test program. An X,Y foot position is generated, converted to hip and knee angles using Inverse Kinematics and sent to the serial to CAN converter (spare PSOC actuator controller). These packets are addressed to the two robot actuators to move to the required position. A second mode just returns Joint positions.

  • Series Elastic Actuator - CAD

    Paul Gould10/13/2018 at 16:02 4 comments

    I have modified the base robot actuator to make it into a full robot leg for a quadruped. It has a parallel linkage with springs and 2 robot Actuators. This means that the spring direction changes from rotation to almost vertically linear when the leg is in the standard walking positions. Different strength springs can be swapped for testing, with the aim to store energy at the end of a jump cycle and use it for the next jump. It will also be used for efficient walking/running.

    Everything rotates around the same joint, for the centre joint, described from top to bottom of the drawing.

    • 6mm shaft running most of the way though
    • Motor bell and flux ring
    • Stator housing with 2 motor bearings
    • Top outer housing
    • Top Thigh plate with 6809 bearing
    • Top Series elastic output plate (also actuator output shaft) with 6809 bearing
    • Eccentric Cam with 2 lobes with 2 bearings each and 2 cycloidal gear
    • Bottom Series elastic output plate (also actuator output shaft) with 6809 bearing
    • Bottom Thigh plate with 6809 bearing
    • Bottom shaft mount with bearing
    • Motor angle Magnet holder
    • Motor magnet (diametric)
    • Bottom outer housing
    • Motor position PCB
    • Silicon steel flux cancellation plate (0.5mm thick)
    • Joint position PCB
    • Joint bearing holder (old protects the PCB)
    • Joint magnet (diametric)
    • Joint angle plate with bearing

  • Quadruped Jump Jig

    Paul Gould10/13/2018 at 15:42 0 comments

    Components

    • 2x Linear rails
    • 4x Linear Bearings
    • wood and screws
    • 11.1V 1500mAh Lipo Battery
    • Analogue Current Meter 20A
    • Digital Current / Voltage Meter
    • Robot Leg

    Assembly

    The rectangular extrusion is the same size as the quad's "backbone" and the robot's hip joins with the following design.

    The robots hip has three mounting points to the rectangular extrusion.

  • PSOC4 - CAN and FLASH

    Paul Gould10/13/2018 at 15:23 0 comments

    The PSOC4 and the IDE Creator helps you to write code for the Arm processor, the CAN peripheral and the FPGA logic. When you use it for standard operations, then you rarely need to go deep into the component datasheets to make the design work as required. The examples are quite useful.

    But, if you want to design something that is not normal, then it is hard to find the information required.

    CAN Problem 

    Each Actuator needs to have it's own individual address or CAN IDs. In my robot there will be at least 12 actuator controllers that will be CAN slaves, with one controller configured as a CAN master.  This requires setting the CAN Receiver Mailbox ID via software. It is normally done in the .cysch CAN configuration page.

    There is a easy for setting the Transmit message ID.

    CAN_SET_TX_ID_STANDARD_MSG(0,(TX_ID));

    There was no such sub-routine for the setting the RX message ID. So after a lot of reverse engineering and some trial and error, the following code is used. Set different IDs for each actuator. 

    CY_SET_REG32((reg32 *) (CYREG_CAN0_CAN_RX0_ACR), (0x0200000u*ID));

    This only works for Message Mailbox 0 and is good for my current needs. A second mailbox maybe required in the future that will be a broadcast message so all the receiver message Mailbox IDs will be the same. This could be used for an Emergency stop message. Send one message and all the actuators stop.

    Additional CAN information

    If the RTR in the Configure 'CAN' is enabled, the CAN transmitter will re-transmit the current packet until the received sends an Acknowledge packet back. 

    FLASH Problem

    As each Actuator needs to have it's own individual ID, this value and some others needed to be stored in flash memory. But when the PSOC4 software is updated it erases ALL of the standard FLASH. It also does not have EEPROM. It has an emulated EEPROM but again this is erased after updating the program. The PSOC4 I'm using has user FLASH but there is no information on how to use it. 

    The set-up code is

    #include <CyFlash.h>
    
    #define CY_TEST_USER_FLASH_ROW       (CY_SFLASH_NUMBER_USERROWS - 1u)
    #define CY_TEST_USER_FLASH_ADDR      (CYREG_SFLASH_MACRO_0_FREE_SFLASH0 + CY_TEST_USER_FLASH_ROW * CY_SFLASH_SIZEOF_USERROW) //255
    uint8 Flash_User_Array[CY_SFLASH_SIZEOF_USERROW];

    To Read from FLASH, I set up a RAM array the same size as a FLASH row.

        uint16 i;
        for (i = 0u; i < CY_SFLASH_SIZEOF_USERROW; i++)
        {
            /* Define source value */
            Flash_User_Array[i] = (*((uint8 *) (CY_TEST_USER_FLASH_ADDR + i)));
        }

     To Write to FLASH, I set up a RAM array the same size as a FLASH row.

        cystatus returnValue = CYRET_SUCCESS;
        returnValue = CySysSFlashWriteUserRow(CY_TEST_USER_FLASH_ROW, Flash_User_Array);

  • Arduino Brushless FOC Controller

    Paul Gould06/18/2018 at 14:04 2 comments

    In an effort to make the controller accessible to more makers I've strated designing an Arduino based system. It uses the SAM D21 G 32-bit micro-controller as used on the MKRZERO board. This chip has an advanced Timer Counter Controller which can generate three centre aligned PWM signals. After sorting out the correct pin mapping it is up and running.

    The code is based on information provided in the Arduino forum by MartinL.

    https://forum.arduino.cc/index.php?topic=346731.0 

    Now with deadband

    I will create a github shortly

    Next up dual SPI. One for the absolute magnetic encoder sensors and the other for the FET gate Driver IC.

    PID and FOC should be easily ported across.

    Voltage and current measurement may be difficult because the Arduino doesn't support free running sequential ADCs very well.

    The SAM D21 does not have CAN commucations so it will need RS485 and USB.

  • Biped Design

    Paul Gould06/09/2018 at 15:22 1 comment

    6x Actuators

    Do all the joints need the same size actuators?

    Are different reduction ratios required? Higher torque less speed.

  • Mini Quadruped Design (CAD)

    Paul Gould06/08/2018 at 16:16 0 comments

    12x Mini Actuators

    Needs a head, tail, electronics and batteries. 

    Two leg has been 3D printed. Some assembly required. See end of YouTube video. It's the little one on the left.

    I have made some 2 axis legs for testing. They have enough torque but the speed is huge, even under load. Jumping is not impossible. 

  • Robot Quadruped Tail

    Paul Gould06/08/2018 at 15:41 0 comments

    Tail Design

    3x Mini Actuators (5 Joints)

    Joint 1 is tail "wag"

    Joint 2 has figure 8 pulley to Joint 3 and the angle is opposite

    Joint 3 is actuated up/down

    Joint 4 is actuated up/down

    Joint 5 has a figure 8 pulley to Joint 4 and the angle is opposite

    Distance between Joints is 100mm

    Motors are Multistar Elite 3508

    The tail is used for the quadruped's balance.

    Was thinking about putting an end effector, camera or gripper

  • Robot Arm Design

    Paul Gould06/08/2018 at 14:38 0 comments

    Robot Arm

    3x Large Actuators

    2x Small Actuators

    1x Gripper ???

    Based on previous testing, it should be able to lift 0.5kg at the end effector.

View all 18 project logs

  • 1
    Actuator Assembly

    1x Brushless motor with extended shaft (6mm Silver Steel)

    1x Custom Brushless Controller Board

    2x AS5147 Absolute Position Boards

    2x 6mm x 2.5mm Diametric Neodymium Magnet

    12x 3D printed parts

    16x 5x8x2.5 bearings

    2x 6809 Bearings

    5x 12x18x4

    1x 6x10x3

    8x 3mm dia x 30mm Silver Steel shaft

    26 (or 13) 3/32" x 28mm Silver Steel shaft

    26 (or 13) 1/8" x 16mm Brass tube (thin wall)

    24x M3 bolts

    10x M2 Bolts

    3x PZT screws

    1x M6 thin wall washer

    1x 2mm dia x 11mm Silver Steel shaft

  • 2
    Sources of information

    Suppliers

    • CCDIY 3D filiment
    • Plaig Bearings
    • MiniTech
    • Digikey/Mouser
    • Apollo Bearings

    Favorite Makers

    • Makers Muse
    • AVE
    • Clickspring
    • Tom Stanton
    • Ivan Miranda
    • James Bruton
    • Gear Down For What?
    • MRB Engineering
    • Thomas Sanladerer
    • ODrive Robotics
    • Benjamin's Robotics
    • Laura Kampf
    • Tested

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Discussions

Jin Han wrote 10/29/2018 at 19:57 point

Hi, just a quick question on the PCB design (specifically for the encoder).  The schematic says "NF" which I know that not-fitted, but does this mean that you don't use any resistors at all? or what are the values for the resistor? I'm trying to make them in bulks and have PCB companies to assembly the board for me as well.

Also are the sizes for capacitors and resistors?

Thanks,

Jin

  Are you sure? yes | no

Dan Royer wrote 10/27/2018 at 17:31 point

Hi!  Saw your latest Youtube (subscribed) and I'm interested in using a version of your actuator design in the next Sixi robot.  Our worst case at the base has a 20Nm holding torque.  Can you advise us how to proceed, please?  We'd love to collaborate.

  Are you sure? yes | no

Dan Royer wrote 10/27/2018 at 17:35 point

Also we're interested in making a version that mimicks ulna rotation between wrist and elbow - so the input and output mounting brackets are coaxial to the rotation.  Have you spent any time on that, and if so what are your thoughts?

  Are you sure? yes | no

Paul Gould wrote 10/28/2018 at 04:00 point

The actuator is very rigid because it has both top and bottom housings and output shafts. I'm not sure how to make it rigid for ulna joints, I'll think about it. What angle of rotation do you need?

  Are you sure? yes | no

Paul Gould wrote 10/28/2018 at 03:57 point

Hi Dan. Yes, we can collaborate. My suggested starting point is with the CAD. I'm using Autodesk Inventor. I think that you are using Fusion360. What format would you like the CAD in? Can you use the "pack and go" files that I have already uploaded?

  Are you sure? yes | no

Dan Royer wrote 10/28/2018 at 18:47 point

fusion would be great. We probably have to make a gearbox at the shoulder that is stronger than your current model. What do you think of https://www.reddit.com/r/robotics/comments/9s134q/i_made_a_cycloidal_drive_for_cheap_robots_git/

?

I’d love to have a parametric way to make the gearbox I need, then follow your wiring recipe. In your experience, will this generator work for among the printable profiles?   Is your package already parametric? Sorry I’m not up to speed yet.

  Are you sure? yes | no

Paul Gould wrote 10/29/2018 at 03:00 point

You should be about to use the ipt files from the "pack and go" as per the instructions https://knowledge.autodesk.com/support/fusion-360/troubleshooting/caas/sfdcarticles/sfdcarticles/How-to-import-or-open-a-file-in-Autodesk-Fusion-360.html?_ga=2.119678216.1803184385.1540780561-1795324385.1536071126

I haven't used Fusion 360 but the script from the reddit should be useful.

I have created a parametric in inventor. I doesn't port well.

  Are you sure? yes | no

Paul Gould wrote 10/28/2018 at 04:07 point

What motor/controller did you have in mind?

  Are you sure? yes | no

Dan Royer wrote 10/28/2018 at 20:07 point

I’ve used a Rumba board until now (6steppers). Since you use dc im looking at odrive x3

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agenthimzz wrote 10/19/2018 at 18:10 point

Hi, Himanshu here from Youtube.. 

So, I have been thinking of this 4 legged robot and have observed that it is much better if the robot has a low Unsprung Mass. 

In Automobiles:

Unsprung mass is: wheels and Suspension.

Sprung Mass is: Body, Engine, Transmission, Chassis.... (and all other things)

It is said Unsprung mass should be very less so that when the car moves and goes over bad roads the Sprung Mass stays at almost level and Unsprung mass only moves and passengers feel very less vibrations

Hence instead of attaching the lower leg directly to the motor.. a Pulley would be connected. this pulley belt will be going above the motor, present above the Hip joint motor. and belt will be having tension using a spring return system like in 3D printer Extruder.

This definitely increases complexity and difficulty in designing and simulating but.. MIT Cheetah 3 is based on this design improvement. 

and the current Hub motor design is followed by ETH Zurich.

I don't know what Spot Mini does but they do it better, so if you find their method then do that.

  Are you sure? yes | no

Paul Gould wrote 10/20/2018 at 00:28 point

Hi agenthimzz, I would like to have as little unspring mass as possible. It was a trade-off between total weight/complexity and unspring mass. The actuators were designed to be integrated into the joint and were made as compact as possible. I found that if I used 3D printed parts as "Fixed-Fixed Beam Moment" instead of the normal "Fixed-Free Beam Moment" they were much better. There were also designed to be linked together in various combinations to form robot arms or humanoid robots. 

This design also meant that I could test my parallel linked series elastic actuator set-up. 

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AVR wrote 10/19/2018 at 17:04 point

Been following this project for a while, its huge for open source robotics!

  Are you sure? yes | no

Paul Gould wrote 10/20/2018 at 13:50 point

Thanks AVR. I'm hoping that making the actuator work with Arduino will make it useful for more makers. I think that a combination of this and the "gear down for what" gearbox will make for a perfect system.

  Are you sure? yes | no

jamma wrote 09/21/2018 at 14:36 point

Beyond the challenge of it, why did you create a custom controller vs the oDrive? Do you see any issues with oDrive and your actuator design for mere mortals like me who would go that route?

  Are you sure? yes | no

Paul Gould wrote 09/21/2018 at 15:21 point

I large part of it was to see if I could make it my own way. Mainly using Voltage FOC with a saddle lookup table on a CPU with no floating point. ( ~10 lines of FOC C code). I wanted full control over the actuator with room to grow as I learn more. The PSOC4 is my favorite uC/SOC, plus it has a little FPGA and easy CAN comms. I can connect any type of encoder up to it for motor and joint control. I didn't like the calibration set-up of oDrive (you have to reference every power-up). Using Voltage FOC motor control and dual magnetic encoders, I can tell where the motor and the joint is at any time. I am currently adding Series Elastic to the actuator. The FETs I use are very low R-on. The board also fit's my space requirement and I can make them with a simple soldering iron.
oDrive are a great idea (I've never used one) and they will add more features over time. There is a large community following and they are currently working on absolute encoders, CAN comms, a smaller single axis board and more control schemes. The oDrive CPU is very powerful and has room to do some much more. YOu can modify the code to suit your needs.
The PSOC4 is not an easy CPU/SOC system to learn. Half the work is done in the CPU and half in the FPGA section. Debugging errors can be a pain.
I started looking at porting the design over to a SAM D21 (Arduino Zero) but I didn't like the SPI. I will work more on this in the future as having an Arduino brushless actuator controller will make the project more accessible to more makers.
I hope this helps.

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Donovan M. wrote 09/07/2018 at 12:44 point

Great work! I like the compactness of the gearbox system.

  Are you sure? yes | no

lion mclionhead wrote 07/08/2018 at 07:09 point

Didn't have much luck with this concept.  Direct drive is a bit too power hungry.  Modern brushless servos can do pretty incredible things for a lot less money.

  Are you sure? yes | no

Paul Gould wrote 07/08/2018 at 09:09 point

Hi lion, this design is not for everyone. It's not direct drive, the cycloidal gearbox is 25:1 reduction. The cost is about $150 (the time is huge). The aim is 20Nm (200kg-cm), with speed of 0.05sec/60deg (testing required). The backlash is extremely low and it also backdriveable. I created this because I couldn't find a servo that suited me, if you know some good servos please let me know.

  Are you sure? yes | no

Alan Timm wrote 07/08/2018 at 02:30 point

I have an updated design.  The bolts helped, but the actuator still binds when the assembly is compressed axially.  This new version includes aluminum spacers.  Switch back to version 50 for a better view.  

https://a360.co/2yxWgQO

I built a prototype of the base design.  Clearances look good, I'll print out a new prototype when the spacer parts come in with PETG.

https://i.imgur.com/kwM9mN0.jpg

It looks like you're planning on using these at 12v.  Would it be reasonable to assume that it should approach 20Nm of force at 24v?

I've been looking at your spring compliance setup.  I don't see a 3rd sensor for spring deflection.  I think... that if you're measuring shaft count / position and joint position after the spring you should be able to derive force and imitate compliance.  I have an idea or two on how to integrate that into the actuator.  

And after futzing about a bit with the stm32f103, and then moving to the stm32f302 after realizing that the 103 makes you choose between CAN and USB, it dawned on me that there is no point.  The PSOC 4 is a good fit for this application and seems to work well.  PCB design isn't my strong suit, but I'm working to convert your design to a circular board integrating an AS5147 in the center, and using one of your carrier boards underneath for the motor shaft.  This minimizes the SPI cable length, and puts the electronics right on the actuator.

Speaking of which, where are you guys finding the AS5147/AS5047?  Mouser and Digikey are out.

  Are you sure? yes | no

Paul Gould wrote 07/08/2018 at 03:29 point

Your actuator looks good. Go to Discord->Odrive->robotics. Others are making this actuator in metal. I plan on running this on 8S (32V) MOSFETs are rated at 40V. I'm hoping for 20Nm with the 5010 motor. I'm looking at the 6215 Brushless 170KV. 

I have just made a jumping stand for one leg. I haven't added the spring feedback sensor yet. I am trying to work out how to store the spring energy. 

The more I work with SAMD21 (Arduino) and STM32fxxxx the more I like the PSOC4. If you are going to redesign the board, you should use the drv8323 instead. It is much better and has in integrated buck reg. I have started a design/PCB with it. Let me know if you want to see it. (What software are you using?)

I purchased a few AS5147 for testing last year. I am trying to find more but nothing yet.

  Are you sure? yes | no

Alan Timm wrote 07/08/2018 at 04:49 point

I thought the 5010 was spec'd out at 6S 24v max?  Maybe temperature dissapation is the limiting factor.  In that case 32v sounds like fun.

I've been using KiCad.  I grabbed Altium Viewer to get a closer look at your board.  Electronics is a weak spot for me, but I can follow most of the datasheets and documentation.  I'm using your Cypress codebase to learn PSOC creator.

Here's a sketch of what I have in mind.  A circular board with an AS5147 mounted dead center, keeping the electronics close.   No long SPI runs, just power and CAN to each actuator.  motor shaft AS4187 mounted on a carrier board underneath.  USBUART for pre-install configuration. 

https://i.imgur.com/rK4i1fp.jpg

  Are you sure? yes | no

Paul Gould wrote 07/08/2018 at 06:53 point

The 5008 is 6S max and the 5010 is 8S max but it is rated at continuous operation with a lot of air flow. The PSOC code needs to be tidied up, sorry. The round board looks good. 

  Are you sure? yes | no

Gabrael Levine wrote 07/08/2018 at 04:31 point

I've been using the AS5047 boards from massmind.org. However, they only sell assembled boards and not the ICs themselves, so I don't know if that's any use to you. 

  Are you sure? yes | no

Alan Timm wrote 07/09/2018 at 23:24 point

Yeah, looks like both the AS5047 and AS5147 are out of stock in the usual places.

Per Paul, the AS5601 isn't suitable.  

There's also a MA700GQ-P that support SPI, but would require a carrier board change and most likely some code changes as well.  Paul says that this one might work.

  Are you sure? yes | no

Gabrael Levine wrote 07/01/2018 at 22:27 point

Great design! Do you have an estimate of the torque-to-weight ratio? I'm working on a similar project (https://hackaday.io/project/159404-opentorque-actuator) that puts out 56 Nm of torque and weighs 1.2 kg. I'm curious how that compares to the specs of your actuator. 

  Are you sure? yes | no

Paul Gould wrote 07/02/2018 at 16:13 point

The actuator weights 536g with all the electronics and cables. (Not including battery). 

Theory - The Multistar 5008 330KV has a Kt of 0.029 (Nm/A). Max Amps = 27A and a gear ratio of 25:1 x 75% Efficiency = 15Nm

Testing  - (As in the video) Lifted 3Kg at 330mm = 10Nm (Voltage and Amps need to be rechecked).

I haven't pushed it to hard yet and I only have 1/2 the external pins in. 10Nm is enough for my application. I am hoping the big quad i'm making with Series Elastic Actuators with be about 10Kg (including some light/small batteries and with minimal sensors)

I like those large high pole count motors. I wanted to use them but I would have to change my control scheme and not use the magnetic encoder (only works well at 7 pole pairs). I just read that you are using the AS5047 magentic encoder with O-Drive. How well does it handle 0RPM holding positions?

  Are you sure? yes | no

Gabrael Levine wrote 07/02/2018 at 21:58 point

The AS5047 has been working perfectly so far. No issues with holding positions at 0 RPM. Currently I'm just using the incremental ABI interface, but I'm going to set up the absolute position interface so it doesn't have to search for the index pulse on startup. 

I see you're using the AS5147. How come you say it's limited to working with 7 pole pairs? That sensor has a resolution of 2048 CPR according to AMS' website, which should be plenty for any high-pole-count motor. 

In regards to the efficiency of your gearbox, have you measured it at 75% or is that just an estimate? 75% seems low for a single-stage cycloidal reducer. If you're having issues with sliding friction, I recommend printing the cycloid disks out of nylon. I used Taulman Alloy 910 for the gears in my design and they run very smoothly. 

  Are you sure? yes | no

Paul Gould wrote 07/03/2018 at 14:19 point

Gabrael - The data sheet of the 5147 says the the UVW interface support 7 poles pairs, so I assumed that it should only be used with 7pole pair motors for all the interfaces. I might be wrong on this and will have to do more research. I would like to use the higher pole count motors with a lower KV and higher torque and lower gear reduction. 

Did you try any other motor? How long does the 9235-100KV take to spin up to full speed? What RPM are you getting?

I haven't measured the efficiency yet, I would rather underestimate. I don't think I am having sliding friction problems, it is all rolling elements. I have some Nylon but I have to upgrade my printer so that it can reach a higher temperature.

  Are you sure? yes | no

Gabrael Levine wrote 07/04/2018 at 02:09 point

The 7 pole pair limitation only affects the UVW interface, since it outputs commutation signals for each of the motor phases. Since you're doing the commutation from the microcontroller, you can use any number of pole pairs you want. 

The 9235-100KV spins up really fast. See here: https://www.youtube.com/watch?v=0rMyaOB3w1Q

The max RPM should be 2400 (24 volts * 100 rpm/volt). I haven't confirmed this though. 

These 9235 motors are great, by the way. They have so much torque you could use them in a quadruped with no gear reduction at all. 

  Are you sure? yes | no

John wrote 07/01/2018 at 20:44 point

Your design is impresive! It got me to thinking about how animals and insects can jump so well. In biological systems that are "jumpers" (fleas, grasshoppers, kangaroos, even humans), the tendons are loaded before a jump, and then the "release" happens ... which basically adds a spring into the system. There are linear wound springs (like in a dog walking leash - but stronger!) that might be usable in your system. You would have to "pre-wind" the spring, and then trigger it. Given a strong enough spring, you could get some pretty decent "instant" burst of power from a "small" addition. You mention backlash; if you have rotational transducers, couldn't you just "fix" that with software? That might introduce too much delay, maybe. Best of luck, and keep posting! Between you and Mike's Robot Dog (see: https://hackaday.io/project/52956-mikes-robot-dog/discussion-108871), y'all are making me want to get a 3D printer, or learn to cast brass, etc!

  Are you sure? yes | no

Paul Gould wrote 07/02/2018 at 15:48 point

I am working on creating a video of the parallel linked series elastic actuator that is on the big quad. I am trying to work out how to store the spring's energy on foot fall/ landing and release it later in the gait. 

The backlash is quite small, it is the main reason I chose the Cycloidal gearbox design. It is more than good enough for e quad, maybe not for a robotic arm. I was after speed and torque rather than precision. 

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Alan Timm wrote 06/28/2018 at 18:05 point

Hey Paul, I've been really studying your actuator model, and I have a few suggestions that I'm incorporating into my build.  The rest of my parts should be arriving next week.  Here's a link to my work in progress model:  https://a360.co/2yxWgQO

 - If you separate the eccentric lobes you can pin them separately during assembly. I'm pretty sure that having a single pin driving both lobes is going to be the weakest point at load.  I'm also looking at 2mm v 3mm retaining pins.

 - I've reduced the drive rollers from 8 to 6, and included 6 through hole bolts.  The locknuts need to be trapped into the motor bracket earlier in the assembly.  This should make the actuator much stronger.  I'm on the fence between having 6 drive rollers and 6 bolts, or going 4 and 4.

I hacked together a parametric cycloidal gear box generator into Fusion 360, changing ratios is almost automatic by adjusting the parameters.  I've been going back and forth between a 26:1 and 32:1 gear reduction for the Gartt 5010.  Out of curiousity, what design software are you using?

https://i.imgur.com/29jmPCP.jpg

https://i.imgur.com/juXSZ5D.jpg

On the electronics side, I have even more catching up to do.  How are things going on your next controller?  Depending on how invested you are in your SAM D21 design, the stm32f103c8 may be another option.  It's what's used in the "STM32 blue pill" board.  They're cheap, fairly well supported, and include CAN as well as a bunch of other features.  

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Paul Gould wrote 07/02/2018 at 15:42 point

Hi Alan, Adding the bolts is a good improvement. Just be careful you still have enough material at that diameter so that it doesn't weaken the gear. I tried to replace the 3mm rods with M3 bolts with a long shank. I had some problems getting the alignment correct. (It also took along time to take the unit apart). 

I have just created a parametric cycloidal gear in Autodesk Inventor (All my CAD is done in this program, I have it on my laptop for my day job). 

Have a look at the testing of the motors here http://build-its-inprogress.blogspot.com/2016/01/motor-characterization-for-small.html

This is main reason I choose the Multistar Elite 5010/5008. I am looking at Foxtech and Arris for their 5010 motors but I just searched the Gartt 5010 on ebay and they are 1/2 the price. mmmm

I got the SAM D21 up to the point where I don't like the CPU. The SPI only has a single buffer TX and RX and requires the DMA to use it properly/efficiently. I wanted to create an easy solution (Arduino) for brushless actuator control. I'm not sure if I can hide the DMA in an Arduino library. I have thought about using the STM32F3 family but the PSOC4 is just so good. The problem is that is too complex for most makers.

I have some STM32Fx development boards so I will see what i can up and running. The stm32f103c8 will still require the use on DMAs for the SPI unless I change the structure of the code/interrupts.

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MattB wrote 06/26/2018 at 04:00 point

Hi! In your video you showed it pulling 10 Nm @ 12 V with a peek of 5 amps. This is with the Multistar elite 5008 & a 26:1 gear ratio, right?

I love the results, but am a bit confused. The 5008 is 330 Kv, which would be a Kt roughly 0.029 (Nm/A). 

0.029 * 5 = 0.14 Nm * 26 = 3.75 Nm

Which is way under 10 Nm. Is there something I'm missing?

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Paul Gould wrote 06/26/2018 at 10:49 point

Hi MattB, Your maths is right. I have never done the Kt calc befre. Where did I go wrong? 

The arm is ~330mm between the centre of the 3kg weight and the pivot point. The battery is 3 cell Lipo ~12V. The cheap current sensor from Hobby King max value shows ~5Amps. I would guess that the filtering / averaging / update rate of the current sensor is not showing the actual peak. Is it the measurement Amps RMS?

I wonder what the efficiency of the cycloidal gearbox is. I would be surprised if it was over 80%. 

Hobbyking claim that the KV is 330. I should be able to check this. Does the KV change as the speed approaches 0RPM/stall?

Does the Saddled Sine wave output of the controller give a different reading than a normal sine wave. Could this effect the Kv and Kt values?

As the arm moves from Vertically down to horizontal the torque required will increase. Maybe it is better to use a large pulley with a weight as this torque will be constant. 

I will set-up the test again with better equipment and post the results. It would be nice if the theory matches the actual.

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Alan Timm wrote 06/20/2018 at 15:51 point

Hey Paul, this is an outstanding project, and congratulations on your win! 

For the parts that require you to center them, it would be a snap to design and print a centering spacer to use during assembly. 

I've been eyeballing the failure modes, and I think the first part to fail at load will be the eccentric bearing lobes. That's based on the stresses between the top and bottom lobe and a single eccentric bearing pin, which I assume is going through the center of the top lobe. Maybe sticking a second pin through the center of the lower lobe would help mitigate that?

And, did I see that you can backdrive the actuator? I wouldn't have expected that.
I have to admit that I've watched this video about 12 times already to understand how you've accomplished this. I'm looking forward to building a couple of these to see how they work. 

Aliexpress would be a great source for most of the parts like the bearings, bulk 2mm x 28mm steel dowel pins, and 3mm (2.6mm internal) brass tubing.  the 2mm pins are a bit smaller (15%) than the 3/32".  Do you think those will work?  Otherwise there are other sources for true 3/32" x 1 1/8" steel dowel pins.

Can you share a bit about the modified axle/shaft?  Did you fabricate it?  Or is that something that can be found and ordered online?

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Paul Gould wrote 06/23/2018 at 15:38 point

Hi Alan, I tried to put a second pin in the eccentric top lode but the "order of assembly" did not allow it to happen (unless I press fit the motor on the main shaft after assembly, this was not going to happen). I get silver steel rods from minitech and the brass tubes from the local hobby shop. I always buy local if I can get the right part (no matter what the cost) and only order overseas if I have too. i prefer to make parts from scratch if I can. Hacksawing a rod and filing the ends are easy to do.

The main 6mm shaft is a silver steel rod from https://www.minitech.com.au/. i just file a flat spot for the motor's bell grub screw and drill a 2mm hole for the pin at the right distance for the second lobe. I don't have many tools. (Cheapest 3D, cheapest drill press, cheapest vice and files and that's it). I can use my vice as an arbor press. I pressed out the motor's original shaft and pressed in longer 6mm silver steel shaft

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neilmbajaj wrote 06/19/2018 at 23:16 point

Can the actuators be backdriven? I suppose if you want it to stand passively, you would not want that, bu it seems like putting the bearing in, on at least the interior pins, could greatly reduce internal friction. 

Since your external pins touch two of the cycloid rings at once, would bearings not help there? Your cad odel appears to have bushings over the exterior pins, but i feel like they might have the same issue.

Backdriveable actuators are especially nice when you want to do force control, or try to map motor current to torque, which might be useful when trying to make this robot walk or jump.

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Paul Gould wrote 06/20/2018 at 01:00 point

The Actuator is backdriveable. It is only a 26:1 ratio. I think that having it backdriveable with regenerative capabilities is a must for overall efficiency. I had a prototype with bearings on the outside pins but the reduction ratio was lower because of the bearing spacing around the circumference. Also that is a lot of additional bearings, which increases cost and weight. Series Elastic actuator is also much better for efficiency (the big quad design has this). I am also trying to work out how to store potential energy from the elastic actuator (spring). Store the energy on a foot fall and release when going to jump/step.

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Dan Royer wrote 06/19/2018 at 19:42 point

I saw a project recently that used an atmospheric pressure sensor (very small) wrapped in a rubber material.  When the rubber touched a flat surface the pressure on the sensor was registered.  Made for a really quick contact sensor for feet.  Might be useful for your quad walker.

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Paul Gould wrote 06/20/2018 at 01:04 point

That is a nice idea but it will only tell you after you have hit the ground. It would be nice to know just before then so that you can turn off the motors so the joints are a bit "softer".

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Dan Royer wrote 06/20/2018 at 01:09 point

how about... if the motor senses resistance and the foot is not touching then we know it's not ground interference.  not sure what the logic branch from there would look like, tho it's an interesting edge case...

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K.C. Lee wrote 06/20/2018 at 08:36 point

The sensor deforms make contact before the leg reaches all the way down and transfers its full weight.  It is like the pads and hairs on the bottom of a cat's feet can provide feedback for the cat to walk quietly.  Or your keyboard keys making contact before it reaches full stroke.
Pressure can travel, so the sensor doesn't need to be where actual impact happens; 

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albertson.chris wrote 06/19/2018 at 18:10 point

Maybe just to re-state one point.  I had the idea that a motor controller should live INSIDE a length of the 25x25 tube.

I had a robot fire a year of so ago.  I has dumb and THOUGH the cable has protected from overcorrect but the design was faulty and I vaporized a #8 cable (3mm diameter copper conductor)  I

I saw that 25x25 tube and though "use it as conduit", "use it as housing for electronics", "use it as a heat sink",  "EMI shielding if you ground it." "I wonder of batteries will fit inside?"   

I thing the most valuable space in the robot might is the inside of the 25mm square tubes

Robots like these and to fall and take bubbles down stairs or worse the limbs get into self-collisions and rip wires from each other.  At least until the motional planner is fully debugged.

I know first hand that LiPo battery fires can't be put out, you just have to wait.

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albertson.chris wrote 06/19/2018 at 03:25 point

I think a good way to design a motor driver is to look at what controls the driver.   I think anyone building a walking robot is going to be using ROS/MoveIt or ROS/TOWR so you'd want to run ROS Serial and a joint controller node on the driver.  It would save a ton of work to build the controller on a CPU platform that can run ROS Serial.    Not looking upward at the higher level controller might mean you later have to introduce a layer between.  A chain of controller adapter PCBs is ugly.   

STM makes the STSPIN32F0 with builtin MOSFET drivers the chip is designed for robotics and BLDC motors.  

The higher level controller likely a "Joint Trajectory Action Controller" attempt to place joints in space at a given time and velocity within the limits of the hardware.  This is the controller this driver will need to interface with.  best to make it seamless.  

All that said.  I'm pretty sure I'll be using a stepper motor at fist.  Steppers are easy to control.

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ꝺeshipu wrote 06/19/2018 at 07:23 point

> I think anyone building a walking robot is going to be using ROS/MoveIt or ROS/TOWR so you'd want to run ROS Serial and a joint controller node on the driver. 

If you look at the walking robot projects (https://hackaday.io/list/29475-walkers), you will notice that very few of them use ROS in any way.

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albertson.chris wrote 06/19/2018 at 17:35 point

But they also lack any interesting behavior other than walking.     What's the point of that?   I would think the reason you use legs rather then wheels is because wheels can't work for you application.    But so many of these robot have no application.

As a minimum, I'd want antonymous navigation and route planning that gets from point A to point B without attempting to walk through solid objects, replannrng aroubd obstacles.

We see this a lot with hobby robots, nice mechanics with no software

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ꝺeshipu wrote 06/19/2018 at 18:37 point

Some of them have pretty interesting behaviors. Also, lack of application is not an exclusive feature of ROS-less robots.

For "antonymous" navigation you need sensors that cost two orders of magnitude more than the whole budgets of those projects. But they usually do obstacle avoidance, so they don't walk into things.

No software is often better than crappy and bloated software. At least you get to learn how it actually is done.

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Paul Gould wrote 06/19/2018 at 12:36 point

Albertson.chris - can you tell me about the joint controller have you have made? 

The PSOC4 controller can be configured to support the following communication protocols PWM,  I2C, SPI, UART, RS485, USB, CAN, Step/Dir and LIN. I like CAN the best, as the packet handling is supported in hardware and it has a higher immunity to noisy environments. The CPU is running at 90% idle, so i'm sure adding some sort of Trajectory planning should be possible. 

The current big quad I'm designing requires interaction between the joints as the two actuators are joined as one parallel linked series elastic actuators. Two controllers, two gearboxes and one series elastic actuator. This will most likely require a centralized controller.

The CPU in the STSPIN32F0 is not a "high end" controller. 4KB of RAM is not much for a Robot controller.

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albertson.chris wrote 06/19/2018 at 17:59 point

I think a robot controller for a robot that has as many degrees of freedom is a quadruped is going to be hierarchical.    Every group of 6 MOSFETS controls one motor has one CPU.   The cPU only has to run a position and velocity feedback loop and monitor temperature.  I would bolt the mosfets to the 25x25 mm tube as a heat sink.  These controllers could live inside the structurel tubes and all connect to a bus and DC power.     4K is easy enough to run a couple loops and control one motor if not get a bigger uP.   

The next layer of control like does coordinated movement.   It receives as input a series of vectors.  The vector has a target position and velocity for every joint and also a time tag for when those positions are to be realized.   It then sends commends to all the motor controllers to update their set points such that every point gets to it's target at the same time.

The next level above os motion planning.  It's output is the vector of joint targets

Above that is the SLAM and navigation.  The robot needs to know where it is and how to get to where it needs to be and hw to rout arub fixed and moving obstacles.  This means a LIDAR, Vision, IMU, Kalman filters and so on.   This is where ROS is best used.  One person could never write a modern SLAM and sensor integration suite in one lifetime.

The level above is what decides the where it needs to be part which is likely based on a high-level command give by the owner of the robot who just said "go downstairs and fetch me a beer"

Using the "fetch me a beer" scenario is a great place holder.  It the design can do that is is general enough.  I don't need a beer fetcher but I do want a personal assistant for a blind person.  A machine that cn read signs and not bump into walls and communicate by voice.

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albertson.chris wrote 06/18/2018 at 16:09 point

I've been looking for motors.  Hoby King has the one in the you tube video.  But then checked industrial brushless motors and found a line of them a StepperOnline.com   One of them has a Kv of 111.  That means 3X more torque per  amp.  Same price but physically larger in the axial direction by a lot, but on the other hand I doubt it needs cooling.  Likely more weight.  

Maybe suitable for an indutrial robot arm that with base fixed to platform

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albertson.chris wrote 06/18/2018 at 05:18 point

I just read the comment about casting.  It is not hard. You can print a mold, pour in the resin then, here is the key:  the mold comes apart in two or three sections.  The mold parts have flanges that bolt together.  and have registration indents for precision alignment.  Coat the inside with some kind of mold release and car past wax.    

I will have access to CNC mill just as soon as I finish my CNC conversion project.  Milling from mild steel is not as hard as it seems

Comercial cscloidic drive this size use grease only the larger ones need a oil bath.  I'd stay with white lithium grease

Now to find suitable motor

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albertson.chris wrote 06/18/2018 at 03:40 point

I have been looking for an actuator for the exact same reasons you were.  My goal was an affordable quadruped that could jump off the ground.  Yours is great and (I say this in a positive way) there is room for modification which means it could be even better.   

I think the electronics can be very much simplified. and I don't know how your position/velocity feedback works yet or anything about the magnetic encoders.  

About cooling, yes, the motors likely can run hotter than you are letting them get and remember that heat flows in proportion th the difference in temperature.   So hot is not bad.  My first try would be putting a fan in the shaft then making the mount out of aluminum and a thermistor some place to measure temperature.   I'm going to have to build one of these and study it.    Thanks for all the hard work

I need to buy a brushless motor -- Sorry I've not read everything yet, do you a source  for the motors? Or a model number i can google.   

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Paul Gould wrote 06/18/2018 at 10:40 point

Any ideas and improvements are always welcome. For the electronics you can get a uC with inbuilt FET driver but then I loose the ability of the PSOC FPGA section (which is very useful). The advantage of the magnetic encoder is you can get full torque at 0RPM at angle angle using FOC. The motors are from Hobbyking Multistar Elite 5008/5010 or from Arris Hobby 5008/5010/5020 or from Foxtech 5010 or T-motor 5008/5012

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