A high speed and high torque robotic actuator using low-cost brushless motors, custom controller, 3D printed parts and bearings.
Step File of the Whole Actuator
stp - 4.09 MB - 06/03/2018 at 06:13
Pack and Go files for Autodesk Inventor
x-zip-compressed - 21.85 MB - 06/03/2018 at 06:11
PSOC4 Project files Hardware and software (code) files
x-zip-compressed - 1.74 MB - 06/03/2018 at 04:17
Altium project Schematic and PCB
x-zip-compressed - 485.14 kB - 06/03/2018 at 04:15
Adobe Portable Document Format - 100.01 kB - 06/03/2018 at 04:14
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.
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.
Do all the joints need the same size actuators?
Are different reduction ratios required? Higher torque less speed.
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.
Testing using the Brushless motor, Cycloidal gearbox, Position sensors, Controller and LiPo Battery
My prototype actuator has a tested no load speed of 60deg in 0.2sec @ 12V, 4A @ 50 Max PWM
I have not tested the max stall torque but 10Nm is currently suitable for my needs.
The motor can handle 600W (22V @ 27A) and the brushless controller can handle 4000W (40V @ 100A) (both dependent on cooling)
In comparison, the Dynamixel MX-106R is the servo I wanted as it had 100kg/cm (10Nm) stall torque but it's no load speed is 55RPM and it cost ~US$500
The software runs on the PSOC4 by Cypress and is a Programmable System on Chip which includes
In the PSOC4 you can choose between fixed blocks (as in normal micro-controllers) or Universal Digital Blocks (as in FPGAs). So features like SPI, I2C, UART, Timers and Counters (PWM), can be either fixed or custom.
I was hoping to say that is it fully configurable but it is really not. There are many limitations and work-arounds in both hardware and software. That being said, it is a brilliant chip, you can do many things without the CPU being involved at all. The sequencing SAR ADC is a good example of this. The IDE is good but hides too much low level functions. Things like being able to change the CAN ID on the fly meant searching through lines of undocumented c code.
The software/configuration went through many iterations depending on the Controller version.
Software Structure for Brushless Controller 2.0
The sub-routine for the controller loop runs at 10kHz and uses about 20% of the CPU time. Not bad for a 48MHz CPU and doing FOC. The main reason it runs efficient is the that FOC array is pre-calculated to suit the number of poles of the motor being used. The other two phases are just offsets in the main array. The output from the PID is just a multiplied by the FOC array and shifted to get the three PWM signals. There are no sin and division calculations in the controller loop.
The FOC array is based on modified sinusoidal waveform, often called saddle profile. It has the benefits of higher voltages between phases (than pure sin waves) and 1/3 less switching losses. The FOC array has 4096 (2^12) points and the Motor's Magnetic encoder has 16384 2^14 steps. Which is fine, because the encoder has some noise.
The hardware and software has been updated to the "file page".
From everything that was learnt from the hardware development a real / custom actuator had to be developed. After some research the best/easiest solution for me was the 3-Phase driver DRV8305 paired with 6x N-MOSFETs and the PSOC4 Processor.
Hardware for the Second Controller
Brushless Controller 2.0
There is not much to say about this controller. All components were "wired" up as per their datasheets. As the two main IC's have a lot of integrated components inside there is not much interconnects required.
The angle of the motor and the joint is read by the AS5147s via SPI. PSOC4 uses PID to determine the required joint position and uses Field Orientation Control from motor angle to calculate 3 Sinusoidal values. These Sinusoidal values are converted to PWM signals to drive the 3-phase Driver. This intern drives the 6x N-MOSFETS to power the Brushless Motor. Control of the Actuator is done by CAN.
As the board was soldered by hand, the SPI link between PSOC4 and the DRV8305 helped to to pick up dry-joints and short-circuits on the MOSFETs, Current Sense Resistors and some DRV8305 pins. It had many fault and error registers which points directly to the bad connection.
The Schematic and PCB has been uploaded to the "file page".
The goal of the Actuator Controller is to
Hardware for the First Controller
Brushless Controller 1.0 (of many)
The control of the Sensored Brushless Motor with Hall effect sensors was done by a "look-up table" in the digital Marcocells of the PSOC4, no CPU overhead is needed for Brushless comutation. Only Direction and PWM duty cycle are passed to the Marcocells. The hall effect sensors only had a resolution of 6 phases per electrical revolution. This worked ok at high speeds but the motors operation would not be smooth a low speeds or when trying to hold a set position.
Brushless Controller 1.1
The Hall effect sensors used had a differential analogue output. In the previous control method they went into the PSOC's internal compactors. In this control method they can be read by the PSOC's differential ADC and a reasonably accurate electrical position can be calculated. Having full motor position information meant that Field Vector Control could be implemented. (More on this later)
The problem with using hall effects in this manner is that the distance between the hall effect sensor and the flux ring has a large effect on the voltage output. Also the type of sensor and thickness of the flux ring varies so much that it is not practical to use this technique for a general purpose robot actuator. For example, when used on the Multistar Elite 5008 the hall effect sensors gave a good sinusoidal output over one electrical rotation, every time took apart system and put back the Hall-effect board the amplitude of the sinusoidal output would change. Moving the hall effect sensors over to the Multistar Elite 3508, the effective diameter had to change but the sinusoidal output was very small.
I came to the conclusion that the thickness of the flux ring on the Multistar Elite 5008 was too thin or the magnets are too strong, either way it is not the most efficient design. It let too much of the magnetic strength travel on the outside of the motor. By contrast the the Multistar Elite 3508 had very little magnetic flux on the outside of the motor. This is not good for using external hall effect sensors but at least the full magnetic strength can be potentially used
Using external hall effect sensor with differential analogue to use Field Vector Control is not relieable for general use. Maybe internal hall effect sensors could work better but they would not be installed easily for different types of motors.
Brushless Controller 1.2
No more Motor based hall effect sensors. Some other brushless controllers use quadrature optical encoders but these need to be referenced on power up. I prefer absolute position sensors, so a diametric magnetic is placed on the motor output shaft and the magnetic rotary encoder chip placed opposite to it. The first chip i used was a AS5043 10-bit SSI, PWM and analogue outputs with a Max 30kRPM (10KHz update rate). This is the "Magnetic Induction" that is used in the Hobbyking HK47360TM Servos, I had several of these in one of my original quadruped robots. The AS5043 and diametric magnetic replaces the normal potentiometer in the servo. It was setup using it's analog output set to 180 degree at full scale and low noise/slow mode.
For use as a absolute angle sensor with Brushless motors a few changes were made. Slow mode was changed to Fast Mode. The SPI from the PSOC was connected the AS5043 SSI which gave a 1024 counts per revolution. With the 5008 having 7 magnetic...Read more »
The position sensor board uses the AS5147 is a 14-bit Magnetic Encoders
One for is used for the motor electrical position/angle. One is used for the joint position/angle. The are connected via daisy chain on SPI communication with 2x 16bit clocking. There is a piece of Silicon Steel between the two sensors boards to prevent cross flux of diametric magnets.
The technical information for the system:-
This basically means that they are quite suitable for this application.
The Schematic and PCB has been uploaded to the "file page".
The Hobbyking range of Multistar Elite Brushless motors are powerful and lightweight. I have paid between AU$25 and AU$75 for the Multistar Elite 5008. They do need to be modified to be integrated into the robot actuator. The shaft has been replaced with a longer one (with a hole) and the base has been reduced in size and weight.
Typical Cycloidal Transmissions consist of four main sections.
The way that the sections are arranged are with the input shaft on one side and the output shaft on the other and the housing in the middle.
This means that the output rollers are based on the "Fixed-Free Beam Moment" and are not suited to high torque operation when 3D printed.
This design's output support pins are based on the "Fixed-Fixed Beam Moment". This means the housing and the output rollers are dual sided. The motor input shaft is only single sided. This all works well on 3D printed parts as the forces on the steel output support pins are spread over a large area in the ABS material. A huge credit goes to Mike Buckingham for his idea on making the input shaft go through both the housing and output shaft. Sure it increased the complexity but it made the design usable.
The CAD files are uploaded to the "files page".
There are two sizes of Cycloidal Transmissions but they are identical in their design.
|Thin wall Bearing 6809 58x47x7|
Hobbyking Elite 5008-330KV (6s 600W)
Shaft size 6mm
Eccentric offset 1mm
Reduction Ratio 1:25
Cycloidal Gear "Teeth" 25
Ring Pins 26
Output Support Pins 8
|Thin wall Bearing 6807 47x35x7|
Hobbyking Elite 3508-268KV HV (8s 300W)
Shaft size 4mm
Eccentric offset 1mm
Reduction Ratio 1:20
Cycloidal Gear "Teeth" 20
Ring Pins 21
Output Support Pins 8
Using Equation Curves in Inventor to draw Cycloidal gears using the epitrohoid equation.
I will have to get the exact numbers for my gears.
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
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