Archive for the 'CNC' Category

DrawBot Badge – Workshop

Woot!

I and Jason Huggins (@hugs), of Tapster Robotics, will be giving a conference at the 2018 Hackaday SuperConference. The conference will be “Getting Started in Small Scale CNC and Robotics (Build a CNC Badge)”. By the end of the workshop you will have built, programmed and learned to use the DrawBot Badge.

The badge is way more than that though. The badge PCB is a very capable general purpose CNC/Robotics controller. We considered every small scale CNC machine we could think of and made sure this badge could control it. Here is what it can control.

  • (3) Stepper Motors
  • (3) Hobby Servos
  • (3) Homing/Limit switches
  • (1) Variable Speed Spindle PWM signal
  • (1) Laser Module (Power control and Safety interlock)
  • (4) Control switch inputs
  • (1) Probe switch control
  • (1) Coolant Control
  • (1) High current (3 Amp) output for relays or solenoids (includes flyback diode)
  • (1) i2C interface (for displays or Sh**ty Add-Ons)
  • (1) SD Card socket to store and run files

You can’t use every feature at once, like the Laser PWM and Spindle PWM are the same signal, but they are broken out logically to multiple, machine specific, connectors.

It is all powered by an ESP32 running Grbl_ESP32, which provides serial, Bluetooth and WebUI interfaces.

The first half of the conference will cover tips and tricks for getting started with making and controlling small machines. We will also learn and use some basic gcode to control some motors and other items. It is a just a basic intro to get you started and discover some resources. During the second half we will build the DrawBot badge.

If you are interested, sign up for the conference and be sure to get into the workshop when that is posted. The workshop will be limited to about 20 people, but I will bring extra badges for sale and will be happy to help build and hack them all weekend. I will also bring extra motors, servos, switches, etc for hacking up simple machines.

Open Source

Within the next couple weeks, all of the documentation will be released. Badge kits will also be for sale on Tindie after the conference.

 

 

DrawBot Badge – Preview

I showed off an early version of my DrawBot Badge at the Hardware Happy Hour last night. Some tweeted pictures generated a lot of questions, so I thought I would write a quick post about it.

I have been following the #badgelife thing for a few years and felt that the mechanical and CNC world needed to be represented. The goal was to create something small, safe, fun and something anyone could learn to use. A drawing machine seemed the perfect fit.

Drawing Surface

I decided to use 3″ square Post-It notes. The main feature is that they are self stick, so I don’t need any clamps or tape. It is also cool that you can stick them to things when the drawing is done. They are cheap and easy to find. I like the kind that have the majority of the back (full stick) covered in adhesive. This means you can use a small stack of them and peel them off as they are done.

 

Drawing Mechanism

I tried to make the drawing mechanism as tiny as possible. The size is about 30mm x 70mm. It uses (3) of the smallest readily available class of hobby servos call micro servos. You can buy these from AliExpress for as low $1-$2 in low quantities.  The choice of the arm configurations was primarily based on tightly packing the the motors, but I also wanted something new and interesting.

The pen lift is simply a cam on one of the motors. The entire mechanism rotates on two 3mm bearings. In my experience with drawing machines, gravity is the best way to engage the pen to the work. The mechanism needed to work hanging from a lanyard or sitting on a table.

Electronics

There is only one electronic item right now. It is an ESP32 dev module. There are also (3) connectors for the servos and (1) for voltage monitoring. The entire back of the badge is a PCB. I made the board full size to enclose the back and the pen lift cam need to push on something. I have big plans for the rest of the area though. The ESP32 provides the Bluetooth and Wifi needed for the remote control.

Firmware

It is running my ESP32 port of Grbl with a little hacking for the servos. The ESP32 uses an RTOS, so I just created a low priority, repeating task (20Hz). The task checks the position of a virtual CNC machine, does some kinematic math and updates the servos. The software supports streaming drawing data (gcode) via serial port, bluetooth, SD card or wifi. I am using the bluetooth option to send via my phone. I will publish the firmware soon.

Crazy Math (Kinematics)

The math is actually not too hard. You know the desired pen location, the axes of the servos and the lengths of the linkages. There are two ways to do the math. The first way is using the law of cosines and the Pythagorean theorem.

The second method uses intersecting circles. Each linkage can sweep a circle from a axis point and radius. Any two linkage’s circles will intersect at two points. It is easy to pick one of the points as more desirable. While it is less deterministic, due to the two points thing, it appears to run about 40% faster on the ESP32.

The Z is very simple. Any Z value less than 0 is pen down and everything else is pen up.

Drawing Quality

The quality is basically “Adorably Wiggly”. I am using the cheapest analog servos. That means I might get about 200 x 200 resolution at best at the servo hubs. If you mapped that grid to the paper it would be a warped grid with dense and sparse areas of the grid due to the non-linear nature of the mechanism. Digital servos with very low deadbands (~1us) it would probably do a lot better, but I am not sure that is worth it. It would only be a little less wiggly. They cost about 4x what analog servos do (still pretty cheap).

There is a calibration feature. Each servo is a little different and it is difficult to precisely mount the arms at the right angle. The calibration adjusts the zero angle and pwm/degree of rotation.

Next Steps

This project is part of a talk proposal for Hackaday SuperCon 2018. If that gets accepted, the badge will become a full featured CNC controller capable of running a router, laser cutters, high res plotter, etc, as well as the Drawbot badge. Only the DrawBot will be populated. The rest will be modular like this board.

I may also make a simple version that is only the draw bot. The BOM cost is currently about $10-$12 (without 3D printed parts)

Regardless of what happens all source files will be publish sometime in October.

 

Follow me on Twitter (@buildlog) or subscribe to this blog if you want to follow the project.

 


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Grbl_ESP32 with Bluetooth Serial Support

Bluetooth Serial has now been added to the master branch of Grbl_ESP32. Bluetooth Serial means the Bluetooth connection looks like a serial port when you pair with the device. This is ideal because it allows all existing gcode senders that support serial ports to use Bluetooth.

This was added using the Bluetooth Serial library that is part of the espressif/arduino-esp32 development environment. This library is relatively new and has a few issues, but I was able to work around them.

  • Sending Characters: The reporting done by Grbl sends characters a character at a time. This means dozens of calls to the library are needed for each response. This caused characters to be dropped. The solution is to send a full string in a single call using .print(), rather than sending individual characters using .write().  Grbl was written using the character at a time method to highly optimize the reporting. The raw speed of the ESP32 does not require that optimization and the resulting code is much more readable.
  • No Password: The library does not support the use of a password for the pairing. This is a big drawback, and needs to be considered before you enable Bluetooth. Apparently this was due to an upstream component that has now been fixed. Hopefully it means the library will be updated soon.

Why and when to use it.

The cost, size and wireless features of the ESP32 are the primary reason I ported Grbl to the ESP32. I make a lot of tiny CNC machines. A laptop is likely to be many times the size of the CNC machine. I wanted to control the machines via a phone or small tablet. I have found the connection to be very reliable, even for jobs lasting more than an hour. The phone can even be in the “off” state in your pocket and it will still stream reliably. The battery drain is accelerated, but not significantly (like listening to audio via Bluetooth) You do need to stay within range of the machine, although Grbl Controller (Android) appears to be able to resume a job.

Using Bluetooth.

It is optional. You enable the feature at compile time. That allows you to save some code space (not really an issue now). Once the feature is enabled, you can still turn it on or off via a serial port command. All Bluetooth traffic is echo’d to the serial port. This allows you to monitor the communications. You can also use the Serial port at any time, but it is not a good idea to do that while running a job. Here are the steps to set it up.

  • Make sure #define ENABLE_BLUETOOTH is not commented out in config.h.
  • Use a serial port terminal to set the Bluetooth name using $I=NAME, where NAME is the Bluetooth name you want. I don’t know all the naming rules, so keep it short and simple. There is no capability to use a password yet. Grbl converts all input to capital letters, so lowercase will cannot be used.
  • Reboot the ESP32 to turn on Bluetooth with that name. Grbl will send Starting Bluetooth:ESP32BT as the first item when booting to let you know Bluetooth is on. ESP32BT is the Bluetooth name I used in this case. Grbl will now respond on either Bluetooth or Serial data. All Bluetooth sends are echo’d on the Serial port if you want to watch the data.
  • You can now pair a phone or PC with Grbl_ESP32.
  • Caution: Do not pair while running a job. The ESP32 will likely interrupt and/or watchdog issues while the stepper timer is running and the pairing process is running.

Next Steps

  • Other modes: A phone also makes an awesome display and control panel. Maybe the PC is the primary sender and the phone acts as a pendant.
  • SD card. SD card is next on the roadmap. This pairs well with bluetooth, because the phone could select and start an SD card job.

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Next Project – Wooden Nickel Engraver

I started working on my next backpack scale CNC project. My backpack scale projects are tiny CNC machines that I can easy carry in a backpack to tech meetups and events. This machine is going to be a wooden nickel laser engraver.

Wooden nickels are small wooden discs. You can buy blanks from various places, including Amazon. You can usually get 100 for less than $10. The goal is to create a machine that loads them from a feed tube, engraves them, then ejects them.

Mechatronics

The basic drive will be an H-Bot. It will be similar to the midTbot, but the motors are at the ends of the X axis. It will be fully enclosed, but I not started work on that part yet.

Disc Feed System

I hope to be able to use a single hobby servo to handle the loading and unloading of the blanks. The servo will control a sliding device that has three positions.

  • In the first position it acts like a support shelf for the disc. The bed slides under the feed tube and a disk drops into the pocket.
  • The next position is the clamping position. This holds the disc still while engraving.
  • The final position retracts the shelf so the disc drops through.

Electronics

The goal is to use a low cost controller like an Arduino Nano.

  • (2) Stepper motors for the X,Y motion
  • (1) Hobby servo for the disc feed system (PWM)
  • (1) Laser power control (PWM)
  • (2) homing switches
  • (1) interlock switch loop if there is a door and/or cover

Status

The major new feature of this design is the disc feed system, so I am primarily working on that right now.


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Coasty Source Files

A lot of people have asked about building their own Coasty Laser Cutter. It takes a lot of work to get the files ready for release. I will release the source files in stages as they are ready so people can get started. Watch this post for updates. Subscribing to this blog or following me on Twitter (@buildlog) is a good way to keep up.

When everything is ready, I’ll probably also post on Thingiverse.

3D Printed Parts

Here are the STL files for the 3D printed parts. The parts are generally pretty easy to print. They require no support and can be printed in low resolution. I print at 0.28mm layer height. You need to watch out for warping on the chassis and front door. If the chassis warps it will stress the PCB and could damage some parts. The door needs to be flat in order to close properly.

I printed my parts in generic PLA. They printed fine, but if you have some crappier PLA or if you don’t have a heated bed, you should probably print with a brim. I would suggest printing the chassis first. If you can print that, the other parts are easier. I have some PETG on order to test. That supposedly warps less that PLA.

The holes used for the 8mm rods are designed to be a press fit. If the rods are hard to install, try cleaning the holes up a little by hand with a 5/16 or 8mm drill. The drive shaft bearing is also a tight fit. Try using a vise or clamp to press it into the chassis.

Zip File containing the STL files.

Mechanical BOM

 

Part DescriptionQtySupplierSupplier P/N
3mm Smooth Pulley (16T equiv dia.) 6mm Wide1
Bearing 3mm x 10mm x 4mm2Generic623-2RS
BEARING 5mm 16mm 5mm1Generic625-2RS
Bearing Shaft 8mm2
Butoon Head Screw M3 x 302
Button Head Screw M3 x 121
Button Head Screw M5 x 201
Coasty Chassis13D Print
Coasty Controller Assy1Buildlog.netCoasty Controller
Coasty Drive Shaft13D Printed Part
Coasty Final Assembly1---
Coasty Laser Carriage13D Printed Part
Coasty Laser Carriage Assy1User Assembly
Coasty X Motor Cover13D Printed Part
Dual Fan Cover13D Printed Part
Flat Head Screw M3 x 64
GT2 Belt 6mm Wide cut to 385mm1GenericGT2 6mm
Hex Nut M33
Laser Module 3.5W 450nm1EleksmakerLA03-3500
Linear Shaft Bearing 8mm Dual2GenericLM8LUU
NEMA14 Stepper Motor2Generic
StepperOnLine
14HS13-0804S
Nylock Locking Nut M22
Nylon Locking Nut M33
Pan Head Screw M2 x 122
Silicon Oring #9052McMaster Carr1283N428
Socket Head Screw M3 x 820
Socket Head Screw M3 x 162
Socket Head Screw M3 x 203
Timing Pulley GT2 18T 6mm Wide 5mm Bore1
Wiring Cover13D Printed Part
Coasty Window149mm x 76mmJ Tech PhotonicsRating: OD+4 @ 445nm
Coasty Door13D Printed Part
Beam Stop (Metal Strip)170mm x 9mm x (0.5mm - 1.5mm)Fabricated

 

PCB Source Files

See this blog post

Firmware

Coming Soon

Build Instructions

Assembly Drawing


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Coasty Version 1.2

Here are some updates to Coasty – The Coaster Toaster,  the tiny laser cutter specifically designed to cut drink coasters.

New Traction Roller

I made the traction roller diameter a lot smaller and moved it behind the beam. A smaller roller has a lot of advantages. It allows the beam to be closer to the contact points of the rubber orings. This improves the usable work area, because you can get closer to the edge of the coaster. With a smaller diameter the coaster travels less per revolution. This increases the torque and resolution.

Smaller Chassis

The chassis is now about 16mm smaller in depth due to the smaller roller and new location. The depth of the machine is quite a bit smaller than the coaster.

Fan Guard and Carbon Filter

I added a fan cover on the back. This acts as a finger guard and also allows a few layers of carbon filter cloth to be used. Bulk carbon filter cloth for use in air purifiers can be purchased on Amazon very cheaply. It removes a good portion of the odor of the smoke.

Carbon Filter Cloth

Door Interlock Switch

There is now a switch that cuts all power to the laser when the door is opened. You can still run the machine to test the motors, homing etc, the the beam cannot turn on with the door open.

IR Coaster Detector

 

I was not happy with the coaster homing switch used on the first version. While it never failed, it did not appear to be very robust and it caused some drag on the coaster. I changed to a IR LED and photo diode. When the light from the LED hits the photo diode, it conducts to the +5V. When the coaster blocks the light, it is pulled down to ground. I used a pot on the pull down because it did not know what he exact value would be. It turns out the value needs to be about 40k. The only catch was the microcontroller input pin pull up resistor on the Nano could not not used because it is less than the 40k.  This required a slight hack to Grbl because Grbl is all or nothing on the pull ups for the limit switches.

I was not sure if ambient light changes might be a problem, like bright sunlight. The photo diode looks down and that appears to be good enough to avoid overhead light. I also have a mounting screw there in case I need to add a little shade/cover.

IR LED / Photo Diode Circuit

 

Bluetooth

I have been using Bluetooth on some other machines and really like it. Skipping USB cords and using a phone instead of a computer is great. I have found it to be very reliable. The real world bandwidth appears to be a little lower than 115200 USB. It has not been a problem, but I don’t do much gray scale engraving on this machine which needs higher bandwidth. Regardless, USB is still an option.

A standard HC-05 or HC-06 module plugs into a right angle connector.

Video

Here is a video of this version.


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ESP32: Step Pulse Experiments with Timers

(Edit: Also check out my better “RMT” way to send the pulse)

I have been playing with the ESP32 microcontroller to see how well it would perform as a small scale CNC controller. The low cost and high performance as well as the built in Wifi and Bluetooth make it very attractive.

One of the challenges is step pulse timing. Most stepper drivers work with a direction signal and step signal. The step signals are a short pulse for each step. If they are too short, the driver will not detect them. If they are too long, it limits the rate at which you can send them.

You first set the direction signal high or low depending on the direction you want the motors to spin. You then send the step pulse. The direction signal has to be stable for a short period of time before the step signal is sent. The process is…

  1. Set direction
  2. Wait a bit (if it changed)
  3. Turn on the step pulse signal.
  4. Wait a bit
  5. Turn off the step pulse signal.

The timing is critical and varies by motor driver. Here is a typical spec.

Here are the specs for a few of the stepper drivers I regularly use.

 Allegro A4988TI DRV8825Toshiba TB6600
Direction Delay200ns650ns?
Step Pulse Delay1us1.9us2.2us

Test Firmware

Typically the firmware motion planner determines when to take a step, then sets an interrupt to occur at that time in the future. This allows the firmware to do other things like interacting with the user and planning future moves while it is waiting for the interrupt.

To simulate a stream of pulses, I created a timer interrupt that would case steps to occur at a constant 5kHz rate. That is onStepperDriverTimer() in the code.

In that interrupt service routine I first set the direction pin. Normally you only need to set it when the direction changes, but it will be easier to see this on a logic analyzer if I change it every time for this test. I then need to wait a little time before setting the step pulse pin. I could use another interrupt to do this, but the time is so short at about 750ns, that it is better to just waste a few cycles. In the CNC software I will only need to do this when the direction changes. That will be at the few beginning of the acceleration when the step rate is the slowest. I do this delay with a few NOP()s. The are “no operations”.

I then setup the the interrupt to end the pulse. That is onStepPulseOffTimer() in the code. I set the step pin after this because those instructions take clock cycles too. I can use those as part of my delay.

When that interrupt occurs, I turn off the step pulse signal. I also turn off the direction in this example. I am only doing it here so I can see that change on the logic analyzer. Normal CNC frmware would just leave it alone because there are typically thousands of steps before the direction is likely to change.

I wrote a program to simulate some CNC firmware so I could play with step pulse timing.

// create the hardware timers */
hw_timer_t * stepperDriverTimer = NULL;  // The main stepper driver timer
hw_timer_t * stepPulseOffTimer = NULL;  // This turns the step pulse off after xx uSeconds

// define the gpio pins
#define STEP_PIN 17
#define DIR_PIN 16

// the step pulse interrupt service routine. 
void IRAM_ATTR onStepperDriverTimer()
{
  // if ... the direction changed from last time (not in this demo)
  digitalWrite(DIR_PIN, HIGH);  // in actual CNC firmware this will go high or low
  for(uint8_t i=0; i<10; i++)
  {
    NOP();  // do nothing for one cycle
  }
  // end if

  // setup the pulse off timer
  timerWrite(stepPulseOffTimer, 0);
  timerAlarmWrite(stepPulseOffTimer, 22, false);  // the alarm point is found by looking at logic analyzer
  timerAlarmEnable(stepPulseOffTimer);  
  
  digitalWrite(STEP_PIN, HIGH); // put it after the timer setup to include the timeto do that
}


// 
void IRAM_ATTR onStepPulseOffTimer()
{
  digitalWrite(STEP_PIN, LOW); // end step pulse 
  digitalWrite(DIR_PIN, LOW); // only here for dem program CNC firmware would leave this until direction change
}


void setup() {  

 pinMode(DIR_PIN, OUTPUT);
 pinMode(STEP_PIN, OUTPUT);

    
 stepperDriverTimer = timerBegin(0, 4, true); // 80Mhz / 4  = 20Mhz// setup stepper timer interrupt ... this will simulate a flow of steps
 stepPulseOffTimer = timerBegin(1, 1, true); // 

 // attach the interrupts
 timerAttachInterrupt(stepperDriverTimer, &onStepperDriverTimer, true);// attach the interrupttimerAttachInterrupt(directionDelayTimer, &onDirectionDelayTimer, true);// attach the interrupt
 timerAttachInterrupt(stepPulseOffTimer, &onStepPulseOffTimer, true);// attach the interrupt
 
// setup the time for the 
 timerAlarmWrite(stepperDriverTimer, 4000, true);  // 20Mhz / 4000 = 5kHz rate ... this is the only one that auto repeats  
 timerAlarmEnable(stepperDriverTimer); 
 
}

void loop() {
  // no loop code required.:
  
}

Results

Here is a picture of my setup.

This is screen shot of what the logic analyzer captured. The upper line is the step signal and the lower line is the direction signal. The direction signal comes on first and then the step pulse signal comes on 0.75us later. The step pulse then lasts for about 2.5us before turning off.

Next Steps

  • I’ll go forward this method to see how well it works in actual CNC firmware.
  • I have been programming in the Arduino-ESP32 environment. This is an easy way to learn about the peripherals and do some quick tests. I may switch to the ESP-IDF  in the future.
  • I would like to investigate the RMT features of the ESP32. It is designed for Remote Controls, but I have heard it is quite flexible and might help with pulse generation.

 


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Using The midTbot Controller

The midTbot Controller is a controller for midTbot pen drawing machines. It is sold on my Tindie store. When fully populated, it has the following features.

  • (2) Stepper Motor Drivers for the X and Y axes with microstepping selector jumpers
  • Servo Connector for Pen Lift
  • Arduino Nano Controller
  • 5V power supply
  • (2) Homing switches
  • Bluetooth connector for HC-05 or HC-06 module.
  • Aux power connector for easy access to 5V and 12V power.

Power Supply

You will need a 12V DC power supply with at least 3A of power. The DC plug should be a coaxial 5.5mm x 2.1mm type with center positive. This type of plug is the most common type and should be pretty easy to find.

Microcontroller.

You should use an Arduino Nano compatible controller. The controller bootloader will need to be changed to the smaller Arduino UNO bootloader in order to fit the firmware. I have instructions on how to do this here. If you would like to buy one already modified, I sell them on my Tindie store. Be sure to install the controller in the correct orientation. The USB connector should face the edge of the controller as noted on the silkscreen of the controller PCB.

Stepper Drivers

You will need 2 stepper driver modules. I recommend ones based on the Allegro A4988 controller or the TI DRV 8825 controller. The drivers often come with heatsinks. The current will typically be set low enough that heatsinks are not needed.

You will need to set the microstepping level. I recommend using 1/8 microstepping. That gives a good balance of accuracy, smoothness and speed. You set this using the jumper blocks. Each stepper driver type has its own configuration. Here are the jumper setting for A4988 and DRV8825

Microstep ResolutionA4988 JumpersDRV 8825 Jumpers
FullNoneNone
1/2MS1MS1
1/4MS2MS2
1/8MS1 + MS2MS1+MS2
1/16MS1+MS2+MS3MS3
1/32Not AvailableMS1+MS2+MS3

Note the MS1, MS2 & MS3 labels next to the jumpers.

The orientation of the drivers is very important. You will break the drivers and likely other parts if you insert them wrong. Most drivers have the pins labeled and the the controller PCB has the corner pins labeled. Be sure the labels match. Here are the correct orientations for the A4988 and DRV8825.

You should set the stepper driver output current to the level required by your stepper motors. The midTbot does not need a lot of power. If you set the power too high, the motor could get hot and damage the 3D printed part it mounts to. You set the current by adjusting a potentiometer and reading a voltage. The drivers need to be powered when you do this. You can install them and power the board to do this. Do not connect the motors when adjusting the potentiometer. Each driver will have a small hole with exposed metal. This is the measurement point. The metal adjustment screw of the potentiometer is also a place you can measure. The reference voltage to current formulea are here.

DRV8825 Current = VREF * 2   (Example 0.4V = 0.8A current)

A4988 is a little more complicated because there are different versions. See this page.

Stepper motors:

The controller is designed for NEMA14 motors. Be sure to get 4 wire motors. Just about any size should work, but I prefer to use smaller ones to keep the size and weight down. You will need to attach connectors to them.  Trim the wires to the correct length so they reach the board connectors when installed. Wires colors should be installed as shown.

The wiring legend on the board that shows where to plug in the motors might not be right. It depends on the way the motor manufacturer wired the coils and which stepper drivers you use. You may need to swap them or rotate the connectors 180° if your axes are moving the wrong way. Here is a photo of how mine are wired. Try this first. Note the wire colors and which motor goes to which connector.

Bluetooth

You can add an HC-05 or HC-06 Bluetooth module. They have different connectors, but just match the pin names on both sides. You will need to setup the module to have a baud rate of 115200, N,8,1. I have detail on how to do that here (HC-05, HC-06).

 

 


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Bus Servo Draw Bot

I wanted to complete a start to finish project with the LX-16A bus servos to do a complete review of their viability for the type of mechatronics projects I do. The low price of the LewanSoul bus servos make them a competitive option over digital servos. I chose to do a remake of the Line-Us clone drawing machine because I would not need to spend too much design time and it would be a good 1:1 comparison with digital servos. Since the bus servos are quite a bit larger, I decided to scale up the machine by 1.5x.

Made My Own Brackets.

I started by buying one servo and it came with brackets. When I went to get more, I noticed the price was lower. I did not realize I was getting these without brackets. These are the brackets you get with the more complete kit.

I requested some 3D models from LewanSoul. They were only able to provide 2D DXF files, but they were easy to convert in Fusion 360. This allowed me to 3D print some brackets. It actually worked out quite well because I was able to thicken up the brackets and integrate some captive square nuts.

They mounted easily to the servos and were plenty strong for this project.

Servo Arms

I made two arms and one short cam for the Z. They were about 4mm thick and had a little pocket that slid over the standard round actuator. They screwed on with the screws that come with the servos. Before mounting anything I turned on the servos and moved them to the center of the range. This put the arms at a known angle.

Support Bracket and Base

The support bracket holds all three servos and there are 2 pockets on the bottom for some shaft bushings.

The base has two shafts pressed into it.

The shafts slide into the bushings and there is a spring to hold the parts together.  The spring prevents the pieces from separating and also provides a little extra pull down force in case the shafts bind a little. The cam provides about 6mm of lift.

Final Assembly

The servos are mounted to the support bracket.

The wires are connected to the servos. They just daisy chain from one servo to the next.

The remaining links are then connected.

Testing

Here is a video of the first run.

Results

I think these are a good alternative to digital servos. They are very strong, easy to mount and accurate. Depending on the design of the controller, using a simple UART might be easier than having multiple PWM signals or extra hardware. The servo’s size might be larger than some machines need but that comes with the higher power.

Source Code

Some people have asked for the source code. Here is what I have. I just tweaked something I had to be good enough for this demo. I can’t offer support for it other than a few quick questions asked on this blog post.

 


 

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LewanSoul Lobot LX-16A Serial Servo Review

I stumbled upon these LewanSoul LX-16A Servos the other day on Amazon while searching for standard digital servos. These servos are digital serial bus servos. That means they use a serial protocol on a bus rather than a PWM signal. I have used bus servos before, but the $12 price got my attention. That competes well with plain old PWM servos.

Serial bus servos have some real advantages over standard PWM servos.

  • Wiring: The wiring can be a lot simpler because you can put all your servos on a single 3 wire bus. Each servo has 2 connectors to make daisy chaining easier.
  • Higher resolution: The resolution is typically higher. It depends on the way they sense position. These use a high accuracy potentiometer and list 0.24° as the resolution.
  • You can set the speed: You can set a destination and speed for each move.
  • Set the range: You can limit the range of the servo. This is great if the using the full range would crash a joint on your machine.
  • Continuous turn mode: There is a speed adjustable continuous rotation mode on these, but absolute distance moves cannot be done.
  • Motor On/Off: Turn the motor off for manual movement.
  • Feedback: The communication is bi-directional so you can query a servo for…
    • Position: If you manually move a servo, you get its position. This is great for recording “poses” or setting range limits.
    • Temperature: You can read the current temperature of the motors.

Compared to Robotis

I have used the Robotis Dynamixel XL320 servos before but these are a lot stronger and the XL-320 has a weird, plastic rivet, mounting systems that I am not fond of.

Here is a size comparison with a XL-320.

Specs

Spec LX-16A XL-320
Max. Torque 17 kg-cm 4 kg-cm
Resolution 0.24° 0.29°
Range 240° and Continuous Rotation 300° and Continuous Rotation
Speed 62 RPM 114 RPM
Weight 52g 16.7g

Usage

To use the servos you need to use one of their controllers. The most basic controller just converts a USB or TTL serial signal to their protocol. The controller is small and low cost ($10). You can send commands via a USB (Com Port) or via the TTL pins.

Setup Program.

They have a setup program.  The program is Windows only and I had to run it in Administrator mode to get it to work. This makes setting up the servos easy, but you could write your own program if you don’t want to use theirs.

TTL Control

They also provide some Arduino Sketches. They worked fine and are a good place to grab some code if you are writing your own program.  The sketches use the Arduino hardware TX and RX pins. That conflicts with uploading, so you need to disconnect the controller every time you upload. I edited the sketch to use SoftwareSerial on some other pins and that made playing with the code a lot easier.

First Impressions

I found the servos very easy to use and they appear to be strong and responsive. I think they will be a good option for my on my machines.

Next Steps

I want to test these in a real machine. I thought I might try to make a slightly larger version of my Line-Us clone. That would be a good comparison of accuracy. I might try one day build on it tomorrow.  I can probably get a machine designed and built in a day, but the controller programming would need more time.