Internet of Things with Intel Edison
Servo motors with Pololu Controller
Pierre Collet
www.Intel-Software-Academic-Program.com
paul.guermonprez@intel.com
Intel Software
2014-10-14
Objectives
Objective
Servo motors are controlled with a signal called PWM.
To interact correctly with a PWM device, you need to generate a strict signal. Obvious
for a basic microcontroller, not so much for a general purpose processor with a regular
linux. Two solutions :
o Use a Real Time OS. On Edison the second processor is running a Real Time OS so
we can do it on Edison itself.
o Use an external PWM card, connected with USB.
A good example : Pololu Maestro card.
Today, we’ll use the Pololu Maestro with 6 PWMs.
Setup
LibUSB
LibUSB
We are connected to the Pololu card with USB, using USB messages.
We need libusb-1.0-dev and we’ll use a package for Yocto, the Linux by default on Edison.
To install :
echo "src/gz all http://repo.opkg.net/edison/repo/all" > /etc/opkg/base-feeds.conf
echo "src/gz edison http://repo.opkg.net/edison/repo/edison" >> /etc/opkg/base-feeds.conf
echo "src/gz core2-32 http://repo.opkg.net/edison/repo/core2-32" >> /etc/opkg/base-feeds.conf
echo "src mraa-upm http://iotdk.intel.com/repos/1.1/intelgalactic" > /etc/opkg/mraa-upm.conf
opkg update
opkg install libusb-1.0-dev
Using the Master USB port
Master/Client
When you plug Edison to your PC with a USB cable,
you see Edison as a serial-over-USB device
and a mass storage device. That’s the client mode of USB.
Switch
But Edison is also a PC, acting as master.
To use the full size USB port on the board as a master,
operate the switch.
Remember to switch it back after this lab.
Hardware
Connecting hardware
Plug and boot your board.
Connect the Pololu card to Edison
on USB host connector
using a standard mini USB Cable.
Connect a servo motor on Pololu channel #1.
In the next slide we’ll see how to power
the Pololu from a USB port.
Hardware – Power from USB
Power ?
The Pololu has 2 power requirements :
o Very minimal for the microcontroller. This power comes from the USB cable.
o High for the servos. We need to get this power from an external source and
connect it to the 2 power pins of the Pololu.
Two methods
Usually, we would get a 6V battery with the right connector. If you have one, use it.
If you don’t, it’s possible to :
o cut a USB cable (it’s not reversible, obviously), separate wires
o get the 2 power wires (red and black)
o connect manually the 2 wires to the Pololu power pins
Hardware
Regular USB cable
DATA
Modified USB cable
POWER ONLY
Vin
#0
#5
Script
Shell script to control a servo
Binaries
We provide you 3 binaries to control 3 parameters :
o The targeted position of the servo motor
o The speed of the servo motor (0 is max speed)
o The acceleration of the servo motor (0 is max acceleration)
For each binary, the first argument is the channel number
and the second parameter is the value.
How to run this files
# ./set_accel
0
# ./set_speed 0
# ./set_target 0
0
0
2000
The order is automatically sent to the Pololu card via USB.
These binaries are C++ program using libusb-1.0.
Shell script to control a servo
A simple bash script to control a servo motor
#! /bin/bash
while :
do
./set_target 0 1000
sleep 1s
./set_target 0 2000
sleep 1s
Done
# infinite loop
# channel 0 targeted position is set to 1000
# we wait a second
It’s just moving the servo from 1000 to 2000 and back, with 1s pauses.
C Code
Set_target source code
A C++ main function using libusb
#include <iostream>
#include <string>
#include <libusb-1.0/libusb.h>
#include <unistd.h>
#include <stdlib.h>
using namespace std;
int main(int argc, char *argv[]){
if(argc < 3)
cout << "Usage: binary servo_number target_value" << endl;
int servo = atoi(argv[1]);
int value = atoi(argv[2]);
Set_target source code
Creating libusb structures
libusb_device **devs;
libusb_context *ctx = NULL;
libusb_device_descriptor descr;
libusb_device *pololu;
string idPololu ("137");
int r;
int found = 0;
ssize_t cnt, i;
Initializing libusb
r = libusb_init(&ctx);
libusb_set_debug(ctx, 3);
Seeking devices
cnt = libusb_get_device_list(ctx, &devs);
Set_target source code
For each device found
for(i = 0; i < cnt; i++) {
r = libusb_get_device_descriptor(devs[i],&descr);
//pololu id is 137, so let’s find it
if(descr.idProduct == 137)
{
pololu = (devs[i]);
found = 1;
break;
Stop looking for devices
and don’t unreference the pololu
}
}
libusb_unref_device(devs[i]);
Set_target source code
Sending target command
if(found) {
libusb_device_handle **handle;
r = libusb_open(pololu,handle);
r = libusb_detach_kernel_driver(*handle,0);
if(r < 0 && r != LIBUSB_ERROR_NOT_FOUND) {
cout<<"Error: detach kernel driver
failed"<<endl;
return 1;
}
r = libusb_claim_interface(*handle,0);
r = libusb_control_transfer( *handle,0x40,//request type
0x85,
//request
value, //value
servo, //servo number
NULL,0,5000);
Set_target source code
Closing USB connection
}
}
r = libusb_release_interface(*handle,0);
libusb_close(*handle);
libusb_free_device_list(devs, 1);
libusb_exit(ctx);
return 0;
Conclusion
Conclusion
Edison being a PC, you can expect to find a lot of great USB solutions to all your
problems. Same thing for webcams.
For PWM, you could do without the Pololu Maestro, both are good solutions.
Idea : Integrate Edison and Android ?
An IoT project which need a remote ?
Use your Android smartphone to control Edison !
We’ve seen how to create a web form with NodeJS,
you can also create an Android app with a native look and feel.
License Creative Commons – By 3.0
You are free:
• to Share — to copy, distribute and transmit the work
• to Remix — to adapt the work
• to make commercial use of the work
Under the following conditions:
• Attribution — You must attribute the work in the manner specified by the author or licensor (but
not in any way that suggests that they endorse you or your use of the work).
With the understanding that:
• Waiver — Any of the above conditions can be waived if you get permission from the copyright
holder.
• Public Domain — Where the work or any of its elements is in the public domain under applicable
law, that status is in no way affected by the license.
• Other Rights — In no way are any of the following rights affected by the license:
–
–
–
•
Your fair dealing or fair use rights, or other applicable copyright exceptions and limitations;
The author's moral rights;
Rights other persons may have either in the work itself or in how the work is used, such as publicity or
privacy rights.
Notice — For any reuse or distribution, you must make clear to others the license terms of this
work. The best way to do this is with a link to this web page.
http://creativecommons.org/licenses/by/3.0/