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Using Motors
This page introduces the most basic motor attributes. It's part of a series of wiki pages that describe using motors with the EV3 running ev3dev that includes:
- Motor synchronization is not yet implemented.
- Motor hold operation is not yet implemented
- Only the standard EV3 and NXT tacho motors, as well as the new Firgelli L12-EV3-50 and L12-EV3-100 motors are auto detected.
The ev3dev motor driver allows you to have complete control of the motor operation. You can run the motor in regulated or unregulated mode, brake at the end of the motor operation, you can specify time or tacho based motor moves. You can specify time or tacho based ramping at either end of the move block.
With this flexibility comes a bit of complexity, but it's all laid out in an easy to understand set of control parameters.
The motor ports support autodetection of the standard tacho motor and the new minitacho motor. Simply plug a motor into any motor port and a new device is created. The name of the device depends on the port it's plugged into.
When the EV3 boots up, there are 4 motor ports created as part of the standard device driver enumeration on the legoev3 bus. They are:
/sys/bus/legoev3/devices/outA/sys/bus/legoev3/devices/outB/sys/bus/legoev3/devices/outC/sys/bus/legoev3/devices/outD
When you plug a motor into one of the ports, a new motor device is created in one of the above motor port directories. For example, if you plug a compatible motor into port B, the new device is available as the directory:
/sys/bus/legoev3/devices/outB/outB:motor
There are further directories underneath this one:
/sys/bus/legoev3/devices/outB/outB:motor/tacho-motor/sys/bus/legoev3/devices/outB/outB:motor/tacho-motor/outB:motor:tacho/
That gets a little unwieldy, but you can always make a link to a more convenient name.
The Linux device model also supports device classes. The supported tacho motors show up in the tacho_motor device class directory here:
/sys/class/tacho-motor/outB:motor:tacho
Notice that the final directory name in the device class tree corresponds to the directory name in the legoev3 bus tree.
The Linux device model communicates with devices using attributes. The tacho motor attributes use text based representatoins wherever possible. What that means is you won't need to remember things like wheter motor type 7 is a standard tacho motor or the new mini tacho motor. More on this later when we discusss the type attribute.
If you do a directory listing on a tacho motor device, you'll see something like this:
user@ev3dev:~$ ls /sys/class/tacho-motor/outB:motor:tacho
device state target_speed
direction stop_mode target_steer
mode subsystem target_tacho
power tacho target_time
ramp_mode tacho_mode target_total_count
regulation_mode target_power time
run target_ramp_down_count type
speed target_ramp_up_count uevent
That's a lot of attributes! (Note that not all these attributes are available or useful. I'm still working on reducing the attributes and getting the names right)
Fortunately, we don't need to deal with all of them at once. Attributes can be read-only, write_only, or read/write. You can read and write to device attributes using standard file I/O in your favorite programming language.
For example, from the Linux command line:
- You read a device attribute using the Linux
catcommand. - You write to a device attribute using the Linux
echocommand.
Let's start with the simplest attributes to get comfortable with manipulating them.
Plug a full sized tacho motor (EV3 or NXT) into motor port B, then read the type attribute from the command line:
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/type
tacho
Now plug the smaller tacho motor into motor port B, then read the type attribute from the command line:
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/type
minitacho
You can override the motor type (if you want to) by writing the desired value to the attribute, like this:
user@ev3dev:~$ echo tacho > /sys/class/tacho-motor/outB:motor:tacho/type
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/type
tacho
Unplug the motor, wait a second or so, and then plug it back in again, and read the type. It should revert back to the minitacho value.
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/type
minitacho
The next attribute is position - it's a read-only indication of how many tacho ticks the driver has counted. Plug a standard tacho motor into motor port B and rotate the hub clockwise by hand.
After each rotation, read the position attribute. It should look something like this:
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/position
0
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/position
116
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/position
-210
You can read the position at any time. The number has a range of +/- 2,147,483,648 which is enough to measure the distance from New York to Los Angeles (3940 km) to an accuracy of about 2 mm.
In other words, it will be a long time before you run out of counts in most applications where the position is important.
The speed attribute is a read-only indication of how fast the motor is turning. It's not measured in any particular units. The speed ranges in value from -100 to +100, where 100 is the maximum practical speed of the motor when it is driven at 100% power on an EV3 with fresh batteries.
In most cases, you will never see a speed near 100%.
You can read the motor speed at any time, even if the motor is being rotated by hand! To see this, connect rubber tires to the motor hub and push the motor forwards and backwards on your desk. While pushing the motor, read the speed like this:
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/speed
0
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/speed
13
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/speed
-20
Positive speeds correspond to increasing position counts, negative speeds correspond to decreasing position counts.
The power attribute is a read-only indication of how hard the motor is being driven. It's measured in duty-cycle percentage. That's just fancy terminology for the percentage of the battery voltage being used to drive the motor. The power ranges in value from -100 to +100, where 100% is driving the motor as hard as posible in the positive direction, and -100 driving the motor hard in the negative direction.
You can read the motor power at any time. Since we're not driving the motor at all yet,the power attribute will simply return 0.
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/power
0
Positive speeds correspond to increasing position counts, negative speeds correspond to decreasing position counts.
The state attribute is a read-only indication of what state the motor is in. Depending on what you're doing, you'll get different results when reading this attribute. The result is a descriptive text string that represents the state of the motor driver.
You can read the motor state at any time. Since we're not driving the motor at all yet,the state attribute will simply return "idle".
user@ev3dev:~$ cat /sys/class/tacho-motor/outB:motor:tacho/state
idle
We'll learn more about the different states that the motor driver can have in more advanced guides.