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Incremental Encoder and Servo Angle Control in STM32 || PWM
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Jun 5, 2021
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View Video Transcript
0:20
hello
0:20
everyone welcome to another video of
0:23
controllers tech
0:25
this video will cover two things
0:28
first we will see how to use the
0:30
incremental encoder
0:32
and in the second half we will use this
0:35
encoder to control the positioning of
0:37
the servo motor
0:39
by controlling the position i mean the
0:41
angle by which the servo rotates
0:44
so let's start with the encoder first
0:48
i have this encoder here and it's rotary
0:51
as you can rotate the shaft
0:56
the shaft is free to rotate in either
0:58
direction
0:59
as there is no limit on the rotation
1:04
it have five pins but we are interested
1:07
in the bottom two pins
1:09
the pin names do not justify their
1:11
functions
1:12
so we will call the clock bin as pin a
1:15
and dt
1:16
pin as pin b the middle one is the
1:19
switch
1:20
and it represents the push button on the
1:22
shaft
1:23
that's it about the encoder now let's
1:25
see how it works
1:32
i got this gif from the wikipedia and it
1:34
shows exactly what happens
1:37
think of the black region as ground and
1:39
white region as vcc
1:42
so it start with both the pins in
1:44
contact with the white region
1:46
so both the pins are high now let's
1:49
say we move the shaft clockwise
1:52
so the outer pin goes low while the
1:55
inner one is still high
1:57
now the inner pin goes low and both the
2:00
pins are low
2:01
and now they are going back to high
2:06
again
2:10
if we move it counterclockwise the inner
2:12
pin goes low first
2:14
then the outer one and then they both
2:17
goes back to high
2:19
this is the entire working basically
2:22
we just have to check which pin goes low
2:24
first and based on that we can figure
2:27
out
2:27
whether the shaft moved clockwise or
2:29
counterclockwise
2:31
let's see one more time with the logic
2:34
analyzer
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here i have connected the channels to
2:38
the pin a
2:39
and pin b
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supply is 5 volts
2:51
let's start this
3:04
pay attention i am rotating the shaft
3:07
counterclockwise
3:09
here we got the signal on both pins
3:16
let's zoom in we have some unwanted
3:19
signals here
3:20
but we will take care of them in the
3:22
code itself
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let's focus on the main part as you can
3:28
see here
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the first pin goes low and after some
3:31
time
3:32
the second one goes low
3:39
now i am rotating it in the other
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direction
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this time the second one goes to low
3:48
first and after some time
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the first one goes low
3:55
this is exactly what's shown in this
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animation
4:02
this is the entire working of this
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encoder and we will use these pins to
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identify the direction of rotation
4:12
let's create a project in cube id
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i am using f103c8
4:19
give some name to the project and click
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finish
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i am selecting the external crystal for
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the clock
4:30
select serial wire debug
4:33
i have eight megahertz crystal and i
4:36
want the system to run at maximum 72
4:40
megahertz
4:43
let's select two input pins which will
4:46
be connected to the encoder
4:52
click save to generate the project
5:05
here first we will read the pin a
5:12
if this pin a is low then we will check
5:15
for pin b
5:24
actually i am considering this situation
5:27
when the pinner goes low at the moment
5:30
pinby was already low
5:35
and if this happens we will increment
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the counter
5:44
now before incrementing this counter we
5:47
will wait for the pin b to go back to
5:49
high
5:50
this is necessary to avoid those
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unwanted fluctuations
5:54
also we need to do the same for pin a as
5:57
there are fluctuations in pin a also
6:00
so we will wait for the pin a to go high
6:07
as you can see the pin goes low again
6:10
and it will change the counter value
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so to avoid this let's wait for some
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time before reading the pins again
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similarly when the shaft is rotated in
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another direction
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we will use the same code with minor
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changes of course
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this time we will check if the pin b is
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set
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this is the part where pinner goes low
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and at this particular time
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if the pin b is high that means the
6:46
movement was different
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from the last time
6:56
now we will wait for the pin b to go low
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this is to avoid these fluctuations
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now when the pin b is low we will
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decrease the counter
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then we will wait for the pin a to go
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high and then for the pin b
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let's say i want to restrict this
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counter value between
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0 and 20.
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this should be integer let's debug it
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now
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note the counter value in the live
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expression
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i am rotating in counter-clockwise
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direction
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and you can see the value is increasing
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now if i rotate in clockwise direction
8:58
the value starts decreasing
9:00
the value is limited between 0 to 20.
9:12
so the encoder is working well but i
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would like the value to increase in the
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clockwise direction
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so i am changing the counter operation
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here
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this is it about the encoder part now we
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will use this encoder to control the
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movement of servo motor
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i have already covered the servo motor a
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long time ago
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and this is the tutorial for that
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i am taking this as the reference for
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today's video
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servo motor works with the pulse width
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modulation
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and like it's mentioned here that a
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pulse of one millisecond will move the
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motor to zero degree
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angle then 1.5 millisecond pulse will
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move it to 90 degrees
10:00
and 2 milliseconds pulse will move it to
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180 degrees
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the pulse width needs to be 20
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milliseconds which
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translates to 50 hertz frequency
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i have also mentioned that some motor
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works between 0.5 millisecond to 2.5
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millisecond
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and i have that kind
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so first of all we will enable the pwm
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i am using timer 1 channel 1 for the pwm
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output signal
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you can see bot the timer clocks are at
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72 megahertz
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but if you want to know where the timer
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is connected you can check the
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controller data sheet
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here you can see the timer 1 is
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connected to the ap b2 clock
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and if you see the clock setup the ap
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apb2 timer clock is running at 72
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megahertz
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now we need to bring this clock down to
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50 hertz
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if we divide 72 megahertz by 50 hertz
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we get four hundred and forty thousand
11:57
we need to split this value between
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prescaler
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and the auto reload register
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i am using the prescaler of 144
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and auto reload of 10 000 keep the auto
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reload value high
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as this is where the percentage for the
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duty cycle will be calculated
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so if we keep it high the percentage
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values will be high
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and hence more accurate
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let's save this to generate the project
12:38
now like i said the motor needs the
12:40
pulse width between 0.5 milliseconds to
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2.5 milliseconds
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this means that the motor covers the
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angle of 180 degrees in the difference
12:55
of two milliseconds
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this would translate to 11.11
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microseconds per degree rotation
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let's start the timer in pwm mode
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with channel 1.
13:17
we will limit the counter between 0 to
13:20
180
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as per the servo rotation now we have
13:25
11.11 microseconds for 1 degree rotation
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and the pulse width is 20 milliseconds
13:31
[Music]
13:33
this is the duty cycle for 1 degree
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rotation
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let me explain this properly
13:42
in between 0.5 millisecond to 2.5
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milliseconds
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the motor covers 180 degrees
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this means the motor will take 11.11
13:54
microseconds for 1 degree rotation
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again we have 20 milliseconds pulse
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width for the servo
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so the percentage duty cycle for 1
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degree rotation is 0.000
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5 5 5 and so on
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this is the percentage from 100
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obviously
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but we have the auto reload of 10 000
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and hence we will calculate the
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zero 0.000 five five percent of ten
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thousand
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this will come around five point five
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five
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this is the value which corresponds to
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the same duty cycle
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as 0.5 from 20.
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let's define a variable to store the pwm
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value now we will multiply this value to
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the counter value
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this will give us the duty cycle
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corresponding to the counter value
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and now we will feed this value to the
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captcha compare register
15:28
i am using channel 1 and that's why ccr1
15:33
if you are using any other channel use
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the respective capture compare register
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there is one last thing the motor timing
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starts from the 0.5 milliseconds
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when the pulse width is 0.5 the motor
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will be at 0 degrees
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so we need to also take this into
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consideration
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the duty percentage for 0.5 is 0.025
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which in our case will become 250
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so we will add this 250 to each of our
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pwm
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values this will make sure that when the
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counter is zero
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the motor must be supplied with 0.5
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milliseconds pulse
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and with the increment in the angle the
16:38
time will be added to this 0.5
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this is the servo i am using it's the
16:48
sg90
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it have three pins the red and brown are
16:56
for vcc and ground
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and the yellow is for the signal
17:07
let's debug it now check the counter
17:10
value
17:11
as it is the angle by which the servo is
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rotating
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clockwise rotation is increasing the
17:30
angle
17:33
it have reached 90 and the servo has
17:35
also rotated by the same amount
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now we are at the maximum 180 degrees
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if we reset the controller here the
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counter will go back to zero
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and the servo will go back to its
17:55
original position
17:57
clockwise rotation is increasing the
17:59
angle
18:00
counterclockwise rotation is decreasing
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the angle
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and the servo is also rotating in the
18:06
reverse direction
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if you want to debug the registers you
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can add the sfr
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here check the value of the ccr1
18:26
register
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it's not changing in the real time but
18:37
if you refresh it
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you can see the value
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this is it for this tutorial i hope
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things were clear
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before using the code check the server
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you are using
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as it might not work within the 0.5 to
18:56
2.5
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range most of the servo motors work
19:00
between 1 milliseconds to 2 milliseconds
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range
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so make sure to check this part that's
19:06
it for today
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you can download the code from the link
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in the description
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leave comments in case of any doubt keep
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watching
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and have a nice day ahead
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you
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#Electronics & Electrical