Servo not reaching full travel

[ruijc]

Hi guys,

I've made a circuit that i use to work with a futaba servo.

The problem is that i cannot get it to reach it's extremes.
The max i can get is a total of 90º from left to right ( and i know these servos move more that this ).

I'm running the code like this...

( sample )

Moving left:
if pos > 25 then
pos = pos - 50
servo =1
pauseus 1000+pos
servo =0

Moving right:
if pos <1300 then
pos = pos + 50
servo =1
pauseus 1000+pos
servo =0

I tryed to change the limits but the servo does not move pass it ( upper limit is 1300 and center is 700 )

Thanks in advance

[Acetronics2]

Hi ruijc

I'd be VERY surprised those lines give something you 'd waited for ...

<<
if pos > 25 then
pos = pos - 50
servo =1
pauseus 1000+pos
servo =0
>>

Now,

Which processor ?
Initialisation lines ???
variables ? ( plus which type: bit, byte, Words, longs ???? )

try a little effort ... for others to undunstand what you've done ( or tried to do ...)

Alain

[ruijc]

Hi Alan,

I tryed diferent pics like 16F84, 16F628, 16F88 and speeds of 3Mhz, 4Mhz and 8Mhz.

Here is the all code ( setting lines learned from you ):

'************************************************* ****************************
'19/12/2007
'
'PWM experience to control servo
'
'PIC 16F628 @ 4Mhz

'************************************************* ****************************
'Config

PIE1 = 0 'off
PIR1 = 0 'off
CMCON = 7 'comparator off
VRCON = 0 'Vref Off
T1CON = 0


'************************************************* ****************************
'DEFINEs
'************************************************* ****************************

'OSC

DEFINE osc 4


'************************************************* ****************************
' variables

va1 var word
va2 var word
ser var word
pos var word
na var word
nb var word

CLEAR

'************************************************* ****************************
'I/O's

PORTA = %00000000
PORTB = %00000000

TRISA = %00000011
TRISB = %00000011

'************************************************* ****************************
'PINS

bu1 var PORTA.0
bu2 var PORTA.1
a2 var PORTA.2
a3 var PORTA.3
a4 var PORTA.4
MCLR var PORTA.5
a6 var PORTA.6
a7 var PORTA.7

b4 var PORTB.4
b5 var PORTB.5
b6 var PORTB.6
b7 var PORTB.7

bu4 var PORTB.0
bu3 var PORTB.1
led var PORTB.2
servo1 var PORTB.3


'************************************************* ****************************

Start:


Low servo1
Gosub center

mainloop:

If bu1 = 1 Then
Gosub left
Endif
If bu2 = 1 Then
Gosub center
Endif
If bu3 = 1 Then
Gosub right
Endif

servo1 = 1
Pauseus 1000 + pos
servo1 = 0
Pause 16

Goto mainloop

left:
high led
If pos < 1800 Then
pos = pos + 75
Endif
servo1 = 1
Pauseus 1000 + pos
servo1 = 0
Pause 16
low led
Return

right:
high led
If pos > 25 Then
pos = pos - 75
Endif
servo1 = 1
Pauseus 1000 + pos
servo1 = 0
Pause 16
low led
Return

center:
high led
High 5
Pause 500
pos = 700
Pause 500
servo1 = 1
Pauseus 1000 + pos
servo1 = 0
Pause 16
low led
Return

end

[Jumper]

Let's just go over the basic idea:

Center position of futaba servo is 1.52ms puls length sent approx every 20 ms. To move it to min position 1 ms and to max position 2 ms.

But when I recently did a thingy for a RC receiver I found out that the pulses are acctually more in the 0.8 ms - 2.2 ms range to allow for trim settings.

If you plan to have more than one servo I would recommend you to have a look at DT software PWM.

If you want to have bigger movement try to change your limit values.

/me

[ruijc]

Hi Jumper,

when you say change the limit values you mean the :

If pos < 1800 Then

and

If pos > 25 Then

right ?

I added a lcd and did some studies. I did increased the upper value but the servo just stops when it reaches +- 1500.

The center is 700

I cant work with numbers bellow 0 i'm affraid.

The plan is to have only one servo, but i need to use it with more travel ( not continuous ) as 90º is too short

.

[Jumper]

Hi,

Hook up a 10k potentiometer (or whatever you have in your drawer) to i.e AN0. Vdd on one side of the pot. GND on the other and the wiper goes to AN0. Put a 0.1 uF cap between wiper and GND to reduce noice .Connect the servo on i.e PORTB.1 ... if you use a voltage meter on the wiper and adjust the voltage to ~3 V you should get a 1.5 ms pulse every 20ms. Now you can turn the pot and then you can try the end positions... your LCD will be great here.


DEFINE ADC_BITS 8

' DEFINE YOUR LCD SETTINGS HERE

SERVO1 Var PORTB.1
AD0 var word
SERVO1_Pulse var byte
Total_Delay var word
n var byte

TRISA 255 ' PORTA to input
?????????? 'set up as analog CHECK DATASHEET for you PIC
?????????? ' turn off comparators CHECK DATASHEET

n=0
Goto main


update_LCD:
n=0 'reset LCD loop counter
'Here you write your LCD code to send the Servo1_pulse value to your LCD
RETURN

main:

ADCIN 0, AD0 'READ POT
SERVO1_Pulse=AD0*10 'Scale value max = 2550 us
SERVO1=1 'Start the servo pulse
Pauseus(SERVO1_Pulse) 'wait a while
CH1=0 'Stop the pulse
Total_Delay=(20000-Servo1_Pulse) 'calculate loop delay so we
Pauseus(Total_Delay) 'get approx 20 ms loop time
N=n+1
if n>=50 then gosub update_LCD 'update LCD every 50 loops i.e once per second
goto main

end


try it and if it doesnt work let me know so I can help. This code is untested but should work if you set up the analog port correctly. After this you will atleast know how far your servo will travel.

/me