HSERIN problem

[s_syafiq81]

hello!

I`m using Rotomotion Attitude and Heading Referance system (AHRS) for my RC airplane stabilization. This AHRS product is based on a inertial measurement unit (IMU) and a three axis magnetometer which output sensors reading on euler angle, quaternion, angular rates, magnetic field and accelerations.

The AHRS outputs ASCII data on its serial port at 38400 N81 with no flow control. Each line consists of a marker indicating the type of data and a comma separated list of values. e.g: i want to read the euler angle line

E:-0.004,0.0123,3.14

The three Euler angles roll, pitch and yaw are output in degrees.
(roll range:-180 < roll angle < 180 , zero deg mean level with positive angle is right wing down)
(pitch range: -90 < pitch angle < 90, positive angle = nose up)
(yaw range: -180 <yaw angle<180, positive yaw angle = easternly heading)

i`m using HSEROUT & HSERIN command to transmit and receive AHRS output since the AHRS have a hardware USART and using simple IF and THEN command, to specifies where direction the servo should move. the code is like this:

DEFINE OSC 20
define HSER_BAUD 38400
DEFINE HSER_CLROERR 1
DEFINE HSER_RCSTA 90h
DEFINE HSER_TXSTA 20h

ADCON1 = 7

kirikanan var portd.0
nose var portd.1
ekor var portd.3

rollH var byte
rollHi var byte
pitchH var byte
pitchHi var byte
yawH var byte
yawHi var byte

b0 var word
b1 var word
b2 var word
b3 var word
b5 var word

low portd.0
low portd.1
low portd.3

b1 = 747
b2 = 750
b3 = 850

pause 1000
hserout ["!00001",13]
pause 2000

hserin [WAIT("E:"),dec rollHi,skip 6,dec pitchHi,skip 6,dec yawHi]

pause 500

start:

hserin [WAIT("E:"),dec rollH,skip 6,dec pitchH,skip 6,dec yawH]

servo:
pulsout ekor,b1
pulsout kirikanan,b2
pulsout nose,b3

;IFs goes here
b0 = b1 + b2 + b3
b5 = b0 / 500
b0 = 23 - b5

pause b0

if rollH < rollHi then
b2 = b2 - 5
goto nex1
endif

if rollH > rollHi then
b2 = b2 + 5
endif

nex1:
if pitchH < pitchHi then
b3 = b3 + 5
goto nex2
endif

if pitchh > pitchHi then
b3 = b3 - 5
endif

nex2:
if yawH < yawHi then
b1 = b1 + 5
goto start
endif

if yawH > yawHi then
b1 = b1 - 5
endif

goto start
end

the problem i`m facing right now is that, the servo is moving only in one direction when i move around the AHRSmovement. for e.g, the roll servo will only roll left, pitch servo will only move so that the aircraft nose moving up. can anyone help me on this.


regards,
Syariful.

[Bruce]

Hi Syariful,

The problem with controlling RC servos is that they require a continuous pulse stream. At least long enough for the servo to move from its current position to the new position.

Also, PULSOUT works by toggling the output pin twice, so the initial logic state of the output pin determins the polarity of the pulse.

For a high-going pulse, you would first need to clear the output pin. Then PULSOUT will toggle the pin twice providing a logic 1 polarity pulse.

Issuing a single PULSOUT command will probably not provide the result you're expecting. You need a loop that will continue the pulse stream for at least the time period it's going to take to move the servo horn to the position you need.

We have several servo control examples in our Micro-Bot robot projects section you might find helpful.

http://www.rentron.com/Micro-Bot/index.htm

For your particular application however, I would opt for a dedicated type servo controller like the Scott Edwards Mini SSC II or something similar.

With a dedicated servo controller, you won't need to run in a continuous loop just for servo control, and can free up processor time for monitoring other tasks.

Also, unless you maintain the pulse stream to each servo, the servo will relax. This will cause major problems in an RC airplane since force against each servo will move the servo, and cause loss of control. Maintaining the servo control pulses will keep servo motors fixed in position, and your RC airplane on a true course.

[s_syafiq81]

hi Bruce!

I would try add a loop in my code as suggested. Another thing I would like to know, are my HSERIN command is correctly used. THe AHRs will output the euler angle as presented below. Can the HSERIN command detect the negative sign since from what i have read from the manual, A variable preceded by DEC will receive the ASCII representation of its decimal value. All non-digits received prior to the first digit of the decimal value are ignored and discarded.

example:

E:-0.01825,12.45123,-5.23564

The three Euler angles roll, pitch and yaw are output in degrees.
(roll range:-180 < roll angle < 180 , zero deg mean level with positive angle is right wing down)
(pitch range: -90 < pitch angle < 90, positive angle = nose up)
(yaw range: -180 <yaw angle < 180, positive angle = easternly heading)




regards,
syariful.

[Bruce]

Hi syariful.

Your hserin looks fine to me, but nothing beats testing it in your application to make sure.