Can I control servos and multitask at the same time?

[ZBWillUser]

I want to multitask, but I also need to control up to 3 servos at the same time. Will there be any "gotchas" I need to look out for in this situation?

I don't want to risk damaging the servos because their timing gets messed up. Would it be better to put servo control in the main task to improve their timing?

"Problems seen in advance and avoided aren't problems at all."

Thanks in advance.

[dkinzer]

I don't want to risk damaging the servos because their timing gets messed up. Would it be better to put servo control in the main task to improve their timing?

If you use hardware PWM to control the servos the only issue you might have is some latency between when you detect/decide that you want to change the position (i.e. the duty cycle of the PWM) and when the code executes that actually changes the duty cycle.

On the other hand, if you're planning to implement the servo pulse streams entirely in software then multi-tasking will almost certainly interfere.

If I recall correctly, you're using a mega1280-based ZX so you have plenty of hardware PWM channels to use.

[ZBWillUser]

You are correct, I have a ZX-1280n (I would think being an n would be a help in this situation).

Where are some good examples of using a harware PWM to drive a servo?

I am used to using the Servo() command in the Arduino which didn't multitask and didn't run Basic, but wasn't bad for what it was.

As usual, thanks for the fast response.

[ZBWillUser]

Got it!
(Basically like this...)

Code: Select all

Dim taskStack(1 to 100) as Byte

sub main()
	CallTask "MyTask", taskStack
	'This works on Channel 2 (B.6)
	Call OpenPWM(2, 50.0, zxFastPWM)' prepare for 50Hz Fast PWM using channel 2
	Call PWM(2, 7.5) ' generate PWM with 7.5% duty cycle (1.5mS)
end sub

Sub MyTask()
	Do
		Debug.Print "Hello from MyTask"
		Debug.Print "Taking a break from Main()"
		Call Delay(2.0)
	Loop
End Sub

(Your SysLib is just awesome!)

[dkinzer]

Where are some good examples of using a harware PWM to drive a servo?

It is helpful to understand how servos work. With that foundational information, you should be able to understand the following code (untested).

Code: Select all

'
'' InitServo
'
' Initialize a PWM channel for driving a servo.  The base frequency
' is set to 50Hz (20mS period) and the duty cycle is set so that the
' "on" time is 1.5mS, i.e., the neutral postion.
'
Sub InitServo(ByVal chan as Byte)
    Call OpenPWM(chan, 50.0, zxFastPWM)
    Call SetServoPos(chan, 0)
End Sub

'
'' SetServoPos
'
' Set the duty cycle on a PWM channel to correspond to a given position
' expressed as a signed value from -100 (corresponding to an "on" time
' of 1.0mS) to +100 (corresponding to an "on" time of 2.0mS).  It is
' assumed that the PWM channel is already open and set to the proper base
' frequency.
'
Sub SetServoPos(ByVal chan as Byte, ByVal pos as Integer)
    ' validate the position
    If &#40;&#40;pos >= -100&#41; And &#40;pos <= 100&#41;&#41; Then
        ' compute the duty cycle corresponding to the position &#40;5% to 10%&#41;
        Dim duty as Single
        duty = 7.5 + &#40;&#40;CSng&#40;pos&#41; / 100.0&#41; * 2.5&#41; 
        Call PWM&#40;chan, duty&#41;
    End If
End Sub

[ZBWillUser]

Yes, I understand. Nice!

In my example I was a lot less elaborate (and precise) and was trying to show a little multitasking as well.

(It also can be done without the SysLib, but that is such a great addition to ZBasic!)

ZBasic is awesome and I wish more people knew that.

[dkinzer]

In my example I was a lot less elaborate (and precise) and was trying to show a little multitasking as well.

I was only attempting to illustrate how PWM can be used to drive a servo. The routine SetServoPos() may not be a good match to your needs. For example, if you are using a standard servo that rotates -90* and +90* from neutral it might make more sense to have the pos parameter range from -90 to +90 rather than -100 to +100. You should be able to modify it to meet the needs of your application.

If you are using a servo modified for continuous rotation, you'll want to come up with a different routine for setting the duty cycle of the PWM signal.

[spamiam]

...The routine SetServoPos() may not be a good match to your needs. For example, if you are using a standard servo that rotates -90* and +90* from neutral it might make more sense to have the pos parameter range from -90 to +90 rather than -100 to +100....

Interesting idea. It can be extended even further. Depending on the geometry of the linkage, you might be converting epicyclic(?) rotational motion of the servo to a linear motion. You might want to work with a linear position, and then you would have to do more sophisticated math to convert to rotational angle, then to duty factor.

I have never done this, but I can see that it might be a useful approach for some purposes.

-Tony