| Posted: 06:59pm 16 Jul 2013 |
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Hi Dylan,
I've been using a little Tower Pro SG90 servo (standard type, not continuous) and I've found that 50 Hz (20mS) is about the fastest rate you can re-issue the PWM command. You can see this if you you put the command inside a DO loop - the servo won't run until you delay each iteration of the loop by about 20mS. You can do that with e.g. a SETTICK interrupt calling the loop or a PAUSE inside the loop. Geoff has noted that this is probably because of the time required to set up registers etc for the PWM output.
This doesn't really change things from the servos point of view anyway though because the pulse rate being sent to the servo is normally about 50Hz, i.e. you shouldn't be trying to get it to change what it's doing any more often than that. The width of those individual pulses going to the servo at 50Hz is what controls its position, not the rate at which the pulses are being sent.
Another thing to watch is that versions of MMBasic before the latest V4.4 release had a bug which created a 'jumpy' output to servos if PWM was used to control them. This has been fixed in the latest V4.4 release.
To control the rate of translation you'll have to re-issue PWM or PULSE commands with a small increment or decrement of pulse width at some regular interval that you've previously determined is appropriate for your travel rate. This 'regular interval' will presumably be considerably more than the standard 20mS pulse update rate of the servo - and this has potential to cause vibration. I guess expensive servos would also have some means of controlling current or voltage to the motor to change the rate. To accurately get it's position you'll probably need to add a pulse or absolute encoder and read that with the program.
The program below which I used to calibrate and test my SG90 model servo might (or not) give you some other useful info. (I've fiddle with this recently so there may be bugs!)
Greg
'SERVO - Program to test the Micro Power SG90 servo motor using
' the original mono-colour Maximite and MMBasic V4.4
' ----------------------------------------------
'Normal analogue servos hold position dependant on the incoming pulse width on
'the control lead. The pulse stream needs to be maintained to hold servo
'position and the normal pulse repetition rate is approx 50Hz but this is not
'critical over quite a wide range.
'The centre (neutral) posn'n for most servos is at a pulse width of approx.
'1.5mS and the full pulse width range for most servos is approx 1mS to 2mS.
'The pulse width range needs calibrating for different servos so as to achieve
'their full movement range but not over-drive and thus possibly damage them.
'Pulse width Vs position is usually not fully linear, i.e. mid pulse width is
'not necessarily the centre servo position. Shorter pulses rotate the SG90 arm
'clockwise but this can be different for other servos.
'
'The SG90 has three leads:
' a) Gnd (usually brown)
' b) 5v supply (usually red - and usually the centre lead). This draws
' typically 50-80mA when moving and 5-10mA when holding or when
' there is no control pulse input. If over-driven the current can be
' above 200mA and damage may occur. Monitoring current on the servo
' lead is a good idea when testing!
' c) pulse control lead (usually yellow).
'
'Testing is conveniently done using a 10K linear pot connected across the
'Maximite's 3.3v supply and connecting the pot wiper to one of the Maximite
'analogue input pins via a 220 ohm resistor. Voltage at that pin can then be
'read by the program and scaled to provide the control pulses. The amplitude
'of the control pulses is not critical but should not exceed the 5v servo
'supply voltage. 3.3v pulses work well.
'
'The servo can be controlled with either the PULSE or PWM command. Control
'using PULSE requires the program to maintain the pulse stream. Control using PWM
'is simple and provides a continuous pulse stream in the program background till 'changed with another PWM command or stopped with a PWM STOP command. Another
'method is to connect the PWM output running at 50Hz to an input interrupt pin
'and issue PULSE commands within the interrupt routine.
'When using the PWM command the first argument is frequency and the second
'argument is a percentage of the period. At 50 Hz the period is 20mS, thus e.g.
'10% of that equates to a 2mS pulse.
'
'NOTE 1: versions of MMBasic prior to V4.4 had a bug which limited the accuracy
'of the pulse width (it's 'resolution') to about 1%, i.e. 0.2mS at 50Hz which
'equates to around 10% of the full servo range. Thus using PWM control with
'MMBasic versions earlier than V4.4 creates very 'stepped' output. With V4.4 the
''resolution' is 0.1%, i.e. 0.02mS at 50hZ which gives fine control.
'
'NOTE 2: when using PWM control, the Maximite's PWM output circuit must have
'the voltage divider network removed to supply the full 3.3v pulse height.
'
' -----------------------------------------------------
'Rough SG90 pulse width Vs position calib'n using the PULSE command.
' Full range on the SG90 is: 2.4 mS (~90o) to 0.6mS (~270o) clockwise.
'
'Rough SG90 PWM command Vs position calib'n when running at 50Hz (20mS period).
' Full range: 12.4% (~90o) to 3.2% (~270o) clockwise.
' -----------------------------------------------------
'
Initialise:
SetTick 0,0: SetPin 11,0: SetPin 5,0: PWM STOP
Print "Input type of servo control to use:"
Print "1=Keybd_PULSE, 2=Keybd_PWM, 3=Pot_PULSE, 4=Pot_PWM"
Input controltype
On controltype GoTo Keybd_PULSE,Keybd_PWM,Pot_PULSE,Pot_PWM
Keybd_PULSE:
SetPin 11,8 'set pin 11 as digital out
Pin(11)=0 'set pin 11 to 0 (in case it's at 1)
Input "Input pulse width in mSecs: ",width 'range approx 0.6 to 2.4 mS
SetTick 20,Pulseout 'set timer interrupt for pulse out every 20mS (50Hz)
GOSUB Wait
goto Keybd_PULSE
Keybd_PWM:
Input "Input pulse width in mSecs: ",width 'range approx 0.6 to 2.4 mS
percent = width*5 'assuming frequency of 50Hz (i.e. 20mS period), thus %=width*100/20
PWM 50,percent 'start 50 Hz continuous pulse train
GOSUB Wait
goto Keybd_PWM
Pot_PULSE:
SetPin 11,8 'set pin 11 as digital out
Pin(11)=0 'set pin 11 to 0 (in case it's at 1)
SetPin 5,1 'set pin 5 as analog in
SetTick 20,Pulseout 'interrupt for pulse out every 20mS (50Hz)
Do
volts = Pin(5) 'read wiper voltage at the pot position
width = (0.5454*volts)+0.6 'from: slope=(2.4-0.6)/3.3 and y=Sx+c
key$ = Inkey$
If key$ <> "" GOTO Initialise
loop
Pot_PWM:
SetPin 5,1 'set pin 5 for analog input
do
volts = Pin(5) 'read wiper voltage at the pot position
width = (0.5454*volts)+0.6 'from: slope=(2.4-0.6)/3.3 and y=Sx+c
percent = width*5 'assuming frequency of 50Hz (i.e. 20mS period), thus %=width*100/20
PWM 50,percent 'start 50 Hz continuous pulse train
Pause 20 'delay needed for PWM at 50Hz otherwise no pulse output
key$ = Inkey$
If key$ <> "" GOTO Initialise
loop
Pulseout:
Pulse 11,width
IReturn
Wait:
key$ = Inkey$
If key$ = "" GOTO Wait 'wait for a keyboard press to input another pulse width
return Edited by paceman 2013-07-18 |
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