I'm kind of new when it comes to using servo's with an Raspberry Pi. Although I do have some experience, I'd like to understand things a bit better. Therefore I would like to share some questions with you and hope to gain some better insights in how to work with servo's.
The questions I'm currently struggling with:
1. Given the specifications of the Servo, how can I deduct the minimum and maximum values (angles) of the servo?
2. Using the PWM Hat described below, and the python library the comes with it, how exactly do I calculate the time that the PWM pulse needs to be set to High to achieve a certain angle? For example: what would be the PWM pulse high length to send to the servo so it will be at a 90 degree position? And what would it be for an 120 degree position?
Any thoughts, remarks or comments are highly appreciated, thanks in advance!
Hardware and software specifications
The servo we have is this one, the Power HD LF-20MG
Torque (4.8V): 16.5 kg-cm
Torque (6.6V): 20 kg-cm
Speed: 0.18sec (4.8V) | 0.16sec (6.6V)
Operating voltage: 4.8 - 6.6 DC Volts
Working frequency: 1520us (=1.52ms) at 333Hz
Max-travel: 165 degrees
The PWM controller
To control the Servo using a Raspberry Pi we use this Adafruit's PWM/SERVO HAT for precise timing. The HAT must be controlled using their Python library in which the following functions control the servo:
The PWM software controller
First, setting the frequency. Based on the servo's specifications I guess it should be 333.
Secondly there's a function that actually controls the PWM pulse. This is were I'm really doubting (see the second question above).
var port = 15 var tickHigh = 0 var tickLow = 1400 #For example; range from 0-4096 pwm.setPWM(port, tickHigh, tickLow)
As stated in the docs (see this link), the values to use are calculated based on the pulse-width needed in microseconds. But I'm really struggling here: I can't seem to determine what the needed pulse-width should be for this servo (hence the to questions).
IF I was to know that, the calculation for the software becomes a lot easier:
(1 / frequency) / 4096 = time per tick
For this servo's frequency that would be: (1/333) / 4096 = 0,00000073315503
Then I could use the following formula, given the needed pulse width length needed:
pulse length needed / time per tick = tickHigh
> If you need to calculate pulse-width in microseconds, you can do that > by first figuring out how long each cycle is. That would be 1/freq > where freq is the PWM frequency you set above. For 1000 Hz, that would > be 1 millisecond. Then divide by 4096 to get the time per tick, eg 1 > millisecond / 4096 = ~0.25 microseconds. If you want a pulse that is > 10 microseconds long, divide the time by time-per-tick (10us / 0.25 us > = 40) then turn on at tick 0 and turn off at tick 40.