Hi,
I have just a simple question that's been on my mind. Or rather..I just need confirmation of my thinking.
Normal airplane servos have speeds from 0.08 seconds to 0.15 seconds per 60° of travel. So the fastest meaningful servo update rate with 0.15s/60° servos is 200 Hz if you're making control corrections that are always +1°/-1°. If you make the deflection bigger than that, for example ailerons that have +15° and - 15° which makes total of 60° of travel, the servo can travel the path peak to peak in 0.15s which limits the maximum frequency of servo update loop to 6.67 Hz. If the update is faster than that, the servos lag behind the update and controls become saturated which in my book is not ok?
Ultra fast servo
0.08 s/60° (control surface +15°/-15° = +15° to high peak -15° to center - 15° to low peak and + 15° to center)
Servo Update Loop Max = 12.50 Hz
Fast Servo
0.12 s (control surface +15°/-15° = +15° to high peak -15° to center - 15° to low peak and + 15° to center)
Servo Update Loop Max = 8.33 Hz
Normal Servo
0.15 s (control surface +15°/-15° = +15° to high peak -15° to center - 15° to low peak and + 15° to center)
Servo Update Loop Max = 6.67 Hz
Am I going correctly about this or am I missing something?
Replies
The maximum update rate is usually around 50Hz because of the servo duty cycle. It receives new commands at a repetition rate of 20ms (the PWM signal), so not 200Hz maximum.
If it matters to calculate this depends on the dynamics of the airframe. If the airframe tracks really well and is ultra stable then minimal changes already have large effects. Servo deflection here is usually pretty minimal. If the airframe however is totally unstable then the servos need to perform a lot of work. Another issue is the speed at which it is operated. Higher speeds require a lot less deflection and this relationship between speed and deflection is not linear. Stability however is a lot more important than speed.
Another issue that could come up on this with PID loops are stalling servos. You could have an aircraft that in certain calm conditions performs really well but due to breakage of the frame or other issues becomes really unstable. Then you have servos that are commanded beyond their normal angles and this could result in stalling servos. Most AP's however have max/min pos's for servos that prevent this condition, same like how this is done on the radio.