can i ignore squar wave in step response of RC servo ? - Page 1
Forum section: electronics / projects

author: ammjy
author_rank: Regular Contributor
timestamp: March 03, 2018, 10:01:12 pm
content: Hello !First, i want say Thanks you always for your help, guys !i have one question about step response of RC servo !for making step response graph. i measured reaction of RC servo from ptentiometer. (i need step response for making transfer function)but, i found one problem.when give physical load to RC servo. RC servo try recover angle position. and in that period, reaction of RC servo making square wave.(feedback action making square wave).this is problem. when RC servo rotating without any physical load. also square wave exist.Please, see my uploaded picture.i measured graphics when RC servo rotating from 1.5ms to 2ms.1 graph is without hand grap power load. 2 graph is with hand grap power load.square wave always exist on the graph.should i ignore square wave component when i use result ?Please, let me know !

author: Benta
author_rank: Super Contributor
timestamp: March 03, 2018, 10:09:30 pm
content: I think you have a problem with a very noisy supply voltage. The square waves are probably caused by the motor PWM. Try filtering the supply to the servo with some capacitors, say, 100 uF in parallel with 100 nF, place the caps very close to the servo (short leads).

author: Andy Watson
author_rank: Super Contributor
timestamp: March 03, 2018, 10:13:55 pm
content: Are you sure that the voltage you are measuring on the "pot" is a steady-state value? I think it is suspicious that the repetition rate of the "square wave" is very similar to the rate of the PWM servo drive signal. I think we need to know more about the electronics surrounding the pot - I would guess that it is part of an oscillator circuit. If this is true, what you need to measure from the pot is the on-time duration of the pulse rather than the voltage.Can you post more details of the servo?

author: Benta
author_rank: Super Contributor
timestamp: March 03, 2018, 10:16:33 pm
content: @Andy Watson: to understand the issue here, you need to read this thread first:https://www.eevblog.com/forum/projects/make-transfer-function-for-rc-servo-is-not-possible/The goal is to find the servo transfer function.

author: ammjy
author_rank: Regular Contributor
timestamp: March 03, 2018, 10:18:17 pm
content: Thanks you again for your help, Mr. Benta !i fully understanded your point like sponge !

author: ammjy
author_rank: Regular Contributor
timestamp: March 03, 2018, 10:22:04 pm
content: Thanks you for your answer, Mr. Andy Watson !my RC servo is microservo type, MG90S.it use KC8801 IC and one potentiometer. my oscilloscope probe correctly connected with wiper pin of potentiometer.KC8801 is servo controller IC.

author: rstofer
author_rank: Super Contributor
timestamp: March 04, 2018, 12:38:39 am
content: There is a bit of feedback from the pulse output back into the potentiometer circuit. What you see is expected. You can ignore it...The other way to do it is to add an external potentiometer with gears from the servo. This will impact the step response.In the end, the step response is a very long exponential. I would do a curve fit with or without the square wave in the dataset. Expand the scale on the scope and eyeball.

author: rstofer
author_rank: Super Contributor
timestamp: March 04, 2018, 12:44:59 am
content: One thing I wonder about: The goal is to get an equation for the step response. But which response? From 1 ms to 2 ms? From 1.5 ms to 2 ms or 1.5 ms to 1.0 ms? Maybe 1.75 ms to 1.8 ms? I have no idea how these would vary. For the assignment, I would go from 1 ms to 2 ms - the entire range of motion.

author: ammjy
author_rank: Regular Contributor
timestamp: March 04, 2018, 08:04:10 am
content: Hello, Rstofer !really Thanks you kind reply, again.Mr. Rstofer and Mr. Benta giving to me lot of sense !yes, i'll try as you said !then, i think i should make plot graph with my hand with program like "Engauge Digitizer" from scope result.if square wave not deleting well.

author: ammjy
author_rank: Regular Contributor
timestamp: March 05, 2018, 08:56:51 pm
content: Mr. Rstofer !i think, that squre wave component of feedback is one of normal signal.when i deleted that squre wave from signal with capacitor.vibration appeared in the rotation of RC servo like ~2hz kind.

author: Benta
author_rank: Super Contributor
timestamp: March 05, 2018, 09:09:38 pm
content: If you placed the capacitor on the feedback pot wiper, you modified the control loop, and instability can be a result.If the power supply decoupling didn't help, you'll have to let the servo drive an external potentiometer.

author: ammjy
author_rank: Regular Contributor
timestamp: March 05, 2018, 09:13:10 pm
content: Hello, Benta !today also Thanks you for your help.Yes... i added lot of decoupling caps for delete noise power supply.but, it don't have effect to result..as you said, i think, i need do add potentiometer for result.or can i use signal from DC motor insde ?pwm -------- ic -------------- motor ---->| |-------feedback---------------hmm. it is same ))..

author: ammjy
author_rank: Regular Contributor
timestamp: March 05, 2018, 09:55:15 pm
content: finally, i got normal step response.Really, Thanks you very much, Mr. Benta, Mr. Rstofer, and other users !now, i should make transfer function with matlab.it's a simillar with realay i think !

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 06, 2018, 02:57:38 am
content: When you apply this transfer function to model system behavior be careful. Remember that a transfer function is a property of a linear system. Model servos frequently are operated in ways that make their operation non-linear. Slew rate limited, or operated into the stops are the two most common non-linear conditions but there are others.

author: ammjy
author_rank: Regular Contributor
timestamp: March 06, 2018, 07:18:43 am
content: Hello CatalinaWOW !Thanks you for reply ! you gave me important information.