should i add capacitor for measure response with scope ? - Page 1
Forum section: electronics / projects

author: ammjy
author_rank: Regular Contributor
timestamp: March 03, 2018, 10:44:49 am
content: Hello !this is my previous question link.(https://www.eevblog.com/forum/projects/make-transfer-function-for-rc-servo-is-not-possible/)Please, see the picture what i added in this post ! (Thanks you very much !! )first, Thanks you for click.now, for make step response of my RC servo motor, i catched voltage move with single shot trigger.graph showing information.1.5ms(1.540V) to 1ms(1.040V).but, graph is quite dirty. for get more clean graph, should i add some capacitor ?

author: danadak
author_rank: Super Contributor
timestamp: March 03, 2018, 11:10:45 am
content: The noise looks like a small ceramic disk, say .01uF or .1uF might help.Larger caps will affect your goal to get to transfer function as the capwill contribute to system dynamic behavior as its an energy storageelement.An alternative way is, if scope is DSO, and has ability to export capturedata to post process it in excel or MatLab. With a LPF or running averagefilter.Or use DSO to average several step response cycles....Regards, Dana.

author: ammjy
author_rank: Regular Contributor
timestamp: March 03, 2018, 11:24:10 am
content: Thabks you a lot, Mr. Danadak !i fully understanded !Thanks you again !

author: David Hess
author_rank: Super Contributor
timestamp: March 03, 2018, 12:40:05 pm
content: The servo itself low pass filters the output from the potentiometer to prevent chatter. I would add a resistor in series with the wiper and then a capacitor to ground. Servos are very slow so the time constant of the filter can be large.

author: ammjy
author_rank: Regular Contributor
timestamp: March 03, 2018, 04:54:59 pm
content: Thanks you for reply, David Hess !aa yes i understanded your point !

author: David Hess
author_rank: Super Contributor
timestamp: March 03, 2018, 07:07:28 pm
content: Long ago I remember assembling Heathkit servos literally from kits and they included a schematic and description:http://vintagercfiles.com/Files/Heathkit/Servos/GDA-1205-8%20servo/Heathkit%20GDA-1205-8%20servo.pdfThese are pulse width modulated servos, which were a great improvement over the analog servos preceding them, so they do not work in the way you might expect by comparing the output of the potentiometer to a voltage level. Instead, the potentiometer controls the charging of a capacitor producing a ramp yielding a pulse width which is compared to the pulse width of the control signal; either it is later or earlier. But the result of this is that the potentiometer position is *integrated* instead of sampled so it has an automatic low pass filter function applied removing wiper noise.I could not find the details on the weird 12 pin integrated circuit Heathkit used but I suspect the potentiometer and capacitor pulse width circuit actually produces a linear ramp similar to how the sweep generator on an oscilloscope works.The application circuit for the On Semiconductor MC33030 analog servo controller shows capacitor filtering on the feedback and input signal.https://www.onsemi.com/pub/Collateral/MC33030-D.PDF