Servo Jitter

Great discussion on an important topic. Just wanted to cross-link to a comment that @Rongzhong Li posted here about how Tower Pro servos (in the original Nybble that I have) are commonly cloned and counterfeited so it is difficult to get genuine Tower Pro servo.

Hi all, I with purchase I received some servos that are not marked with any brand, in plastic casing and I am considering replacing them and servo arms as well. Namely after 3 min. Kitty legs are unscrewed and I have to mount them again. I wanted to replace servo arms with alu that are stronger and hold much better. I have some quesitons for you:

• Did anyone tried this and is it really helping?

• Did anyone mounted better servos such as Savos or similar?

• For Alu arm how many tooth have your recommended servos that is which servo spline is used? I think you said somewhere 25 teeth which should be then B1 mini spline. - is that correct?

Best regards,

Alex

@eric.chanussot

Thank you, I will try this and get back to you.

Cheers

Chris Gomersall

@eric.chanussot

You were right my PWM ranges were initialy wrong I got them from some documentation on a site that sells the servo. But I don't trust them to be correct. It says they use standard PWM range. The Sketch program via the Arduino UNO works well and the servos moved, and responded to the inputs. After following the text prompts from the serial window, The program reported these PWM ranges for the 2 servos. As in my above post.

I'm still not there, I think I have the PWM ranges correct NOW, but the Nybble moves so SLOWLY and the movements are REDUCED in there range. For example in the WAVE or HI behavior. I can see Nybble attempting to wave and his left paw moves up and down but the shoulder position is not moving far enough, and the paw is not moving up and down enough. it just wiggles up and down a small amount.

It looks like I need to increase the range of movement and speed up the movement.

Can I increase the amount of movement per position say in 6 degree increments instead of 5 degree? I'm not sure how to achieve this.

Kind Regards

Chris Gomersall

@Rongzhong Li

@eric.chanussot

Hi There,

I used the Sketch code you provided with my Arduino UNO to get the PWM ranges of the two servos:

Shouldder servos K-Power DMC809 = 283 - 2080 replaces MG92B

Knee Servos Power-HD HD-1810MG = 490 - 3100 replaces MG90D

Uploaded modified sketch code to cat.

Then recalibrated.

Cat performs all behaviors VERY SLOWLY and with LIMITED RANGE OF MOTION

Should I change the constant in the code ?

#define MG92B_RANGE 150 to #define MG92B_RANGE 180

#define MG90D_RANGE 150 to #define MG90D_RANGE 180

Is there a way to speed up the servo movement? at the moment it is very slow?

Thanks for your support and help.

Kind Regards

Chris Gomersall

@Chris Gomersall

Hello,

Have you calculated the minimum and maximum pulse values of your DMC809 yourself? or did you find them somewhere?

In your October 12 message, you say you use the values: 1000µS at 1500µS.

It seems that the right values are: 900µS at 2100µS.

Source:

https://french.alibaba.com/product-detail/k-power-dmc809-coreless-servo-high-speed-3kg-torque-micro-digital-servo-for-450-rc-helicopter-60778400736.html

Maybe your problem is there?

Good cordiality

Yes, imagine you are the cat and the joints are arranged as clockwise spiral from body to limbs.

@Rongzhong Li

The index of the servos is the same as that of the representation below?

@Chris Gomersall

If you own an Arduino UNO.

Try this test sketch with the Servo library to see how your

servo responds to different settings, type a position

(0 to 180) or if you type a number greater than 200 it will be

interpreted as microseconds(544 to 2400), in the top of serial

monitor and hit [ENTER], start at 90 (or 1472) and work your

way toward zero (544) 5 degrees (or 50 micros) at a time, then

toward 180 (2400).

#include <Servo.h>

Servo servo;

void setup() {

// initialize serial:

Serial.begin(9600); //set serial monitor baud rate to match

servo.write(90);

servo.attach(9);

prntIt();

}

void loop() {

// if there's any serial available, read it:

while (Serial.available() > 0) {

// look for the next valid integer in the incoming serial stream:

int pos = Serial.parseInt();

pos = constrain(pos, 0, 2400);

servo.write(pos);

prntIt();

}

}

void prntIt()

{

Serial.print(" degrees = ");

Serial.print(servo.read());

Serial.print("\t");

Serial.print("microseconds = ");

Serial.println(servo.readMicroseconds());

}

then use the corresponding servo index after the ‘m’

That is a good idea, however I still have the Original servos' for the Head pan and tilt and the Tail.

I change the servos over for some testing.

On the cat, try to use the ‘m’ token to check the angle range. For example, “m0 -45” and “m0 45” should make the cat pan it’s head by 90 degrees. You can also use it to check the max angle ranges on Botha sides.

I've uploaded the altered code, and the cat is performing the behaviours strangely. The movements are shortened and the limbs don't extend fully

The wave for example. Nybble doesn't sit properly, and her front legs don't extend fully.

Maybe I need to alter the angle range of the servo's for the shoulder and for the knees?

Unfortunately I don't have any servo range testing equipment.

I must admit I feel a bit out of my depth here. Any further help is appreciated.

Cheers

Chris G

It’s the angle range of the servo. The tolerance is actually quite wide.

@Rongzhong Li

What is the value of 150 (MG92B_RANGE) located in: OpenCat.h \ // servo constants?

A small sketch usable with a simple UNO that can help to find the minimum and maximum values.

Rename .txt.en .pde

The parameters still work with the new servos, so I didn’t bother to fine calibrate them. I’m going to use DMC808 for MG90D. It’s a ODM model I requested from the manufacturer.

@Chris Gomersall Nice job, can you explain the method used to find the angle and the PWM range for these new servos? @Rongzhong Li It seems to me that you have tested the "K-Power DMC809", with you kept information? In the same quality as the "DMC809", which servo have you tested to replace the "MG90D"?

Yes, you’re right. Kudo was a servo brand I once tested but not satisfactory. I’ll remove it in later updates.

@Rongzhong Li

@eric.chanussot

Ok Good news I have found Pulse width range for new servos.

Shouldder servos K-Power DMC809 = 1000 - 1500 replaces MG92B

Knee Servos Power-HD HD-1810MG = 750 - 2250 replaces MG90D

Am I on the right track? what is KUDO what does it refer to?

These are the changes I have made to the code in OpenCat.h my changes are underlined

//servo constants

#define DOF 16

#define PWM_FACTOR 4

#define MG92B_MIN 250*PWM_FACTOR // changed from 170 to 250 to give min of 1000

#define MG92B_MAX 375*PWM_FACTOR // changed from 550 to 375 to give max of 1500

#define MG92B_RANGE 150

#define MG90D_MIN 188*PWM_FACTOR //changed from 158 to 188 to give min of 752

#define MG90D_MAX 562*PWM_FACTOR // changed from 515 to 562 to give max 2248

#define MG90D_RANGE 150

#define KUDO_MIN 115*PWM_FACTOR // I'm not sure what KUDO refers to ?

#define KUDO_MAX 621*PWM_FACTOR

#define KUDO_RANGE 220

// called this way, it uses the default address 0x40

Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver();

// you can also call it with a different address you want

//Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver(0x41);

// Depending on your servo make, the pulse width min and max may vary, you

// want these to be as small/large as possible without hitting the hard stop

// for max range. You'll have to tweak them as necessary to match the servos you

// have!

#ifndef KUDO

#define SERVOMIN MG92B_MIN // this is the 'minimum' pulse length count (out of 4096)

#define SERVOMAX MG92B_MAX // this is the 'maximum' pulse length count (out of 4096)

#define SERVO_ANG_RANGE MG92B_RANGE

#else

#define SERVOMIN KUDO_MIN // this is the 'minimum' pulse length count (out of 4096)

#define SERVOMAX KUDO_MAX // this is the 'maximum' pulse length count (out of 4096)

#define SERVO_ANG_RANGE KUDO_RANGE

#endif

Kind Regards

Chris G