# Servo problem



## Joesmo1958 (Jan 29, 2012)

I have an unused Hitec 475HB servo that when I run it thru a VSA program, it jerks and seems to be moving one click at a time. If I manually move it in VSA, it seems to operate fine. Do I have a bad servo or is there a problem somewhere else?


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## hpropman (Jul 27, 2008)

hard so say - do you have a another thing that you can test it on? A radio control setup, a servo tester, or maybe a microcontroller. If not if you take to a hobby shop they should be able to test it for you.


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## Joesmo1958 (Jan 29, 2012)

It appears to be a timing issue. The shorter the event, the smoother the movement. If you program a long event, the servo movement is like watching the second hand of clock, ticking off the seconds.
I've got to go to the hobby store tomorrow, so I'll see if they can check it for me.


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## scubaspook (Dec 16, 2010)

Are you controlling/programming in vsa with a joystick? I like using trackskull as I can make the movements as fast or slow as I want over a time period.


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## spinman1949 (Jun 29, 2009)

I think you are seeing normal servo moverment. Depending on the servo there is resolution. The 475HB is a high torque servo. So it may not have the resolution that a lighter duty servo might have. So when you program VSA you set a movement of degrees over a period of time. The less movement over the longer period of time, results in a series of small jerks in movement. 

I have edited this response. I made a claim that is incorrect. Here is a good explanation from Servo City.

" Servos are constructed from three basic pieces; a motor, a potentiometer (variable resister) that is connected to the output shaft, and a control board. The potentiometer allows the control circuitry to monitor the current angle of the servo motor. The motor, through a series of gears, turns the output shaft and the potentiometer simultaneously. The potentiometer is fed into the servo control circuit and when the control circuit detects that the position is correct, it stops the motor. If the control circuit detects that the angle is not correct, it will turn the motor the correct direction until the angle is correct. Normally a servo is used to control an angular motion of between 0 and 180 degrees. It is not mechanically capable (unless modified) of turning any farther due to the mechanical stop build on to the main output gear. 


The amount of power applied to the motor is proportional to the distance it needs to travel. So, if the shaft needs to turn a large distance, the motor will run at full speed. If it needs to turn only a small amount, the motor will run at a slower speed. This is called proportional control. "

The bottom line is that at some point the servo can only move so slow. Drop the voltage to the motor too low and the servo under load will stall. So if you program a movement over a long period of time, at some point the servo will start to appear to be moving like a clock. One thing that is not clear in the explanation from Servo City is if the motor is given higher or lower voltage or if the motor is given faster or slower pulses of power. I suspect what is occuring is the latter, which would explain the chatter type movement often seen with servo's.

And if I had read further. LOL !!!

" The purpose of this information is to give an overview of how servos operate and how to communicate with them. Though we have taken steps to assure the quality of information here, ServoCity makes no guarantees about the information presented. ServoCity cannot be held liable or accountable for any use or misuse of the provided information.


Servos are controlled by sending them a pulse of variable width. The control wire is used to send this pulse. The parameters for this pulse are that it has a minimum pulse, a maximum pulse, and a repetition rate. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has exactly the same amount of potential rotation in the clockwise direction as it does in the counter clockwise direction. It is important to note that different servos will have different constraints on their rotation but they all have a neutral position, and that position is always around 1.5 milliseconds (ms).




The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse width Modulation. The servo expects to see a pulse every 20 ms. The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral position).

When these servos are commanded to move they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position. The maximum amount of force the servo can exert is the torque rating of the servo. Servos will not hold their position forever though; the position pulse must be repeated to instruct the servo to stay in position.

When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs. The minimal width and the maximum width of pulse that will command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maximum and minimums. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide.




Another parameter that varies from servo to servo is the turn rate. This is the time it takes from the servo to change from one position to another. The worst case turning time is when the servo is holding at the minimum rotation and it is commanded to go to maximum rotation. This can take several seconds on very high torque servos. "


From reading this I suspect the control is a combination of voltage to the motor to control speed and the pulse is setting the desired position. So in the standard stick type control IE model airplane, the servo will move quicker if the stick is moved quickly. Slower if the stick is moved to the same position slower. Again there must be a limit as to just how slow the servo can move. Extend beyond that and the result is your clock movement.


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