Stalling torque for a converted DC Servo

Hi guys,
An ordinary DC motor can be converted into a servo motor using an encoder connected to it's shaft.A driver/controller takes the
signals from the encoder and acts accordingly to control the speed of the motor(using PWM or something like that).The question is that when the motor is stopped at a certain angle how can it maintain it's stalling torque(like a stepper motor does) or how does the driver keep the motor at that certain angular position without applying any power to the motor(An ordinary motor can be stopped by cutting the power) and also not letting the load to move it's shaft.
Thank You, prizark.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 18 May 2004 23:51:34 -0700, prizark wrote:

Basically the servo loop will only apply power if needed:
If there is no external torque applied, and the servos position matches the desired position, it will just stay there without any power applied.
As soon as any external torque is applied, causing the actual motor position (as measured by the encoder) to be different from the desired position, the servo loop will apply a torque in the reverse direction of the external torque until the desired position is restored.
This is the P (Proportional) part of the PID loop commonly used for servo control, something like this:
MOTORDRIVE = (DESIREDPOSITION-ACTUALPOSITION)*KP
Where KP is the "gain"
Peter Wallace
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

By its nature, a servo is a closed loop device, when the motor shaft begins to move away from its commanded position, the error signal biases the drive amplifier to compensate for that movement. The compensation consists of feeding a current into the motor in a direction that cause the motor to turn in a direction against the change.
Basically your servo amplifer ALWAYs applies power to the motor, it just applys a enough current to keep it where it is.
--Chuck
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Interestingly enough, the most common method I have seen is a brake/locking device. The brake engages with no power applied to it, and disengages when you apply power to it. So you would apply power to the brake to unlock it, then apply power to the motor, and when the device moves to the correct position, you would stop the motor and engage the brake to lock it in place. You typically see the brakes on DC gearmotor/encoder combinations on the more sophisticated antenna rotator units. The most common setup I saw like this is a DC gearmotor/brake combo unit that is sold on ebay from time to time. That is a 25 rpm gearmotor, with a brake unit. It appeared to be out of some kind of a radar antenna unit. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category &209&item816830841&rd=1

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.