Servo Loops

Please 'Boom' Responsibly As most of you have noticed, the noise ordinances have become much tougher lately. Most of this is due to idiots, yes IDIOTS, who drive through residential areas with their windows down while their system is playing at full power. To make things worse, the music they listen to has all sorts of foul language that's not suitable for small children, (who may be playing outside). There are even a few people, who are even beyond idiot status, that play their systems at full power through residential areas after 10:00 PM (when many people go to bed). I don't believe that this type of behavior is good for the industry. If the fines get too stiff, people will stop buying large systems. If this happens, more people will get out of car audio (who wants a mediocre system). People get interested in things because they're exciting. A deck and four 6.5" speakers are not going to interest many of the younger car audio enthusiasts. If car audio enthusiasts keep annoying more and more people, the fines will keep getting tougher. All of this will only reduce interest in the equipment that fuels the industry. If you want to listen to your system at full volume, get out on the highway where there's little chance of bothering anyone. When you get to a red light, turn it down. If the only thing attractive about you is your 'system', you have some work to do. Bottom line... Think about what you're doing. Think about other people. It's not the end of the world if you have to turn the volume down for a little while.

Servo Loops:

Servos can be electronic or mechanical or a combination of the two. Servos are used in devices that rely on precise operation under varying loads or conditions. In audio, the servo looks at the output signal (via a feedback signal) and corrects if the audio output signal varies from it's intended output. For amplifiers, it means that the internal drive circuits will drive harder to make up for losses of any electrical/electronic components in the output circuit. In the examples here, the correction is slow. In electronic circuits, the correction is instantaneous. The example below will help you to understand one type of servo controlled system.

A servo controlled system has a point of reference (generally set by the user), a feedback sensor and a means to correct for errors. In the following demo, the valve assembly is the sensor. The point of reference is the height at which the green arrow (on the valve assembly) is set. If it were set higher, the lever in the demo would be set to stay at an angle instead of parallel to the floor. The device that will correct for an unlevel condition is the hydraulic system.

Click HERE to make the demo fill this window.

In the previous demo, the weight was placed on the left side of the fulcrum. If it were set on the right side, the system would still correct as the green valve was pushed from its point of rest. Instead of the hydraulic fluid going into the bottom of the cylinder and pushing up as it did before, the fluid would flow into the top of the cylinder and the cylinder would pull down. As soon as the lever was again parallel to the floor, the the green valve would return to its point of rest and the hydraulic cylinder would stop.

If you've ever used a tractor with a 3-point hitch, you've probably seen a similar type of servo. The control for the height of the 3-point hitch is a servo mechanism. You set the handle to a reference point and the hitch will quickly move to the desired level. To compare the tractor to the device shown here, the servo control on the tractor would be analogous to the entire valve assembly here.

Another very imprecise version of a servo type feedback loop is you when you're driving. If you're trying to maintain a steady speed when driving on hills, you have to keep an eye on the speedometer and vary the position of the gas pedal to maintain the target speed. If you're lucky, you have a 'cruise control' and can set it to maintain the correct speed. The cruise control uses sensors on the transmission or the wheels as the feedback signal to maintain the correct speed. When you set the cruise control, it takes note of the number of pulses per second from its wheel/transmission speed sensor. If the pulse rate drops below a certain point, it will open the throttle a little more. If the rate gets too high, the throttle will be cut back.

Servo Controlled Speakers:

Some subwoofers use servo feedback to virtually eliminate distortion. Velodyne speakers use an accelerometer mounted on the woofer's voice coil former to monitor the speaker's output. If there is any deviation from the desired output of the speaker, the servo circuit corrects. Kenwood has some amplifiers out that allow the amp to monitor the signal at the speaker. This allows the amplifier to correct for losses in the speaker wire.

I will cover other uses of servo/feedback loops on the op-amp page.

You should remember: 1.A servo controlled system employs some sort of feedback and error correction to precisely control the output of some device.

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