OT: what is a "servo" if not a motor?

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Servo usually means ..those funny little motors.
But I've seen reference to servo in electronics which have nothing to do with motors.. "synchronize the servo [ in the sensor circuit ]".. OK what is this meaning of servo?

thx

Regards,
Scott

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Definition of Servo-
a control system that converts a small mechanical motion into one requiring much greater power, may include a negative feedback system.

Most people consider Servos to be motors. I think that hydraulics, pneumatics, and solonoids using feedback can be considered Servo Systems as well.

Matt

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Electrical servo motors are smaller diameter usually... less rotational iniertia.... respond quicker.. like for a robot arm... often have encoders or tachs built in.... as opposed to a regular motor to run a fan, etc

Imagecraft compiler user

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With reference to the little black boxes called "Hobbie" servos, these are pulse width to position transducers.

I.E. Set up a fixed period of the electrical signal driving the servo and adjust the on/off duty cycle of that the electrical signal within the established period.

I would prefer view these as servo positioners. These are not within them selves, a servo system. They are only one of the components making up a complete servo system.

The system becomes a servo system when the feedback loop is closed and the physical property is precisely controlled around some fixed or variable reference point.

Most gas fired industrial ovens employ a servo positioner to control the phsysical opening and closing of a gas valve in response to the temperature controllers target setpoint temperature and corrections to any errors within the process to the target setpoint.

This is a complete servo system.

This servo positioner in an industrial temperature control system is "Way Big" with reference to the "Hobbie" servo positioner.

Servo systems don't neccessarily mean that PWN is involved, either..

A servo system is a "Closed Loop" system whose intended purpose is to precisely control velocity, position, temperature, pressure, direction, etc. It is a control system, where by, mechanical properties are precisely controlled by electrical stimuli.

I have a lot of invilvement with industrial based servo systems. Typically they incorporate the following components:

1) Servo Amplifier - Power converter/translator
2) Servo Motor - Electrical to mechanical transducer.
3) Feesback system - Usually an incremental encoder.
4) Power cable between the servo amplifier and motor.
5) Encoder cable between the servo amplifier and motor.
6) Command input.
7) Control inputs and outputs.

These things comprise a basic industrial servo system.

In addition, there are motion controllers.

Motion controllers come in two flavors:
1) Motion controllers without internal servo amplifiers and feedback inputs.
I.E. Motion Control Technology Systems Model XDC.
2) Motion controllers with internal servo amplfiers and feedback inputs.
I.E. Allen Bradley Ultra Drive 3000 & 5000 series.

The servo "System" provides a means of providing precise velocity/position control in response to external analog or digital commands.

The "Motion" controller keeps track of the commanded velocity/position and coordinates that velocity/position amung multiple axes. Axes being plural, axis being singular.

I hope this has helped provide a little better understanding of what the diferences in terms are.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Quote:

With reference to the little black boxes called "Hobbie" servos, these are pulse width to position transducers.

I.E. Set up a fixed period of the electrical signal driving the servo and adjust the on/off duty cycle of that the electrical signal within the established period.

I would prefer view these as servo positioners. These are not within them selves, a servo system. They are only one of the components making up a complete servo system.

The system becomes a servo system when the feedback loop is closed and the physical property is precisely controlled around some fixed or variable reference point.

The feed back loop is closed in a hobby servo, you have pwm in(external reference) the you have motor control, with analog feed back to the internal motor driver.

The main difference between a normal DC motor and a DC Servo motor is the very fact that the rotor feedback is built in(resolver, encoder, hall effect sensor etc..). I know that there are other differences.

Matt

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That is true! But in the end, it is still only a PWM to position transducer. There is no feedback to the source of the PWM controller to correct for position errors. You can only assume that the position is correct based on the PWM duty cycle being sent to the positioner. A true "Closed Loop" servo system provides position feedback to adjust/compensate the PWM signal for those position errors.

A true servo system will have an "Inner" feedback and "Outer" feedback control loop.

The inner feedback loop is the feedback that you are talking about. The inner feedback loop ensures that the motor in the servo positioner tracks the PWM, or command signal being applied to it. I.E. 40% PWM = 70 degrees, or some such angle. But that doesn't guarentee that the actual mechanical position is, in fact at the required 70 degrees. All it does is tell the motor that, for this PWM signal, move that far. But just how far is that? And, if I push on the servo transducer and move it "Off-Position", is it guarenteed to return the the target position? And can I prove it?

Your typical little "Rug Rover" doesn't really need a full servo system because, who cares if it runs into the wall?
CNC machine tools, burner control systems, robot "Pick & Place" equipment, they can't leave to chance that the required mechanical movement accurately took place solely based on the "Inner" feedback loop. They have to monitor and correct for position/process errors based on unforseen external influences - things such as mechanical slop, change in product ldensity, airflow, pressure, temperature, etc.

The "Outter" feedback loop monitors the "Real" mechanical position ( process variable) and makes PWM (command signal) corrections to actually put the servo positioner in the desired position, and keep it there, even when external forces attempt to move it out of position.

So, even though the "Little Black Box" that is called a servo encorporates a feedback loop, it is only the electrical to mechanical transducer component of a complete servo system.

Read some articles by Allen Bradley, Pacific Scientific, Indramat, and the hundred of other servo equipment manufacturers, and they all describe a "Closed Loop" servo system under the same terms.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Servo comes from the Latin, meaning "slave" or "servant".

Servo as an adjective means "acting as part of or pertaining to
a servomechanism".

Servo as a noun is the informal version of servomechanism,
which is "an electronic control system in which a hydraulic,
pneumatic, or other type of controlling mechanism is actuated
and controlled by a low energy signal". (although I take issue
with the "electronic" requirement)

Servo as a verb means to "connect a mechanism to another, as
in a servomechanism"

A system where a valve position is set by a motor, which is
controlled by an error amplifier which receives a feedback from
a sensor, is a "closed loop control system" in which a "servo"
(the actuator that moves the valve in response to a small control
signal) is just a part.

A servomechanism by itself does not necessarily include a closed
loop control scheme, but probably most commonly is part of
a closed loop control scheme. A "hobby servo" is a box that
contains both a servo and a closed loop control system for it.

IMO, calling a closed loop control system a "servo system" seems a
technically incorrect use of the term.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Tom,
When I saw you come on to the forum, I just knew that you wouldn't be able to pass this one up.

While the "Hobbie" servo does have a feedback loop, I have issues with it being called a servo, with reference to the command signal, it is still "Open Loop" Maybe Servo Transducer would be better, as this implies only one component in a complete servo system.

Did you ever work at a place, in or near Broken Arrow, that produced automated inventory/material handling systems? These systems used a wire in the floor to guid the material pickers.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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I know who you mean but I have a temporary block in recalling the company name! They
had a building on East 51st which I think is now occupied by LaBarge. I never worked there,
my only "real job" was 1973-1976 with Mobilfone Service, a radio common carrier operation.
I designed and built IMTS terminal and toll ticketing equipment, manual operator control
and phone patch control consoles. I started my little company in 1976 first providing
electronic assembly services and then adding industrial controls design and OEM manufacturing.

Hey I just remembered the name- Clay Bernard Systems!

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Yep! That's them. I worked for them back in 1979. Ine Friday afternoon that called all of the technicians into the office and laied all of us off. A few weeks later, they went under. It's a shame! They were a fun place to work.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston