Servo tuning basics - Revision history

Tero K at 09:51, 29 June 2015

2015-06-29T09:51:21Z

Tero K at 23:31, 19 March 2015

2015-03-19T23:31:48Z

Tero K at 22:40, 15 April 2014

2014-04-15T22:40:14Z

Tero K: /* Driver as a controller */

2013-08-12T20:38:26Z

‎Driver as a controller

Tero K: /* Driving (a.k.a servoing) a car */

2013-08-12T20:37:28Z

‎Driving (a.k.a servoing) a car

Tero K: /* Driving (a.k.a servoing) a car */

2013-08-12T20:35:38Z

‎Driving (a.k.a servoing) a car

Tero K: /* Gain tuning (car) example */

2013-08-12T20:26:18Z

‎Gain tuning (car) example

Tero K: /* Gain tuning (car) example */

2013-08-12T20:22:45Z

‎Gain tuning (car) example

Tero K at 20:22, 12 August 2013

2013-08-12T20:22:00Z

Tero K: /* Driver as a controller */

2013-08-12T20:21:16Z

‎Driver as a controller

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-Driving a servo motor is much like driving a car. Driving a car has many similarities including the key concepts of torque, velocity and position control. Most of this happens in the driver's head the same way than a servo drive does with a motor.
+{{parent|Servo motor tuning guide}}Driving a servo motor is much like driving a car. Driving a car has many similarities including the key concepts of torque, velocity and position control. Most of this happens in the driver's head the same way than a servo drive does with a motor.
 For those who are already familiar with the basics, see the principles of real world servo motor drive from article [[Signal path of motor drive]].
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 See the principles of real world servo motor drive from article [[Signal path of motor drive]].
 {{next|[[Tuning torque controller]]}}
-[[Category:Tuning guide]]

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 ==Real servo motor controller==
 See the principles of real world servo motor drive from article [[Signal path of motor drive]].
 [[Category:Tuning guide]]

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 Driving a servo motor is much like driving a car. Driving a car has many similarities including the key concepts of torque, velocity and position control. Most of this happens in the driver's head the same way than a servo drive does with a motor.
 ==Driving (a.k.a servoing) a car==
 Driving a car to a destination is much like driving a servo motor to it's destination position. The analogies in between are:
 {| class="wikitable"
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 The response can be improved further by introducing a limit to the slew rate of setpoint signal. The controller behaves optimally when the system is able to follow the setpoint continuously with little tracking error. In this case the gains could be same as before, P=50 I=25.
 [[Category:Tuning guide]]

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 *All actions the driver makes, is based on comparing the setpoints and the actual state
-==Driver as a controller==
+==Servo controller basics==
-===What is a controller gain and a PI controller===
+===Controller gains and a PI controller===
 Controller gain means '''sensitivity to change output''' due to ''tracking error'' (the difference between setpoint and feedback).
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 In servo drive case, this means that each motor type and mechanical axis need to be tuned separately. However, if axis and motors are identical, then the same gains should work equally.
 ==Gain tuning (car) example==
 The following series of images illustrate an imaginary car driving scenario where the driver acts as velocity controller of the car. The magnitude of PI gain values equal the driver's aggressivity of controlling the pedal to reach the desired speed.

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 *Controls car's velocity based on the speed meter value and speed limit
 *Controls car's position based on trip meter's reading or by observing location trough the windshield
 ==Driver as a controller==

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 ===Low gains - sluggish response===
 [[File:Carexample sluggish.png|800px]]
 If gains are set too low, the system response tends to be sluggish and sometimes leave a static tracking error (not reaching setpoint in any time). In this imaginary case we could think PI gains to be P=20 I=10.
 ===Too high gains - oscillation & instability===
 [[File:Carexample unstablepng.png|800px]]
 If gains are set too high, the system becomes [[overshooting]], oscillating and less stable. Here the comparable PI gains could be P=200 I=100
 ===Optimum gains - only little overshoot===
 [[File:Carexample stable.png|800px]]
 When gains are tuned correctly, the response shows rapid response with low overshoot and no ringing or oscillations. The comparable gains here P=50 I=25.
 ===Optimum gains with realistic setpoint - optimum response===
 [[File:Carexample response accellimit.png|800px]]
 [[Category:Tuning guide]]

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 ==Driver as a controller==
-===What is a feedback gain and PI controller===
+===What is a controller gain and a PI controller===
-Feedback gain means '''sensitivity to change output''' due to ''tracking error'' (the difference between setpoint and feedback).
+Controller gain means '''sensitivity to change output''' due to ''tracking error'' (the difference between setpoint and feedback).
-The simplest form of feedback controller is a '''proportional''' gain controller (P controller) where output follows the formula ''output = Pgain*(setpoint-feedback)''. The problem of proportional gain controller is that it may never reach the setpoint because output starts approaching zero when the following error is reaching zero.
+The simplest form of feedback based controller is a '''proportional''' gain controller (P controller) where output follows the formula ''output = Pgain*(setpoint-feedback)''. The problem of proportional gain controller is that it may never reach the setpoint because output starts approaching zero when the following error is reaching zero.
 Because of this, it's better to add in '''integrating''' component to the controller (forming PI controller). Integrator accumulates the tracking error to a integrator variable. Integrator variable is like a bucket of water, when you add water, the water level rises and when you take out water, the level lowers. In controller the equation becomes: ''output = Pgain*(setpoint-feedback) + Igain*IntegralOf(setpoint-feedback)''.
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 *I-gain - reacts slowly over time, adjusts output until tracking error is zero
-When driving a car, human brain closely resembles the operation of a PI controller.
+When driving a car, human brain closely resembles the operation of a PI controller. For in-depth info about PI controllers and its variants, see the Wikipedia article [http://en.wikipedia.org/wiki/PID_controller PID controller].
-===Low feedback gains - sluggish response===
+===Tuning the gains===
 [[File:Carexample sluggish.png|800px]]
-===Too high feedback gains - oscillation & instability===
+If gains are set too low, the system response tends to be sluggish and sometimes leave a static tracking error (not reaching setpoint in any time). In this imaginary case we could think PI gains to be P=20 I=10.
 [[File:Carexample unstablepng.png|800px]]
-===Optimum feedback gains - only little overshoot===
+If gains are set too high, the system becomes [[overshooting]], oscillating and less stable. Here the comparable PI gains could be P=200 I=100
 [[File:Carexample stable.png|800px]]
 ===Optimum gains with realistic setpoint - optimum response===
 [[File:Carexample response accellimit.png|800px]]
 [[Category:Tuning guide]]