Tuning Servo Motor

Introduction
To paraphrase an adage, there are two types of motion
control engineers, those that are comfortable tuning a
servo loop, and those that aren’t. And if you are one of
those engineers that aren’t comfortable, you in turn, have two
options. The first is to use a non-servo device such as a step motor,
and the second is to get comfortable!
Whether you are a relative novice, or an experienced hand with
servo tuning, this article will help. It provides an overview of
PID (proportional, integral, derivative) based servo loops, and
introduces two standard manual tuning methods that work well
for a large variety of systems. It will also provide an introduction
to the increasingly popular technique of auto-tuning, which, despite
the name, isn’t necessarily as automatic is it may seem. Finally,
we will look at advanced servo techniques such as feedforward
and frequency domain bi-quad filtering.

Using your in-tune-ition
One of the reasons PID compensators are so popular is that it
is easy to conceive of how each term contributes to the overall
output. The D (derivative) term introduces resistance or drag,
the P (proportional) term introduces a linear restoring force,
and the I (integral) introduces a time-dependent windup term.

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Tuning a Servo System
Any closed-loop servo system, whether analog or
digital, will require some tuning. This is the process
of adjusting the characteristics of the servo so that
it follows the input signal as closely as possible.
Why is tuning necessary?

A servo system is error-driven, in other words, there
must be a difference between the input and the
output before the servo will begin moving to reduce
the error. The “gain” of the system determines how
hard the servo tries to reduce the error. A high-gain
system can produce large correcting torques when
the error is very small. A high gain is required if the
output is to follow the input faithfully with minimal
error.

Now a servo motor and its load both have inertia,
which the servo amplifier must accelerate and
decelerate while attempting to follow a change at
the input. The presence of the inertia will tend to
result in over-correction, with the system oscillating
or “ringing” beyond either side of its target (Fig. 3.1).
This ringing must be damped, but too much
damping will cause the response to be sluggish.
When we tune a servo, we are trying to achieve the
fastest response with little or no overshoot.

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Tuning the P.I.D. Loop
There are two primary ways to go about selecting the P.I.D. gains. Either the operator uses a trial and error or an analytical approach. Using a trial and error approach relies significantly on the operator's own prior experience with other servo systems. The one significant downside to this is that there is no physical insight into what the gains mean and there is no way to know if the gains are optimum by any definition. However, for decades this was the approach most commonly used. In fact, it is still used today for low performance systems usually found in process control.

To address the need for an analytical approach, Ziegler and Nichols [1] proposed a method based on their many years of industrial control experience. Although they originally intended their tuning method for use in process control, their technique can be applied to servo control. Their procedure basically boils down to these two steps.
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Servo Motor Motion Profiles - S-Curve

All servo systems consist of some kind of movement of a load. The method in which the load is moved is known as the motion profile. A motion profile can be as simple as a movement from point A to point B on a single axis

The S-curve motion profile allows for a gradual change in acceleration. This helps to reduce or eliminate the problems caused from overshoot, and the result is a great deal less mechanical vibration seen by the system. The minimum acceleration points occur at the beginning and end of the acceleration period, while the maximum acceleration occurs between these two points. This gives a motion profile that is fast and accurate.

advancedmotioncontrols
http://www.advancedmotioncontrols.com

Servo Motor Motion Profiles - Trapezoidal

All servo systems consist of some kind of movement of a load. The method in which the load is moved is known as the motion profile. A motion profile can be as simple as a movement from point A to point B on a single axis

The trapezoidal motion profile slopes the velocity curve to create predictable acceleration and deceleration rates. A trapezoidal motion profile is shown in figure 3. The time to accelerate and decelerate is precise and repeatable. Ta and Td still exist, but they are now specified values instead of random values


- If ta = td = T/3 for a trapezoidal move profile, the overall power used is a minimum
- Overshoot error still exists for a trapezoidal move, but this error is negligible for many systems.
- Higher precision machines require a different motion profile.

advancedmotioncontrols
http://www.advancedmotioncontrols.com/

DC Servo motor Controller with Microcontroller Project

This is an experiment on the closed loop DC servomotor control system (SMC). It will able to be used for practical use with/without some modifications. The closed loop servo mechanism requires real-time servo operations, such as position control, velocity control and torque control. It will be suitable for implementation to any embedded 32 bit RISC processors as a middleware. In this project, these operations are processed with only a cheap 8 bit microcontroller.

Hardware

Figure 1 shows the block diagram for SMC. This is built with only an AVR microcontroller and a PWM mode power amplifire. Whole of servo operation is processed by software implemented servo processor. Any analog component for servo operation is not used.Software

Figure 5 shows the servo operation for SMC. It is multiple feedback configuration which is most popular and fundamental servo mechanism at now. The servo operation which has multiple feedback, is called "Cascaded control". It is a kind of "Advanced servo mechanism". At the cascaded control, one or more control term whoes response is faster than major loop is chosen, and put it into the major control loop as minor control loop, the overall servo performance is increased.
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DSPIC Servo motor Controller Project

This project was developed as an inexpensive way to drive small dc brushed motors as positioning servos for use on a desktop sized CNC machine. The board is interfaced to the PC through 2 pins of a parallel port. The drive signal on these pins is known as quadrature drive. The power stage consists of a power op amp driven in constant current mode. The internal PIC processor ( a 30f4012 from Microchip ) is programmed in C through the C30 compiler and the Microchip IDE. The servo loop parameters are programmed through a serial port connection and are saved in the dspic eeprom. Once set for a particular drive, they should not need to be changed.
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Servo Control of a DC-Brush Motor
The PIC17C42 microcontroller is an excellent choice for
cost-effective servo control in embedded applications.
Due to its Harvard architecture and RISC features, the
PIC17C42 offers excellent computation speed needed
for real-time closed loop servo control. This application
note examines the use of the PIC17C42 as a DC brush
motor servo controller. It is shown that a PID (Proportional,
Integral, Differential) control calculation can be
performed in less than 200 us (@16 MHz) allowing control
loop sample times in the 2 kHz range. Encoder rates
up to 3 MHz are easily handled by the PIC17C42's high
speed peripherals. Further, the on-chip peripherals allow
an absolute minimum cost system to be constructed.

The servo system discussed in this application note
uses a PIC17C42 microcontroller, a programmable
logic device (PLD), and a single-chip H-bridge driver.
Such a system might be used as a positioning controller
in a printer, plotter, or scanner. The low cost of implementing
a servo control system using the PIC17C42
allows this system to compete favorably with stepper
motor systems by offering a number of advantages:
• Increased Acceleration, Velocity
• Improved Efficiency
• Reduced Audible Noise
• True Disturbance Rejection

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PICmicro DC Motor Control Tips ‘n Tricks
INTRODUCTION
Every motor control circuit can be divided into the
drive electronics and the controlling software.
These two pieces can be fairly simple or extremely
complicated depending upon the motor type, the
system requirements and the hardware/software
complexity trade-off. Generally, higher
performance systems require more complicated
hardware. This booklet describes many basic
circuits and software building blocks commonly
used to control motors. The booklet also provides
references to Microchip application notes that
describe many motor control concepts in more
detail.

Content
TIP #1: Brushed DC Motor Drive Circuits ................2
TIP #2: Brushless DC Motor Drive Circuits..............5
TIP #3: Stepper Motor Drive Circuits .......................9
TIP #4: Drive Software...........................................13
TIP #5: Writing a PWM Value to the CCP Registers
with a Mid-range PICmicro® MCU.............17
TIP #6: Current Sensing ........................................19
TIP #7: Position/Speed Sensing ............................23
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AC Servo Motor Control Algorithm

A precise control of AC servo motor using neural
network PID controller
A new control technique based on a neural network, is proposed
here for control of AC servo motors. The PID control is widely
used in servo systems as it has simple structure, safety and
reliability. However, it has certain problems in a complex system,
resulting in imperfect action in the presence of uncertain
parameters. To solve these problems, a new hybrid control
algorithm of the PID controller is proposed, which could
prove the adequacy of the proposed control algorithm through
simulation and experiments after driving the AC
servo motor system using neural network PID controller.

Structure of PID controller using neural network control



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Position Control of an AC Servo Motor Using
VHDL & FPGA

Abstract
In this paper, a new method of controlling position of
AC Servomotor using Field Programmable Gate Array (FPGA).
FPGA controller is used to generate direction and the number of
pulses required to rotate for a given angle. Pulses are sent as a square
wave, the number of pulses determines the angle of rotation and
frequency of square wave determines the speed of rotation. The
proposed control scheme has been realized using XILINX FPGA
SPARTAN XC3S400 and tested using MUMA012PIS model
Alternating Current (AC) servomotor. Experimental results show that
the position of the AC Servo motor can be controlled effectively.

INTRODUCTION
A servo motor is an Electro-mechanical device in which the
electrical input determines the position of the armature of
a motor. The shaft of the servo motor can be positioned to a
specific angle by sending the coded signal. The AC servo
motors have been widely used in the industrial fields and
various approaches have been made to realize high
performance motion control. These can be effectively utilized
in many position control systems subjected to external
disturbances such as friction.
With successively improving reliability and performance of
digital controllers, the digital control techniques have
predominated over other analog counter parts. The advantages
of digital controllers are:


• Reconfigurability
• Power saving options
• Less external passive components
• Less sensitive to temperature variation
• High efficiency

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New Digital Hardware Control Method for High
Performance AC Servo Motor

Abstract:
Today’s motor drives widely use Digital Signal
Processor (DSP) or Microcontroller to
implement the digital control algorithm. Most
recently new requirements have arisen. These
include faster torque control update with flexible
design capability of motion peripherals for high
performance military servo drive applications.
A Complete digital hardware based AC servo
drive development system has been developed to
satisfy increasing demand for performance
enhancement. Based on the FPGA, the system is
configurable for either induction or permanent
magnet machine servo control. The detail design
of complete hardware based high performance
AC servo drive system is discussed.

Control Block Diagram

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FPGA Based Speed Control of AC Servomotor
Using Sinusoidal PWM

Abstract
This paper presents a Xilinx Field Programmable Gate Array
(FPGA) based speed control of AC Servomotor using sinusoidal
PWM technique. Xilinx FPGA is a programmable logic device
developed by Xilinx which is considered as an efficient hardware
for rapid prototyping. It is used to generate 50 Hz sine wave, the
triangular wave and the sinusoidal PWM signals. The sinusoidal
pulse width controls the speed of Motor. The proposed control
scheme has been realized using Xilinx FPGA SPARTAN
XC3S400 and tested using SM115 model Alternating Current
(AC) servomotor. The result provides a controllable speed with
satisfactory dynamic and static performances.

Control Block diagram

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Control and Automation Home

Servo Motion Control

Control

Servo Motion Control - PID Control

Servo Motion Control - PIV Control

DC Servo motor control

AC Servo Motor Control Algorithm

Servo motor control - Feedforward with PIV control

Tuning

Servo Motion Control Tuning the PID Loop

Modeling

MODELING OF A DC SERVOMOTOR

System Modeling - Linear Permanent Magnet Motors

Servo Motion Control Tuning the PID Loop

There are two primary ways to go about selecting the PID gains.
Either the operator uses a trial and error or an analytical approach.
Using a trial and error approach relies significantly on the
operator’s own prior experience with other servo systems. The one
significant downside to this is that there is no physical insight into
what the gains mean and there is no way to know if the gains are
optimum by any definition. However, for decades this was the
approach most commonly used. In fact, it is still used
today for low performance systems usually found in process control.
To address the need for an analytical approach, Ziegler and Nichols
[1] proposed a method based on their many years of industrial
control experience. Although they originally intended their tuning
method for use in process control, their technique can be applied to
servo control. Their procedure basically boils down to these two steps.

Step 1:
Set Ki and Kd to zero. Excite the system with a step command.
Slowly increase Kp until the shaft position begins to oscillate.
At this point, record the value of Kp and set Ko equal to this value.
Record the oscillation frequency, fo.

Step 2:

Set the final PID gains using equation (6).



Loosely speaking, the proportional term affects the overall response

of the system to a position error. The integral term is needed to force
the steady state position error to zero for a constant position
command and the derivative term is needed to provide a damping
action, as the response becomes oscillatory. Unfortunately all three
parameters are inter-related so that by adjusting one parameter will
effect any of a previous parameter adjustments. As an example of
this tuning approach, we investigate the response of a Compumotor
BE342A motor with a generic servo drive and controller.

This servomotor has the following parameters:

Motor Total Inertia J = 50E-6 kgm^2
Motor Damping b = .1E-3 Nm/ (rad/sec)
Torque Constant Kt = .6 Nm/A

We begin with observing the response to a step input command with
no disturbance torque (Td = 0).

Step 1:
Fig. 2a shows the result of slowly increasing only the proportional term.
The system begins to oscillate at approximately .5 Hz (fo = .5Hz) with
Ko of approximately 5E-5 Nm/ rad.

Step 2:

Using these values, the optimum P.I .D. gains according to
Ziegler-Nichols (Z-N) are then (using equation (6)):

Kp = 3.0E-4 Nm/ rad
Ki = 3.0E-4 Nm/ (rad/sec)
Kd = 7.4E-5 Nm/ (rad/sec)

Fig. 2b shows the result of using the Ziegler Nichols gains.
The response is somewhat better than just a straight proportional gain.
As a comparison, other gains were obtained by trial and error. One set
Of additional gains is listed in Fig. 2b. Although the trial and error gains
gave a faster, less oscillatory response, there is no way of telling if a
better solution exits without further exhaustive testing.

One characteristic that is very apparent in Fig.2 is the length of
the settling time. The system using Ziegler Nichols takes about
6 seconds to finally settle making it very difficult to incorporate
into any highperformance motion control application. In contrast,
the trial and error settings gives a quicker settling time, however
no solution was found to completely remove the overshoot.

Source ( pdf )
http://www.compumotor.com/whitepages/ServoFundamentals.pdf

Control and Automation Home

DC Servo motor control

NEURAL ADAPTIVE TACKING CONTROL OF A
LOW SPEED DC SERVO SYSTEM

Hu Hongjie Chen Jingquan Er Lianjie

DC SERVO SYSTEM
The low speed system’s hardware setup is composed of a
permanent dc motor, driving circuit, servo amplifier
(PWM), a mechanical frame as an inertial load, interface
circuit (A/D and D/A), an encoder for position sensing,
and a personal computer (PETIUM I 133) is used as the
programming environment, using Borlandc31 as
programming language for the real-time control
application. Sampling time is defined as 5ms. The block
diagram of the hardware setup is shown in figure

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Two Adaptive Friction Compensation for DC Servomotors

Abstract
Two advanced control strategies of adaptive friction
Compensation For DC servomotor are presented in this paper,
the first is used for The direct on-line friction compensation in
the velocity control system, The second is making use of an
adaptive inverse neural network controller In the position control
system. Both are composed of an adaptive Compensator for
the nonlinear stiction and Coulomp friction in Parallel with a
PID regulator. Experiments show that much improvement
Of performance has attained respect to conventional controller

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Feedforward and IMP Control Applied to a DC Servo Motor

1.0 Introduction
The purpose of this report is to compare feedforward and internal
model principle (IMP) control applied to a DC servo motor.
These control schemes will be tested with known sinusoidal inputs.
The performance of the control schemes will be compared to the
Open loop performance of the system. System identification of
the motor is another task that will be performed.

Feedforward Control
Feedforward control was implemented by inverting (2) to yield:

this gives an overall transfer function of one for the system as
can be seen from figure 3. Even though H(s) is not a proper
transfer function, the control system could be implemented
because the input signal is a known sine wave so the first and
second derivatives can be readily calculated.

Internal Model Principle Control (IMP)
The internal model principle [Control System Design, Goodwin
et. al.] can be used to design a controller when the input to the
system is know and can be modeled in the Laplace domain.

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MODELLING AND CONTROL OF A DC SERVO MOTOR
WITH LABVIEW

OBJECTIVES
This is a hands-on session on the application of computer-based
control to a voltage-controllable electro-mechanical system – the
DC motor. The session is mainly concerned with the modelling
and control of a DC servo motor system, fully instrumented with
position and velocity measurements. National Instrument’s
LabVIEW will be the control software for the experiment. At the
end of the experiment, you should have some experience in

• Simple static and dynamic modelling of the DC motor system,

• Manual and feedback control of the system for velocity tracking

To benefit more fully from this session, students should read the
manual and answer the pre-laboratory questions (Q1-Q3) before
going to the laboratory.

Fig1. DC Servomotor

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Real –Time DC Motor Position Control by Fuzzy Logic
and PID Controllers Using Labview

Abstract
This paper presents the position control of a DC
motor using Fuzzy Logic and PID Control algorithms. Fuzzy
Logic and PID controllers are designed based on labview
program, and the real - time position control of the DC motor
was realized by using DAQ device. The experimental results
demonstrate that the responses of DC motor with FLC show a
satisfactory, well damped control performance.

Fig .3. The block diagram of proposed PID Controller structure

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DC Servomotor Controller
This is an experiment on the closed loop DC servomotor control
system (SMC). It will able to be used for practical use with/without
some modifications. The closed loop servo mechanism requires
real-time servo operations, such as position control, velocity
control and torque control. It will be suitable for implementation
to any embedded 32 bit RISC processors as a middleware. In this
project, these operations are processed with only a cheap 8 bit
microcontroller.

Figure 5. Operation diagram for the SMC (Cascaded control)

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Control and Automation Home

MODELING OF A DC SERVO MOTOR

MODELING OF A DC SERVO MOTOR
Electric motors are the most common actuator used in
electromagnetic systems of all types. They are made in a
variety of configurations and sizes for applications ranging
from activating precision movements to powering diesel-electric
locomotives. The laboratory motors are small servomotors,
which might be used for positioning control applications in a
variety of automated machines. They are DC (direct current)
motors. The armature is driven by an external DC voltage that
produces the motor torque and results in the motor speed. The
armature current produced by the applied voltage interacts with
the permanent magnet field to produce current and motion.
A simplified schematic of the motor is shown in Figure 1 below.

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DC Servo Motor Parameter Estimation
This example demonstrates the process of estimating the
parameters of a multi-domain DC servo motor model constructed
using various physical modeling products.
Contents
1.Description of the DC Servo Motor System
2.Estimating Parameters of the DC Motor Model
3.Importing Experimental Data
4.Selecting Parameters for Estimation
5.Defining an Estimation
6.Running the Estimation
7.Validation
8.Summary

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DC Motor System Identification
Objective The purpose of this lab is to experimentally determine
the frequency response of a DC servomotor system, which
includes the DC motor and amplifier. Experimental results will be
obtained to create a Bode plot for the servomotor system.
A transfer function can also be derived by fitting the Bode plot.
These results then can be used to design a suitable controller.

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