Control 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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Control servo motor control-Velocity Profiling

A velocity profile is a graph of the velocity of a motor vs. time. The area inside the curve that the velocity profile creates is the distance traveled. Velocity profiling is useful for applications where specific velocities are necessary at specific times. Two typical velocity profiles are shown in the following figures.

These two figures are both examples of velocity profiles that can be implemented using the FlexMotion hardware and software. In the first example, the motor simply accelerates to a target velocity at a specified acceleration, runs at the target velocity, and then decelerates after a certain amount of time. In the second example, the motor accelerates to a certain velocity, runs at that target velocity for a period of time, accelerates to a higher velocity, then travels at that velocity for a period of time, and then decelerates to zero.

National Instruments - Fundamentals of Motion Control
http://zone.ni.com/devzone/cda/tut/p/id/3367

Creation of velocity profile using s-curves

In this paper an approach is proposed for velocity profile control of an AC motor. The dynamic control algorithms for calculation and estimation of the S-curve profile adapt in real time to variations in system behavior to improve their performance.

The S-curve velocity profile is similar to trapezoidal, and in this case, trapezium sides are replaced by S-curves, which enables smoother velocity transitions in acceleration and deceleration periods [1, 9].

The first order trapezoidal velocity profile is a typical point-to-point move. An
axis accelerates from rest to a given velocity at a constant rate. Then traverses, or slews, to a certain point where it decelerates at a constant rate until finally, the end position is reached and the axis will come to a rest. Sometimes the slew velocity and the end position can be changed on the fly. The S-curve velocity profile can be represented as a second-order polynomial in velocity. We have an extra term here – jerk (jerk is a derivative of acceleration and a measure of impact). The second order S-curve provides complete flexibility in the control of profiles for smoothing motion and eliminating jerk from mechanical systems. The degree of S-curve on a motion
profile is controlled by separate acceleration and deceleration smoothing (jerk-limit) factors.

Fig. S-curve profile with symmetrical acceleration and deceleration periods

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Control Types of Stepper Motors

Stepper motors come in two varieties: permanent magnet and variable reluctance. (The reader may be familiar with hybrid motors, which are indistinguishable from permanent magnet motors from the controller's point of view.) Permanent magnet motors usually have two independent windings, with or without center taps. Center-tapped windings are used in uni polar permanent magnet motors. This can you see in the figure (1).


Bipolar permanent magnet and hybrid motors are constructed with a mechanism similar to that used in uni polar motor, except that the two windings are wired without center taps in the Figure 2. The motor itself is simpler, but the drive circuitry needed to reverse the polarity of each pair of motor poles is more complex.

Stepper motors come in a wide range of angular resolutions. The coarsest motors typically turn 90 degrees per step, whereas high resolution permanent-magnet motors can commonly handle 1.8 or even 0.72 degrees per step. With the appropriate controller, most permanent magnet and hybrid motors can be run in half steps, and some controllers can handle smaller fractional steps or micro steps. For permanent magnet and variable-reluctance stepper motors, when one winding of the motor is energized, the rotor (under no load) snaps to a fixed angle. It holds that angle until the torque exceeds the holding torque of the motor, at which point the rotor turns, trying to hold at each successive equilibrium point.

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Control A Digital Thermometer Using AT89C2051

The system presented in this application note implements a simple digital thermometer that includes a built-in LCD and RS-485 communicate ion Port. It is designed around Atmel’s AT89C2051 processor, a digital thermometer/thermostat from Dallas Semiconductor, a small 8 X 2 LED backlit LCD, and an RS485 line interface. The system, shown in Figure, can be used as the basis for developing custom solutions for networked and stand alone data collection and control equipment. It can be centrally powered due to its low current requirement and its small size allows it to be placed almost anywhere.


Temperature acquisition is handled using the digital thermometer/thermostat IC from Dallas Semiconductor. The digital contains all temperature measurement and signal conditioning circuitry on-chip and presents the processor with a 3-wire digital interface composed of a bi-directional data line DQ, a reset input \RST, and a clock input CLK. The temperature reading is provided in a 9 bit, two’s complement format. The measurement range spans from -55°C to +125°C in .5°C increments. Data transfers into and out of the DS1620 are initiated by driving \RST high. Once the DS1620’s reset is released, a series of clock pulses is emitted by the processor to actually transfer the data. For transmission to the DS1620, data must be valid during the rising edge of the clock pulse. Data bits received by the processor are output on the falling edge of the clock and remain valid through the rising edge. Taking the clock high results in DQ assuming a high impedance state. The sequence can be immediately terminated by pulling \RST low which forces DQ into a high impedance state and concludes the transfer. Temperature data is transmitted over the 3-wire bus in lsb first format. A total of nine bits are transmitted where the most significant bit is the sign bit. If all nine bits are not of interest, the transfer can be terminated at any time by asserting \RST.

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Control USB Protocol as implementation for microcontroller

USB provides a serial bus standard for connecting devices. USB 1.1, though USB 2.0 is more prominent. The host detects addition and loads the appropriate driver if installed. There are two types of connectors;
- Type A.
- Type B.

Type A sockets found on hosts and hubs. It is always upstream. Type B plugs are found on devices. It is always downstream. A USB device is detected by the host, if any of the lines D+ or D- are pulled up through a resistor. A full speed device pulls up the D+ pin through a resistor to 3.5V. A low speed device pulls up the D- pin through a resistor to 3.5V. The figure will explain the statement. Figure (1);
There are three classes of USB functions;
- Low-power bus powered functions – It is draw the power from the V bus and cannot draw any more than one unit load.
- High-power bus powered functions - It is draw the power from the bus and cannot draw more than one unit load until it has been configured
- Self-powered functions -These may draw up to 1unit load from the bus and derive the rest of its power from an external source.

For all schematic circuit is shows the figure;

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Control Precision Temperature Sensor with LM335

The LM335 series are precision, easily-calibrated, integrated circuit temperature sensors. Operating as a 2-terminal zener, the LM335 has a breakdown voltage directly proportional to absolute temperature at +10 mV/°K. With less than 1W dynamic impedance the device operates over a current range of 400 μA to 5 mA with virtually no change in performance. When calibrated at 25°C the LM135 has typically less than 1°C error over a 100°C temperature range. Unlike other sensors the LM135 has a linear output.

The LM335 operates from −40°C to +100°C. The LM135/LM235/LM335 are available packaged in hermetic TO-46 transistor packages while the LM335 is also available in plastic TO-92 packages.

The schematic basic figure for Temperature Sensing is;

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Control Digital clock as Implementation of microcontroller

Electronic clocks have predominately replaced the mechanical clocks. They are much reliable, accurate, maintenance free and portable. In general, there are two kinds of electronic clocks. They are analog clock and digital clock. But digital clocks are more common and independent of external source. It would be needed the controlled devices and implementation of software for microcontroller control system because the hardware devices cannot do any desired task to execute. In this paper, the microcontroller-based digital clock is constructed with PIC16F877A and its software program is written with CCS C program language. Various types of digital clocks and modules are available in the market nowadays but this clock is different at least in the accurate time. To be controlling in microcontroller is only the feature of the clock. The input frequency is taken from the 50 Hz clock frequency circuit. To show the time, seven - segment Light Emitting Diodes (LEDs) and four LEDs are used.

Time is such a fundamental concept that it is very difficult to define. To measure time is needed something that will repeat itself at regular intervals. The number of intervals counted gives a quantitative measure of the duration.

In the software implementation process, initialization processing, LED display processing, time adjustment processing and time signal processing are considered. In this article, the microcontroller-based digital clock is mainly controlled by the clock pulse frequency. The clock pulse frequency can be generated by using the IC1 555. The clock pulse frequency can be obtained from other methods such as the power line frequency and the internal oscillator IC with RC circuit and so on. The power line frequency will not get more accuracy than the quartz crystal. The 555-timer unstable mode can be used for this purpose. In the display, there are needed to give the outputs of seconds, minutes and hours and AM/PM. In this display system, the output of PIC is connected with the input of decoder (CD4028) to drive the seven-segment LEDs. The decoder (CD4028) has four inputs and ten outputs. But, in this circuit three inputs and six outputs are used. So, one input pin is grounded and four outputs pins are not used. The circuit is shown by the figure;
The microcontroller-based digital clock can be provided with the date, month and year circuits. The output of day indicator can be shown by connecting with the output pins of CD4028, pin seven and eight. But the required instructions are added to the existing program. If the month indicator is wanted to show, the remaining input pin, pin D is connected with the output pin of PIC, RA3 and then to drive the seven-segment LEDs output pins of CD4028, pin zero and nine are used.

For more information, you can access;
PROCEEDINGS OF WORLD ACADEMY OF SCIENCE

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