Servo Motor Control with AdndroiDAQ

Servo motors are common motion control actuators that are used in robotics, radio controlled devices, 3D printers, and other motion drive systems. Using servo motors with AndroiDAQ is easy. This article contains a brief history of the servo motor, some basic operational background for servo motors, and how to connect and use a servo motor with the AndroiDAQ data acquisition and control module.

The steam engine governor is considered to be the first servo mechanism using a powered feedback system. Its first recorded use was in 1868, by JJL Farcot, who described steam engines and hydraulics for use in steering a ship. Servo mechanisms are considered to be closed-loop systems, as the mechanism uses position feedback to control its motion and final position.

Solution

Use PWM to control the width of pulses to a servo motor to change its angle. Although this will work, the PWM generated is not completely stable, so there will be a little bit of jitter with the servo.

You should also power the servo from a separate 5V power supply because peaks in the load current are likely to crash or overload the Raspberry Pi.

To make this recipe, you will need:

    5V servo motor (see “Miscellaneous”)

    Breadboard and jumper wires (see “Prototyping Equipment”)

    1kΩ resistor (see “Resistors and Capacitors”)

    5V 1A power supply or 4.8V battery pack (see “Miscellaneous”)

The breadboard layout for this is shown in Figure 5-1.

The 1kΩ resistor is not essential, but it does protect the GPIO pin from unexpectedly high currents in the control signal, which could occur if a fault developed on the servo.

The leads of the servo may not be the same as the colors indicated in Figure 5-2. It is common for the 5V wire to be red, the ground brown, and the control lead orange.

You can, if you prefer, power the servo from a battery pack rather than a power supply. Using a four-cell AA battery holder with rechargeable batteries will provide around 4.8V and work well with a servo. Using four alkali AA cells to provide 6V will be fine for many servos, but check the datasheet of your servo to make sure it is OK with 6V.

The user interface for setting the angle of the servo is based on the gui_slider.py program intended for controlling the brightness of an LED (“Controlling the Brightness of an LED”). However, you can modify it so that the slider sets the angle, between 0 and 180 degrees (Figure 5-2).

Open an editor (nano or IDLE) and paste in the following code. As with all the program examples in this book, you can also download the program from the Code section of the Raspberry Pi Cookbook website, where it is called servo.py.

Note that this program uses a graphical user interface, so you cannot run it from SSH.

You must run it from the windowing environment on the Pi itself or via remote control using VNC (“Controlling the Pi Remotely with VNC”). You also need to run it as superuser, so run it with the command sudo python servo.py:

from Tkinter import *
import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 100)
pwm.start(5)

class App:

    def __init__(self, master):
        frame = Frame(master)
        frame.pack()
        scale = Scale(frame, from_=0, to=180,
              orient=HORIZONTAL, command=self.update)
        scale.grid(row=0)


    def update(self, angle):
        duty = float(angle) / 10.0 + 2.5
        pwm.ChangeDutyCycle(duty)

root = Tk()
root.wm_title('Servo Control')
app = App(root)
root.geometry("200x50+0+0")
root.mainloop()

A Servo Motor consists of three major parts: a motor, a controller circuit, and a feedback system, which usually consists of a potentiometer which is connected to the motor’s output shaft. The motor typically drives a set of gears, which turns the output shaft of the motor and the potentiometer simultaneously. The potentiometer’s measured resistance controls the output angle of the motor shaft. This resistance is fed into the servo controller circuit and when the controller circuit detects that the motor position is correct, it stops the servo motor. If the controller circuit detects that the angle is not correct, for whatever the motor it trying to control, it will turn the servo motor the correct direction until the angle is correct. Normally a servo motor is used to control an angular motion of between 0 and 180 degrees. They typically can not turn any further unless they are modified, due to a mechanical stop which is build onto the main output gear or the potentiometer itself. Servo motors are generally used as a high performance alternative to the stepper motor.

Servo motors are controlled by sending to the servo’s control wire a pulse train of variable width, or better, a pulse width modulated signal. This pulse train has to have specific parameters such as a minimum pulse, a maximum pulse, and a repetition rate. This signal is typically derived and sent to the servo control wire by a microcontroller such as the one on AndroiDAQ. The shaft angle of the servo is determined by the duration of the pulse train sent. Given the pulse train’s constraints of having a minimum pulse, a maximum pulse, and a repetition rate, a neutral position is defined. This neutral position is where the servo has exactly the same amount of potential rotation in the clockwise direction as it does in the counterclockwise direction. This neutral position is typically always around 1.5 milliseconds.

Stepper Motor in Cllosed Loop Mode

Due to the limitations of open loop contorl, a closed loop control of stepper motors is used in practice. In a closed loop control, the input controller gets the information about the output through the feedback element. Hence the driver circuit receives the control signal which is based on the feedback information. So switching of the motor takes place by means of train of input pulses, which is generated on the basis of feedback from rotor. Such a switching of the motor is called as closed loop mode of operation of the stepper motor. The block diagram shown in the Fig.A.17 illustrates the closed loop operation of the stepper motor.

Let us consider a closed loop temperature control system. The temperature of the tank is required to be kept constant with the help of controlling the steam flow. There exists a valve whose position is controlled by a stepper motor, to control the steam flow. The actual temperature is sensed by using temperature sensor and feedback is given to the input controller. The input contorller has the reference information corresponding to the desired ideal temperature. It compare the feedback with this reference to generate the appropriate contorl signal. This control signal inturn is given to the driver circuit. The driver circuit control the excitation and logical sequence of the excitation of the phases. This drives the stepper motor and hence valve opening gets controlled appropriately so that steam flow gets controlled. This maintains the temperature of the tank constant. The logical operation of the system is illustrated in the block diagram shown in the Fig. A.18.

In a speed and position control systems, optical encoders coupled to the rotor shaft are used. Simlarly use of microprocessors as an input controller for better accuracy is very commen now a days. The Fig. A.19 shows the use of microprocessor in the closed loop control of the stepper motor.

What are the functionalities of these closed-loop stepper systems?

Closed-loop stepper with step-loss compensation will be the most typical kind of closed-loop stepper manage. The stepper drive operates as a micro-stepping drive and usually receives pulse and path commands to move towards the preferred position. An encoder tracks shaft or load position. If lost actions are detected, a compensation algorithm inserts extra actions to ensure that the motor shaft (or load) arrives in the preferred position. Usually, the stepper-motor drive has settings for two currents: The motor gets operating present when in motion and gets resting present when stopped.

In closed-loop stepper with load-position manage, the stepper drive operates as a typical microstepping drive and usually receives pulse and path commands to move towards the preferred position. The encoder (usually mounted around the load) monitors the load's position. The closed-loop algorithm dynamically tracks the load position and compensates all through the move profile. Usually, the motor gets operating present when in motion and gets resting present when stopped.

Closed-loop stepper servo manage treats the stepper motor like a high-pole-count brushless motor, turning it into a servomotor. A shaft-mounted encoder detects shaft position to figure out the correct present vector. A pulse and path interface might be provided within this kind of drive, however the position controller does not use actions to obtain towards the preferred position. Rather, closed-loop algorithms manage motor torque to servo the shaft into position utilizing a position manage loop (a PID loop for instance). Within this mode, the present setting is dynamic. The stepper drive delivers only the quantity of present required to move the motor shaft and load into position.

Do you have the right VFD for Your Application

Choosing the right variable frequency drive for an application involves several important considerations. For example, based on acceleration requirements, sensorless vector control may be more suitable than volts-per-hertz (V/f) control. While V/f control is effective in dragging logs up a slope, it’s not appropriate for dockside hoists that position 12-ton shipping containers to within inches.

What is your load type: constant or variable torque?

To get a constant-torque load, the torque is independent of speed (ignoring momentary shock loads). Examples consist of conveyors and hoists. To get a variable-torque load, torque varies as a function of speed. Examples consist of fans and pumps. This main distinction underlies each choice you will make concerning the kind of drive.

What are your acceleration requirements?

Does it matter how fast your load accelerates up to speed? For a fan, probably not. For a centrifuge, almost certainly. In the latter case, you may want to select sensorless vector control, rather than volts-per-hertz (V/f) control. While the V/f approach is effective for many applications, it doesn’t allow a motor to develop near-full torque at near-zero speeds (unlike sensorless vector control). V/f control can be appropriate for dragging logs up a slope, but not for a dockside hoist that needs to position a 12-ton shipping container to within inches.
Controlled deceleration presents its own challenges.

Throughout decelerations, the motor acts as a generator. The resulting power must go someplace, and is usually dissipated as heat inside a braking resistor. Controlled-deceleration capability is really a great answer for constant-torque loads, altering loads, or perhaps unbalanced loads.

What is your speed range?

Although a conveyor belt may operate consistently at 60 Hz, for an unspooling module on a printing line, the motor needs to deliver torque as effectively at 0.5 Hz as 60 Hz. This is another application where garden-variety V/f control won’t do the job. Sensorless vector control will (and most VFDs these days include it). Keep in mind, however, that not all offerings are created equal. Be sure to double check specifications against your requirements.

How do I select the proper size VFD?

For most applications, select a VFD that has an output current rating that equals or exceeds the nameplate current rating of the motor. Do not just select the VFD based on the motor Hp rating, as this can result in a VFD that is too small. Some severe applications, such as punch presses, vibratory conveyors, mixers, etc, may require oversizing the VFD in order to handle the peak current demands of the load. Please contact AC Tech's Application Engineering Department for more information.

VFDs have a wide variety of programmable functions. Not all manufacturers call these functions by the same name. Research is required to properly utilize a VFD to its best performance. We are fortunate to live in an age where a great wealth of information is available on the Internet. Utilizing manufacturers’ websites and educational information, plus a little time, can make most any controls person an expert in VFD selection.

How Do VFDs Save Energy and Money

VFDs are developed to supply variable speed manage. They preserve the optimal speed needed for an application to improve production and save power. Low- and medium-voltage drives steadily accelerate and decelerate motors and pumps, assisting shield mechanical elements and extend their life, whilst decreasing inrush currents, which assists save power. VFDs are utilized inside a broad selection of industries and applications?aincluding HVAC, water and wastewater, and oil and gas.

VFDs are a critical component of motor speed control. They improve efficiency, reduce wear on mechanical components and improve system performance. Fundamentally, they are used to control the frequency and voltage supply to the motor and match the application’s speed requirements.

All VFDs are going to take 3 phase AC power and convert that 3 phase power to DC power inside the drive and pulse it out in a simulated AC wave form to the motor.  The motor still thinks it has AC power but the DC power conversion now lets us control the speed of the motor without harming it.  Now we are in control to save energy and money.

Variable frequency drives are widely used in many areas such as fan, pump, compressor, elevator, tower crane, CNC, paper making machine and more. There are many benefits of applying frequency inverter:

    Saving energy: when your production don't need the motor to run at full speed, then you can use a variable frequency drive to adjust the motor running speed, so energy can be saved during the process.

    Improving the control accuracy: in variable torque or tension control applications, the production needs variable toruqe or tension, such as paper-making industry, textile industry and etc. By using a frequency inverter, it can help users to control the torque or tension accurately according to the application requirements. Therefore, the production efficiency can be greatly reduced.

    Reducing the maintenance cost of your equipments: when your production control system is without a frequency inverter, the motor will start instantly with large current which is harmful to your mechanical equipments. A frequency inverter which is developed with motor soft-start function can prevent this phenomena, so the equipment wear will be reduced and the equipment maintenance cost will be decreased also.

How much can you save using a VFD?

In these centrifugal pool pumps, (according to energy conservation experts) energy consumption is proportional to the cube of the flow rate. In plain English that means that even small reductions in speed and flow can result in significant energy savings. In pool applications, significant energy and cost savings can be achieved by reducing the operating speed when the filters are clean, or when the pool is experiencing off-peak loads.

How great are the savings?

A motor running at 50% of full speed capacity has a motor torque of 25% of full speed. In addition, electricity required to operate the motor at 50% of full speed is 12.5% of the amount of electricity required if the motor was running at 100% full speed capacity. Thus, reducing motor speed can significantly reduce the electrical energy consumption.

Plenty of applications which will save a lot of power by installing a VFD (Main, secondary air, exhaust/flue gas extractor fan, cooling pumps for certain, motors utilized for automatic adjustment of parameters). Anyway, in case from the plant/motor is utilized for energy balance regulation when the load is most likely to increase/decrease much more often, you are able to set up a VFD for saving energies. In case your plant is operating below nominal speed a lengthy time, the power saving using the motor is really a minor issue for you personally. The large loss will come from other points like electrical energy consumption, motor efficiency, upkeep and so on. Fitting a VFD for such motor won't assist as well a lot.

How to Select PLC for Motion Control

Almost every modern industry uses programmable logic controllers, commonly known as PLCs. These compact electronic devices enable much of the automation that is prevalent in today’s manufacturing processes. PLCs are utilized to get a myriad of purposes in company and business, such as every thing from climate manage systems to complex item assembly and packaging processes.

Buyers unfamiliar with PLCs and their benefits may feel overwhelmed as they begin the process of selecting and purchasing Delta PLC for their business or industry. This guide will offer a short overview from the history and function of PLCs, such as the programming specifications and a few important elements to help keep in thoughts whilst buying for PLCs.

The factors to consider when choosing a PLC:
I/O
Servos
Size of Memory
Communications
Compatibility to HMI
Format
Speed

Format of PLC

You can go away using the format of Little PLC Omron simply because the count of I/O is much less than one hundred and also you don't have servos or analog to compete with; although, from a view of studying point and becoming that Medium Format of PLCs would be the business function horse you'll make use of a Medium Format PLC having a genuine of back-plain.

Speed of PLC

The speed of PLC uses to be a main concern when planning controls but does not truly affect to controls now. Nowadays PLCs are rapid enough for the majority applications with full programs scan times usually less than 4 msec. This is the scan time needed for the PLC to seem at the service and program all the Inputs/Outputs. The Cycle Time is 4 seconds so the majorities PLCs today are glowing when think this requirement of time.

Size matters - Sizing a PLC is crucial towards the achievement of one's project. As well little and also you might max out your I/O on modifications and additions. As well big and also you might blow your spending budget. Be sure you leave space for expansion but do not break the bank.

Count up your:

Discrete input points
Discrete output points
Analog input points
Analog output points

Communications. ALWAYS have a port available on the PLC to communicate with it from your laptop without disconnecting other devices.  With modern PLCs with multiple communications methods, there is no reason for this to happen.

        Will you need remote I/O?  This can reduce installation time and troubleshooting in the long term.
        Will you system utilize an HMI?  How will you communicate with it?
        Will you have a way to remotely monitor it from either the office across the plant or across the country?  It is becoming a standard practice.  Personally I like Ethernet but there are many options.

Brand - I'll be unpopular when I say this, but a PLC is a PLC when it comes to most capabilities.All of the major players such as Allen Bradley, Automation Direct, Mitsubishi,Omron, and Siemens have small, medium, and large scale PLCs. Always consider when brands the end user already is using. Things will always go smoother if the maintenance personel are already familiar with the brand of PLC you choose. Also try to choose a brand that will have good local  support for the end user if you are not in the area.

Conclusion

Whether buyers are looking for a replacement PLC for an existing system or are interested in automating a new process, they will likely be able to find PLCs that are appropriate for their needs on Fasttobuy. Fasttobuy’s user-friendly policies and search options allow buyers to enjoy a secure and profitable shopping experience. With their new PLCs, buyers will be able to implement time saving automation technologies for their business or industry.

The Brief Introduction of DC Servo Motor

What is the meaning of servo?

In modern usage the term servo or servo-mechanism is restricted to a feedback control system in which the controlled variable is mechanical position or time derivatives of position such as velocity and acceleration.

A servo is a device, electrical, mechanical or electro mechanical, that upon receipt of a stimulus or input, will employ feedback for velocity and/or position control, creating a closed loop.

A feedback system is a control system which tends to maintain a prescribed relationship between a controlled quantity and a reference quantity by comparing their functions and using the difference as a means of control

There are mainly two types of servo-motors

1)AC Servo motor
2)DC Servo-motor

AC servo-motors are generally preferred for low-power use. And for high-power use DC servo motors are preferred because they operate more efficiently than comparable to AC servo motors.

Unlike large industrial motors, dc servomotors are not used for continuous energy conversion. The basic operating principle is same as other electromagnetic motors.

Construction:

1.It has construction as same as dc motor. It is consist of stator and rotor and controlling parts.2.It has feedback generator for generating feedback for controlling the speed & torque.
3.It has two ports one for dc supply and other for controlled dc supply.
Field Controlled DC Servo Motor Theory:

The figure below illustrates the schematic diagram for a field controlled DC servo motor. In this arrangement the field of DC motor is excited be the amplified error signal and armature winding is energized by a constant current source.

The field is controlled below the knee point of magnetizing saturation curve. At that portion of the curve the mmf linearly varies with excitation current. That means torque developed in the DC motor is directly proportional to the field current below the knee point of magnetizing saturation curve.

Armature Controlled DC Servo Motor Theory:

The figure below shows the schematic diagram for an armature controlled DC servo motor. Here the armature is energized by amplified error signal and field is excited by a constant current source.

The field is operated at well beyond the knee point of magnetizing saturation curve. In this portion of the curve, for huge change in magnetizing current, there is very small change in mmf in the motor field. This makes the servo motor is less sensitive to change in field current. Actually for armature controlled DC servo motor, we do not want that, the motor should response to any change ofPerformance Specifications:

DC servomotors share many performance specifications that are applicable to all types of?DC motors. To properly size a motor, these specifications must be matched according to the load requirements of the application.

Shaft speed (RPM) defines the speed at which the shaft rotates, expressed in rotations per minute (RPM). Typically, the speed provided by the manufacturer is the no-load speed of the output shaft, or the speed at which the motor's output torque is zero.

Terminal voltage refers to the design voltage of the DC motor. Essentially the voltage determines the speed of a DC motor,and speed is controlled by raising or lowering the voltage supplied to the motor.

Torque is the rotational force generated by the motor shaft.The torque required for the motor is determined by the speed-torquecharacteristics of thevarious loads experienced in the targetapplication.

Starting torque - The torque required when starting the motor,which istypically higher than the continuous torque.
Continuous torque - The output torque capability of the motor under constant running conditions.

Some ratings of dc servo-motor available:

Shaft Speed:

Less than 1,610 rpm
1,610 to 3,187 rpm
3,187 to 4,700 rpm
4,700 to 7,090 rpm
7,090 rpm and up

Terminal Voltage:

Less than 20 VDC
20 to 50 VDC
50 to 100 VDC
100 to 180 VDC
180 VDC and up

Continuous Current: 

Less than 1 amps
1 to 4 amps 4 to 8 amps
8 to 17 amps
17 amps and up

Continuous Torque:
Less than 0.45 Nm
0.45 Nm to 1.70 Nm
1.70 Nm to 5 Nm
5 Nm to 17 Nm
17 Nm and up

Continuous Output Power:

Less than 0.4 HP
0.4 to 1 HP 1 to 2 HP
2 to 6 HP
6 HP and up

Advantages:

• High output power relative to motor size and weight.Encoder determines accuracy and resolution.
• High efficiency. It can approach 90% at light loads.High torque to inertia ratio.It can rapidly accelerate loads.
• Has "reserve" power. 2-3 times continuous power for short periods.
• Has "reserve" torque. 5-10 times rated torque for short periods.
• Motor stays cool. Current draw proportional to load.
• Usable high speed torque.
• Maintains rated torque to 90% of NL RPM
• Audibly quiet at high speeds.Resonance and vibration free operation.

Disadvantages:

• Requires "tuning" to stabilize feedback loop.Motor "runs away" when something breaks. Safety circuits are required.
• Complex. Requires encoder.Brush wear out limits life to 2,000 hrs. Service is then required.
• Peak torque is limited to a 1% duty cycle.Motor can be damaged by sustained overload.
• Bewildering choice of motors, encoders, and servo-drives.
• Power supply current 10 times average to use peak torque. 
• Motor develops peak power at higher speeds. Gearing often required.
• Poor motor cooling. Ventilated motors are easily contaminated.

Applications:

DC servomotors finds its applications in various domain. Some of them are given below:

• For very high voltage power systems, dc motors are preferred because they operate more efficiently than comparable ac servomotor.
• It has also find its application in inkjet printers and RC helicopters.
• To drive conveyors used in Industrial manufacturing and assembling units to pass an object from one assembly station to another.
•  It is also used in solar tracking system.DC servomotors are widely used in robots, toy cars and other position controlled devices.
• Widely used in radars, computers, robots, machine tools tracking system, process controllers etc.

Fasttobuy co.,ltd is a professional and experienced company which specializes in producing and researching,located in changzhou city,jiangsu province, china. Our main products are stepper motors and drivers,servo motor and drivers,gear motor and other automatic device, applied widely in printing equipment, engraving machine textile machine, computer external application equipment, medical instruments, stage light equipment, robot, CNC machine and other automatic controlling system.  Exported to United States, Canada, Germany, United Kingdom, France, Switzerland, Italy, Russia, Korea and so on more than ten countries and regions in the world.

Variable Frequency Drive Sizing

Have anyone done much vfd sizing for motors? I know you have to take torque and and horsepower into consideration alot. It appears most of the vfds I have seen tend to have larger conductors feeding it than one would calculate for a motor load without a vfd. Maybe its because the vfd is more oversized, or they used 150% NEC 2008 430.6(C). from the motor tables unlike only 125% sizing for conductors to the motor. If you want to buy VFD, you can go to our offical website: Fasttobuy.com. We provide We carry various VFD manufacturers from Mitsubishi to Delta VFD to provide you with a brand you know and trust. We supply both single-phase and three-phase drives in various voltages to help meet the specifications of your current system.

Any one have an instance where they had to size the vfd? Was the feed to the vfd allowed to be smaller than it would be for the motors itself? How did you size it? 

When a single VFD is utilized to manage numerous motors, VFD sizing and choice turn out to be much more complicated unless all the motors are began simultaneously. With numerous motors connected to 1 VFD, adding the horsepower of every to acquire a total load and choosing the VFD accordingly might not be adequate based on operating circumstances.

1 of much more motors cannot be began up whilst 1 or much more motors are currently operating unless the chosen drive is sufficiently oversized. To illustrate this point, think about the following instance with 3 460 VAC motors connected to 1 VFD.

Two from the motors are rated at five HP having a complete load amp (FLA) rating of six.two amps. The third motor is rated at ten HP with an FLA of 14 amps. If all motors are accelerated, decelerated and run in unison-the sum from the connected motor FLA enables use of a 20 HP drive. But if it had been essential to accelerate and run the five HP motors and after that begin the ten HP, the sum from the FLA would need to be recalculated.

The FLA for every from the five HP motors could be utilized within the calculations, however the locked rotor amps (LRA) for the ten HP would need to be taken into account. The LRA will be the quantity of present drawn by a motor at startup.

Simply because the ten HP motor wouldn’t be accelerated from zero frequency and voltage to its operating situation, it would appear in the drive as a fixed voltage/frequency line starter and would need its complete LRA rating to rapidly accelerate towards the drive’s output frequency.

Therefore, the amp draw around the drive when the ten HP drive is coming on line could be the FLA of every from the five HP motors, plus the LRA of ten HP motor that is 86.five amps. The total amp figure to become utilized for VFD sizing is therefore six.two FLA + six.two FLA + 86.five LRA = 98.9 Amps. This amp load would need upsizing to a 75HP VFD having a minimum continuous output rating of 99 Amps, a 50% improve in VFD size.

Final Recommendation

With modern power electronics and advanced microprocessor technology, Delta's AC Motor Drives are able to efficiently control motor speed, improve machine automation and save energy. Taking advantage of our strong position in power electronics technology, Delta's VFD Series of AC motor Drives has evolved rapidly. Each Drive series is designed to meet specific application needs. Our AC Drives accurately control speed and torque, smoothly handle an increased load, and provide numerous custom control and configuration operating modes. Our AC Motor Drive product line provides a full range of motor control technologies and is used throughout a wide range of industries, to enhance and improve machine automation. 

This guide is intended to become of use for common application sizing and isn't intended to become a complete guide. You will find applications and loads that might need unique sizing and consideration. When you're sizing or specifying a VFD for any application it pays to become conservative and leave some buffer space within the FLA and overload ratings. This really is particularly accurate in case your load is difficult to begin or sees heavy loading throughout operation. When you have any concerns about your application or in sizing a brand new VFD contact and speak to 1 of our application specialists before buying.