Pseudo-coherent Radar

A requirement for any Doppler radar is cohenrence; that is, some definite phase relationship must exist between the transmitted frequency and the reference frequency, which is used to detect the Doppler shift of the receiver signal. Moving objects are detected by the phase difference between the target signal and background clutter and noise components. Phase detection of this type relies on coherence between the transmitter frequency and the receiver reference frequency.

If the transmitter output stage is a self oscillating device, the pulse to pulse phase is random on transmission. In coherent detection, a stable cw reference oscillator signal, which is locked in phase with the transmitter during each transmitted pulse, is mixed with the echo signal to produce a beat or difference signal. Since the reference oscillator and the transmitter are locked in phase, the echoes are effectively compared with the transmitter in frequency and phase. This phase reference must be maintained from the transmitted pulse to the return pulse picked up by the receiver.

Pseudo-coherent Radar sets are sometimes called: „coherent-on-receive”

Duplexer
The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.

Mixer Stage
The function of the mixer stage is to convert the received rf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 op to 74 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal.

IF-Amplifier
After conversion to the intermediate frequency, the signal is amplified in several IF-amplifier stages. Most of the gain of the receiver is developed in the IF-amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF-stages.

Automatic Frequency Control
As in all superheterodyne receivers, controlling the frequency of the local oscillator keeps the receiver tuned. Since this tuning is critical, some form of automatic frequency control (afc) is essential to avoid constant manual tuning. Automatic frequency control circuits mix an attenuated portion of the transmitted signal with the local oscillator signal to form an IF signal. This signal is applied to a frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity to any change in IF-frequency. If the IF signal is at the discriminator center frequency, no discriminator output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF-signal.

The output of the discriminator provides a control voltage to maintain the local oscillator at the correct frequency.

Stable Local Oscillator
As the receiver is normally a super heterodyne, a stable local oscillator known as the StaLO down converts the signal to intermediate frequency.

Most radar receivers use a 30 up to 74 megahertz intermediate frequency. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000 megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal to the bandwidth of most receivers and would greatly decrease receiver gain.

The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power.

The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000-megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 up to 74 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable.

Phase-sensitive Detector
The IF-signal is passed to a phase sensitive detector (PSD) which converts the signal to base band, while faithfully retaining the full phase and quadrature information of the Doppler signal. This means, the phase-sensitive detector produces a video signal. The amplitude of the video signal is determined by the phase difference between the coho reference signal and the IF echo signals. This phase difference is the same as that between the actual transmitted pulse and its echo. The resultant video signal may be either positive or negative.

Signal Processor
The signal processor is that part of the system which separates targets from clutter on the basis of Doppler content and amplitude characteristics.

Directional Coupler
The directional coupler provides a sample of the transmitter output on every pulse. This signal adjusts the STALO frequency via the AFC but more importantly, it adjusts the phase of the COHO, locking it to the phase reference from the magnetron. The phase synchronization of the COHO by means of a sample of the magnetron output is mandatory because there is no phase correlation between two successive RF pulses of the magnetron.

Mixer Stage
The function of this mixer stage is to convert the sample of the transmitter output to the intermediate frequency. This coho lock pulse synchronize the coho to a fixed phase relationship with the transmitted frequency at each transmitted pulse.

Coherent Oscillator
The second local oscillator known as the coherent oscillator (COHO) enables the down conversion process into the phase sensitive detector, whilst maintaining an accurate phase reference. The coho lock pulse is originated by the transmitted pulse. It is used to synchronize the coho to a fixed phase relationship with the transmitted frequency at each transmitted pulse.

The COHO takes over the phase of the transmitter tube and provides it to the receiver part of the system. This is the reason why the pseudo-coherent radar is also called „coherent on receive”.

Modulator
The oscillator tube of the transmitter is keyed by a high-power dc pulse of energy generated by this separate unit called the modulator.

Disadvantages of the pseudo-coherent radar
The pseudo-coherent radar is a retired one today, but some older radar sets are still operational. The disadvantages of the pseudo-coherent radar can be summarised as follows:

The phase locking process is not as accurate as a fully coherent system, which reduces the MTI Improvement factor.
This technique cannot be applied to a frequency agile radar. Frequency change in a magnetron relies on the mechanical tuning of a cavity and it is essentially a narrow band device.
It is not flexible and cannot easily accommodate changes in the PRF, pulsewidth or other parameters of the transmitted signal. Such changes are straightforward in a fully coherent radar because they can be performed at low level. It is also impossible to perform FM modulation (which is mandatory for a pulse compression radar) with this type of system.
Second time around echoes are returns from large fixed clutter areas located a long distance from the radar. Because they originate from a large distance, such echoes are returned after a second magnetron pulse has been transmitted. However, they pertain to the first pulse transmitted by the magnetron. Such echoes are range ambiguous but, in addition, second time around clutter will not cancel. This is due to the fact that the phase locking of the COHO applies only to the last transmitted pulse.

Microcontrollers

Introduction - It is all Faraday's fault
In the beginning there was an electricity... People lived in an ignorant bliss, having no idea that energy was everywhere around them and that it could be tamed. It was good... And then came Faraday, and the ball started to roll, picking up the speed slowly but surely...

Eventually, first machines using the new energy source were built, and with these came the first people who could "understand the electricity". Time passed on and just about the time civilized world got used to this new concept and let the self-proclaimed experts toy with it, somebody came up with an idea to put electrons in a glass tube. At the first glance, it seemed like a good idea... but there was no turning back as electronics was born and the ball kept on rolling faster...

New science called for new scientists. White collars took the place of blue collars and there came the people who could "understand the electronics". And while the world stared in awe, the conspirators split up in two factions: those who specialized in software and those who specialized in hardware. Younger and more enthusiastic than their teachers, both groups carried on their efforts, but following the different paths. While the first advanced steadily to this day, those dedicated to hardware got carried away by their success and eventually discovered a transistor.

Up to that crucial moment, the situation could have been controlled to a certain extent - but the public underestimated the importance of this breakthrough and made a grave mistake! In a naive attempt to slow down the technological revolution, world market opened itself for a flood of electronic products, thus closing the circle. Market value of components rapidly decreased, electronics making its way to the younger populace. The ball started to fall freely...

Discovery of the integrated circuits and the processors only sped up the things, making the computers as available as ever. Computers started to upgrade themselves. The resulting deflation brought electronics new wave of devotees, making it omnipresent nowadays. Yet another circle closed - computers fell into the hands of the masses and their rule supreme had begun...

While this drama took place, nameless hobbyists and professionals across the world kept working on their projects, although still divided into two large factions.

And then somebody came up with another great idea: why not create an all-purpose component? A cheap and universal integrated circuit that could be programmed and used in any device, anywhere. Technology was up to the task, there was an interested market... why not? Soul and body were united into one powerful MICROCONTROLLER.

What are microcontrollers and what are they for?
As with everything that is good, this powerful component is actually very simple in its essence. It was built using the tested solutions and ingredients by the following recipe:

Processor was removed from the simplest of computers to be used as the "brain" for the upcoming system.

Depending on the manufacturers' taste, some memory was added, a few A/D converters, timers, I/O communication lines, etc.

It was all placed in a standard casing.

Simple software that everybody could learn was developed for controlling the thing.

A variety of microcontrollers has been constructed in this manner, becoming a man's subtle yet indispensable companion in everyday life. Their incredible simplicity and flexibility has earned our trust awhile ago - if you have an idea of utilizing a microcontroller for the most trivial of tasks, know that somebody has already been there...

There are three decisive facts responsible for such a success of microcontrollers:

Their powerful, cleverly chosen electronics is able to control a variety of processes and devices (industrial automatics, voltage, temperature, engines, etc) independently or by means of I/O instruments such as switches, buttons, sensors, LCD screens, relays...

Their low cost makes them suitable for installing in places which attracted no such interest in the past. This is the fact accountable for today's market being swamped with cheap automatons and "intelligent" toys.

Writing and loading a program into microcontroller requires practically no previous schooling. All that is required is: any PC (software is very friendly and intuitive) and one simple device (programmer) for loading a written program into microcontroller.

So if you are into electronics, you will undoubtedly want to master the great potential of microcontrollers and put it to good use.

How does microcontroller work
Although there are plenty of different microcontrollers and programs available, they all share a lot in common between them. This means that if you master one model, you will be able to handle them all eventually.

Typical scenario goes something like this:

Power is off and all is still... Program is loaded, everything is set, with no indications of what is about to take place...

As soon as the power is on, it all happens too quickly to follow! First one to register the change is the control logic. It halts all the circuits except the quartz oscillator. First few milliseconds pass in hurried preparations and loading of parasitic capacitances...

As voltage reaches its maximum, oscillator frequency stabilizes. SFR registers are loaded with bits indicating the states of the subsystems, and all the pins are designated as input. Impulse sequence starts dictating the pace and the electronics is now fully operational. From this point on, time is measured in microseconds and nanoseconds.

Program counter is reset to zero address of the program memory. Instruction is sent from this address to the instruction decoder where it is interpreted and executed promptly.

Program counter is incremented by one as the whole process plays out again... (several millions of times per second)

Obviously, it all works at tremendous speeds, and is also pretty simple. But it would not be especially useful if it was not for additional systems which round out the MCU:

Read Only Memory (ROM)
ROM is the memory which stores the program to be executed. Apparently, its size dictates the maximal program length. It can be internal or external with respect to the microcontroller. Both options have their pros and cons; external memory chip makes the microcontroller cheaper and allows for longer programs. At the same time, number of available free pins is reduced as external ROM uses MCU's I/O ports. Internal ROM is usually smaller and more costly but it provides more possibilities for connecting to the outside world. ROM spans from 512 bytes to 64kB, which is an equivalent to the number of possible program instructions.

Random Access Memory (RAM)
RAM is used for temporary storage of data during runtime. RAM memory does not retain its stored information when the power is interrupted. RAM spans from tens of bytes to several kilobytes.

EEPROM Memory
EEPROM is a special kind of memory not available with all MCUs. Its contents can be changed during runtime (as with RAM), but is also saved after the power is off (as with ROM).

SFR Registers
Special Function Registers are special elements of RAM, their purpose predefined by the manufacturer. Each of these registers is named and controls a certain subsystem of MCU. For example: by writing zeros and ones to the SFR register which controls an I/O port, each of these pins can be designated as input or output (each register bit corresponds to one of the port pins).

Program Counter
This is the "engine" which starts the program and points to the memory address of the instruction to be executed. Immediately upon its execution, value of counter increments by 1. Due to this automatic increase, program executes one instruction at a time, just the way it was written. However... value of a counter can be changed anytime with a "jump" to a new program memory location as a result. This is how routines and branch instructions are carried out. Upon performing a jump to the specified destination, Program Counter increase carries on steadily, +1, +1, +1...

In case you forgot...
Bit is the term often found to be confusing by those starting out in electronics. In practice (and in practice only) bit determines if voltage exists between two points of a circuit. If it does, we say that logic level is 1, i.e. bit is 1. If not, logic level is 0, i.e. bit is 0.

In theory, things are a little bit more complicated - bit actually represents a digit of binary system. As such, it can still have only one of two values - zero or one, set or clear (unlike the decimal system we are used to).

Control Logic
As the name implies, this is the "Big Brother" which supervises and controls every aspect of operations within MCU, and it cannot be manipulated. It comprises several parts, the most important ones including:

Instructions Decoder – part of the electronics which "recognizes" different program instructions and directs the other circuits accordingly.

Arithmetical Logical Unit (ALU) – is in charge of all mathematical and logical operations over data. Capabilities of this circuit are represented through the so called "instruction set" which is different for every MCU.

Accumulator – a special SFR register highly interconnected with ALU. It can be described as a working desk for performing operations on data (adding, shifting, etc). Also, this is the register which holds the result of the operation. Of SFR registers, Status Register is specifically associated with the accumulator. It monitors the status of the accumulator's content at all times (is zero, is greater than zero, etc).

In case you forgot...
Byte is a set of 8 adjacent bits. As bit represents a digit, it is logical to assume that byte represents a number. All standard mathematical operations that can be applied to decimal numbers can be applied to bytes. These are carried out by ALU.

It is important to note that byte has "two sides", i.e. not all positions are of the same significance. The leftmost bit is the Most Significant Bit (MSb). The rightmost bit is the Least Significant Bit (LSb). As there are 256 possible variations of 8 bits, the greatest decimal number byte can represent is 255 (zero is included!) .

A/D Converter
Another convenient instrument not available with all MCUs. Usually there are several separate channels so that multiple analog values can be measured simultaneously. Commonly, converters are 8, 10, or 12-bit and this is sufficient for most tasks.

As shown on the image below, A/D conversion (converting analog value to a numeral) starts with changing the bit in the appropriate SFR register. Hence, voltage on input pin is measured (this sampling takes a few microseconds) and stored in another SFR register as a numeral.

In our example, 8-bit A/D converter is used and the default voltage is 5V. This divides the scale in 256 units, with the smallest voltage range that can be represented of 19.53mV (5V / 256 = 19.53mV). On such a "crude" scale, voltage of 2.204V is represented by numeral 113 (113 x 19.53mV = 2.207V).

Converters of higher resolutions (10 and 12-bit) have much finer scales of 1024 or 4096 units, respectively. Using one of these instruments under the same conditions from the previous example would significantly reduce the inaccuracy (as shown on the image below).

In case you forgot...
Register is a byte physically realized as electronic component, and represents a basic memory cell. Beside the 8 bits available to the user, there is also an address part not readily accessible.

ROM and RAM registers are nameless and are usually peers.

SFR registers are named (differently with different MCUs) and each has its role.

When writing a code for MCU in one of the higher programming languages, each register can be given an arbitrary name, which proved to be useful.

I/O Ports
To be of any practical use, microcontroller needs to be connected to other electronics and to the environs. For this purpose, every MCU has one or more registers (also known as ports) which are connected to the pins on its case. Why I/O? Because every pin can be designated as eiher input or output to suit user's needs.

Oscillator
This is the rhythm section of the miniature orchestra. Stable pace provided by this instrument allows harmonious and synchronous functioning of all other parts of MCU. Commonly, oscillator frequency is stabilized using a quartz-crystal or a ceramic resonator. It can also work without an element for stabilizing frequency (as RC oscillator). Note that instructions are executed at a rate several times slower than the pace dictated by the oscillator. This happens because instructions take several steps to be accomplished (execution period of different instructions may vary on different MCUs). Therefore, if your system uses 20MHz quartz-crystal, execution period of one program instruction will not be 50 ns, but more likely 200, 400 or a "whole" 800 ns.

Timers
Majority of programs utilizes these miniature electronic "stopwatches". Commonly, timers are 8 or 16-bit registers whose value automatically increases upon an impulse. If timer is stimulated by an internal MCU oscillator, it can be used for measuring time between two occurrences (register value at the start of measuring = T1, register value at the end of measuring = T2, elapsed time = T2-T1). If timer is triggered by impulses from an external source, instrument behaves like a counter.

In both cases, filling the register resets the counter and the occurrence is recorded in a particular bit of SFR register (Overflow flag). This makes possible to measure longer time periods. Also, if enabled, this flag can cause an interrupt for breaking the program and carrying out a designated routine (see the image).

Watchdog Timer
The name itself implies the role of this instrument. It is a timer with input connected to an independent MCU RC oscillator. If Watchdog is enabled, MCU is reset every time it overflows, and the program execution starts anew (much as if the power had just been turned on). The idea is to prevent that from happening by means of special instruction. Whole concept is based upon a fact that every program technically "goes around", i.e. executes within a number of simpler or more complex loops. If, beside the regular instructions, key places in the program were added an instruction for resetting the Watchdog, its safeguard function would pass unnoticed. If, for some reason, (in industrial environment, electrical interference is common) Program Counter "gets stuck" at a memory location with no return, the ever-increasing Watchdog Timer eventually overflows and voila - MCU is reset.

In case you forgot...
Interrupt ; electronics is generally way faster than physical processes in the environment it controls. Therefore, microcontroller spends most of its time in expecting an occurrence or waiting for tasks to be accomplished. To avoid the repeated checks of input pins and internal registers, interrupt was introduced. It is a signal which breaks the standard order of program execution. In case of an interrupt, MCU comes to a halt, and determines the source of interrupt. If certain action is required, current value of Program Counter is set aside to Stack and the appropriate routine (interrupt routine) is executed.

Stack is a special part of RAM for storing the current value of Program Counter, so that program could "know" where to pick up after the interrupt routine. It can be multi-leveled, i.e. routines can be executed within other routines.

Power Supply Circuit
Two features of the MCU power supply circuit deserve your attention:

Brown out – this is a potentially dangerous state which occurs during the shutdown or in situations when voltage "oscillates" on the edge of the allowed range due to strong interference. Since microcontroller comprises a number of elements with various voltage ranges, this can cause their uncontrolled behavior. To prevent this from happening, brown out reset circuit is included within MCU. If voltage drops below the specified value for high-voltage elements of MCU, brown out promptly resets the entire electronics.

Reset pin – frequently marked as MCLR (Master Clear Reset) is used for "external" reset of microcontroller by bringing the logical zero or one (depending on the MCU). If not preinstalled, a simple external brown out reset circuit can be connected to this pin.

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Development of an integrated robotic laser welding head - Ir. D. Iakovou

Project Description

The advancement of laser technology allows welding procedures to be implemented with high velocities (up to 250 mm/sec). Industrial robots can be used to perform such high velocity laser welding processes. Still, the positioning accuracy of the industrial robots is insufficient for open-loop (first teach then weld) laser welding. Therefore, additional sensor systems must be used to overcome the positioning accuracy problem by measuring and correcting the position of the welding tool in real time.

Systems already in existence, make use of “of the self” sensor that are mounted on the welding head to perform a certain operation. The addition of separate sensor systems onto the welding head increases its volume and restricts its usability. Therefore there is need for laser welding heads that have sensors integrated in them, and that can perform all the required processes like seam tracking, seam inspection and laser welding process control.

Objectives

The aim of this research project is to develop a prototype welding head with integrated optical sensory systems (optical sensors, structured light, etc) that will enable it to:

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Assist and teach an industrial robot to track a weld joint (butt, corner, overlap, etc.) in 3-dimensional space.

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Correct in real time any positioning errors

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Monitor and control the laser welding process.

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Inspect the welded seam and provide information about the weld surface quality.


The final integrated head must be designed for compactness which will improve the accessibility of complicated products and avoid deterioration of the dynamic performance of the robot.

Experimental Setup

The first prototype laser welding head (Fig.1) was designed to provide the ability to easily mount and test each of the required processes.

The setup consists of:

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a collimator lens system and holder

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a fiber link

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a coaxial mirror with a mirror holder

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a hexagonal shaped main body

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the main body side plates

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a high power laser focus lens holder

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cooling rings

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three adjustable holders for structured light laser diodes


The purpose of the coaxial mirror is to provide a camera system with a coaxial view to the high power laser. The purpose of the mirror holder is to enable the mirror to rotate towards each of the six sides of the main body where sensory systems can be integrated.

The camera with a coaxial view to the welding laser is to monitor the changes of the structured light shape on the seam and the joint plates. It can be easily understood that changes of the orientation or distance of the joint plates result in changes of the shape of the structured light. By processing the images that are captured by the camera it is possible to measure the features of the projected structured light. Thus, it is possible calculate the actual orientation and position of the welding head towards the work piece as well as the position of the seam

The prototype is designed so that the size, shape and position of the structured light are adjustable, because the influence of the process illumination is yet to be determined.

Currently, the laser structured light projectors used are 3mW line projectors. The structured light shape is projected around the tool center point of the laser head.

Sensor-guided robotic laser welding for 3-D seam welding (CRAFT project SenseLasBot) - Ir. M.W. de Graaf

Project background

Current laser welding techniques put high demands on the manipulator that is used. For most applications, accuracies in the order of 100 µm have to be reached at velocities up to 250 mm/s. It is not feasible to program industrial robots that accurate in a production environment. Sensors are needed to increase the positional accuracy and ease-of-use of a robotic welding cell.


OBJECTİVES
The main objective of this research project is the development of a fully automated robotic laser welding cell by integrating a Nd:YAG laser, an industrial 6-axis robot, laser welding optics, an optical seam-tracking sensor and process sensors (Figure 1). Both a commercial seam-tracking sensor (Falldorf GmbH) and the developed integrated welding head of Iakovou will be considered. The system uses sensor information for teaching of the welding trajectory. Furthermore it measures the seam trajectory in real-time during the robot motion and compensates for positioning errors, hereby making robot programming easier and faster.

Seam tracking
An optical seam-tracking sensor is used to accurately measure the seam trajectory in the product to be welded. The commercial sensor uses the principle of optical triangulation (Figure 2) to measure four coordinates (3D position and 1 rotation angle) of the weld joint with respect to its coordinate frame. The integrated welding head of Iakovou can even measure six coordinates (3D position and complete orientation). To be able to use the seam-tracking sensor while the robot is moving, the sensor measurements and the robot joint position measurements have to be synchronized. Algorithms are being developed that can teach an unknown seam trajectory while the robot is moving and track an approximately known seam trajectory during laser welding. For this development and subsequent testing, a simulation environment has been realized, where the effect of non-ideal behavior like sensor errors, robot geometric errors etc can be simulated.

System architecture

The system components are shown in Figure 3. There is a separation between tasks that have to be carried out in real-time (hardware control during welding, seam tracking) and tasks that don’t necessarily have to be in real-time (seam teaching, tool calibration, product frame calibration). The software framework is designed in such a way that it is independent of the hardware used. Different robots and sensors can therefore be used

LASER
A Graphical User Interface, called 24-Laser has been developed. With this GUI the robot can be programmed and welding jobs can be carried out. Furthermore a number of dedicated tasks have been implemented
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Laser and sensor tool calibration

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Manual robot motion control

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Sensor-guided seam-teaching using point-to-point movements

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Real-time seam-tracking

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Product frame calibration



24-Laser communicates with the robot and sensor through commonly used Ethernet-sockets. Only a part of the software, necessary for communication and real-time control of the hardware, is located at the robot controller, making this GUI robot and sensor independent.

Research objectives

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Development of intelligent seam-teaching algorithms for easy robot-programming.

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Development of real-time seam-tracking algorithms for faster robot-programming and to compensate for product clamping errors, etc.

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Development of a Graphical User Interface for work-preparation and job-processing

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Implementation of easy and reliable calibration procedures for the calibration of laser and sensor tool center point

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Integration of the software framework on multiple robot and sensor platforms