In this assignment, we will be given a task to find a few type of sensor that will be use in Electrical Discharge Machining(EDM) for process monitoring. EDM is a type of advance machining which is use to remove surface material to produce a desire product with specific shape and dimension. EDM can be use to cut harder material of work part such as Titanium and metal tungsten. EDM process is normally use for good surface finishing product because the cutting tool does not touching the workpiece surface as this machining application use spark erosion for material removing process. There are a few types of sensor in EDM to keep the process running with less possible error or to prevent malfunction of the machine before the machine breakdown.Besides, the function of the sensor is use to alert the operator regarding the whole operational process so that it is easier for operator to detect the faulty area to repair. In the following discussion, we have mention about some sensor that is presence in this machine and the specific function of the sensor.
2.0 Explanation of process
2.1 Process for Electrical Discharge Machining. (EDM)
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Figure 1 : EDM machining operation.
Electrical Discharge Machining (EDM) is the thermal or spark erosion process that start disintegration with electrical short that consumes a little opening in a bit of metal it contacts. This basic process is fast and simple. In the process of Electrical Discharge Machining, both the workpiece material and the anode material must be in channels of power which means they must be can conduct electricity. These are the basic steps for the Electrical Discharge Machining (EDM) works:
1.The electrode is formed and shaped by using graphite or copper and it is molded to the type of the pit it is to recreate.
2.The erosion and the electrical is started between the workpiece and the electrode.
3.Two sections are united to inside a small amount of an inch, the electrical strain is released and a start hops over. Where it strikes, the metal is warmed up so much that it dissolves.
4.Countless such starts shower, in a steady progression and step by step shape the coveted frame in the bit of metal, as per the state of the terminal. A few hundred thousand flashes must fly for each prior second disintegration happens.
5.The exist of the electricity will induced a visible spark and these visible spark will produce extreme warmth within them.
6.At the point when this happens, a current flows through an infinitesimally little zone, vaporizing the surface of the anodes.
7.This vaporization isolates some material from the bigger workpiece, leaving a pit at first glance. As various circular segments happen, this setting dissolves the surface over an expansive territory, molding it in the coveted way.
8.At the point when the voltage between the two anodes is expanded, the force of the electric field in the volume between the terminals ends up plainly more noteworthy than the quality of the dielectric, which separates, permitting current to stream between the two cathodes.
9.This marvel is the same as the breakdown of a capacitor. Accordingly, material is expelled from the anodes.
10. Once the present stops, new fluid dielectric is typically passed on into the between terminal volume, empowering the strong particles to be diverted and the protecting properties of the dielectric to be reestablished.
11.Including new fluid dielectric in the between anode volume is ordinarily alluded to as “flushing.”
12. Likewise, after a present stream, the difference of potential between the terminals is reestablished to what it was before the breakdown, so that another fluid dielectric breakdown can happen.
13.The graphite or the copper electrode is encouraged and moved vertically down and the opposite direction of the hole is formed.
14.Computer system or PC is control by operator so that the nonstop moving vertical electrode with the thin needle can cut and erode the ways through the metal to formed a desired workpiece.
2.2 Sinker EDM Process
Figure 2 : Sinker EDM Operation.
The sinker EDM machining prepare utilizes an electrically charged terminal that is designed to a particular geometry to consume the geometry of the anode into a metal segment. The sinker EDM process is usually utilized as a part of the creation of dies and molds.
1.Sinker EDM, likewise called hole sort EDM or volume EDM, comprises of a cathode and workpiece submerged in a protecting fluid, for example, oil or other dielectric liquids.
2.The cathode and workpiece are associated with an appropriate power supply.
3.The power supply produces an electrical potential between the two sections.
4. As the anode approaches the workpiece, dielectric breakdown happens in the liquid, framing a plasma channel and a little start hops.
5.Two metal parts submerged in a protecting fluid are associated with a wellspring of current which is turned on and off consequently relying upon the parameters set on the controller.
6.At the point when the current is exchanged on, an electric pressure is made between the two metal parts.
7.These sparkles as a rule strike one at a time, because it is impossible that diverse areas in the between cathode space have the indistinguishable neighborhood electrical attributes which would empower a start to happen at the same time in every single such area.
8.These flashes occur in gigantic numbers at apparently arbitrary areas between the anode and the workpiece.
9.As the base metal is disintegrated, and the start crevice accordingly expanded, the terminal is brought down consequently by the machine so that the procedure can proceed continuous.
10.A few hundred thousand sparkles happen every second, with the genuine obligation cycle deliberately controlled by the setup parameters.
11.These controlling cycles are now and then known as “on time” and “off time”, which are all the more formally characterized in the literature.
12.The on time setting decides the length or term of the start.
13.Consequently, a more drawn out on time delivers a more profound cavity for that start and every single ensuing flash for that cycle, making a rougher complete on the workpiece.
14.The turn around is valid for a shorter on time.
15.Off time is the timeframe between flashes.
16.A more extended off time, for instance, permits the flushing of dielectric liquid through a spout to get out the dissolved flotsam and jetsam, accordingly maintaining a strategic distance from a short out.
17.These settings can be kept up in microseconds. The run of the mill part geometry is a mind boggling 3D shape, often with little or odd molded edges.
18.Vertical, orbital, vectorial, directional, helical, cone shaped, rotational, turn and ordering machining cycles are likewise utilized.
2.3 Wire EDM Process
Figure 3 : Wire EDM operation.
1.In wire electrical release machining (WEDM), likewise known as wire-cut EDM and wire cutting, a thin single-strand metal wire, usually brass, is nourished through the workpiece, submerged in a tank of dielectric liquid, ordinarily deionized water.
2.Wire-slice EDM is normally used to cut plates as thick as 300mm and to make punches, apparatuses, and bites the dust from hard metals that are hard to machine with different techniques.
3.The wire, which is always bolstered from a spool, is held amongst upper and lower diamond guides.
4.The aides, usually CNC-controlled, move in the x–y plane.
5. On most machines, the upper guide can likewise move autonomously in the z–u–v axis, offering ascend to the capacity to cut decreased and transitioning shapes.
6.The upper guide can control hub developments in x–y–u–v–i–j–k–l–.
7.This permits the wire-slice EDM to be modified to cut exceptionally mind boggling and fragile shapes.
8.The reason that the cutting width is more noteworthy than the width of the wire is on the grounds that starting happens from the sides of the wire to the work piece, bringing about erosion.
9.This “overcut” is essential, for some applications it is enough unsurprising and along these lines can be adjusted
10.The wire-cut process utilizes water as its dielectric liquid, controlling its resistivity and other electrical properties with channels and de-ionizer units.
11.The water flushes the remove flotsam and jetsam from the cutting zone.
12.Flushing is a critical calculate deciding the greatest sustain rate for a given material thickness.
13.Alongside more tightly resiliences, multi hub EDM wire-cutting machining focuses have included components, for example, multi sets out toward cutting two sections in the meantime, controls for forestalling wire breakage, programmed self-threading highlights in the event of wire breakage, and programmable machining techniques to upgrade the operation.
14.Wire-cutting EDM is normally utilized when low remaining anxieties are coveted, in light of the fact that it doesn’t require high cutting powers for expulsion of material.
15.In the event that the control per heartbeat is moderately low, little change in the mechanical properties of a material is required because of these low lingering anxieties, albeit material that hasn’t been stress-assuaged can misshape in the machining procedure.
16.The work piece may experience a critical warm cycle, its seriousness relying upon the mechanical parameters utilized.
17.Such warm cycles may bring about arrangement of a recast layer on the part and lingering malleable weights on the work piece.
18.In the event that machining happens after warmth treatment, dimensional precision won’t be influenced by warmth treat distortion.
2.4 Fluid EDM Process
Figure 4 : Fluid EDM Operation.
1.The correct utilization of EDM dielectric liquids is typically a compelling strategy for controlling the airborne era of beryllium containing particles.
2.Care ought to be given to liquid control and to counteract sprinkling onto the floor territories or administrators’ garments.
3.The reusing of EDM liquids containing finely isolated beryllium particles in suspension can bring about the development to a point where particles may end up noticeably airborne amid utilize.
4.EDM dielectric liquids ought to be separated or changed consistently to lessen the gathering of beryllium-containing particulate.
3.0 List of Sensor use in the process.
a)Tool Wear Sensor.
b)Current Sensor.
c)Wear Debris Sensor.
d)Sensor Monitoring System.
e)Sensor Signal Data Processing.
f)Safety Sensor.
4.0 Explanation of it’s functions use in Electrical Discharge Machining.
4.1 Tool Wear Sensor.
The process of metal removing involve in Electrical Discharge Machining are thermal erosion and usually electrodes will be the tools that eroded in the process. The work piece will be connected to anode and electrode will connect to cathode. Tool wear that happen on the cathode does not occur in constant rates during the discharge process, however in the first few hundred nanoseconds during the spark ignition are the period where the tool wear happen the most frequently. In order for us to track for the tool wear, a sensor can be use to identify a real time evaluation depends on the power density. This thickness is represented by the development speed of the plasma channel and is corresponded with the voltage fall time.
4.2 Current Sensor.
More attention are considering considering current sensor due to high-frequency of electrical discharge currents.The current sensor that commonly used for bearing-current measurement that include shunt resistor, oscilloscope current probe and Rogowski coil. This sensor requires reasonably high bandwidth and its its affects the initial electrical conditions in bearing currents of the circuit has to reduce as low as possible.It is necessarry that stray capacitances and inductances of the resistor are as low as possible and known.Shaft voltage was also measured.Bearing currents were measured by using the shunt resistor. Parameters of bearing-currents circuit between motor shaft and motor frame were estimated by using the simplex algorithm.Inputs for that estimation were measured shaft voltages and bearing currents. Measurement and analysis of bearing-currents’ trend line were required to carry out during half-hour test and bearing endangerment was estimated.
4.2.1 Shunt resistor
Shunt resistor can be called as Current shunt resistors. Sometimes called an ammeter shunt, it is a type of current sensor. This resistors are low in resistance accuracy resistors. Resistor used to measure Alternative current or direct current electrical currents by the dropping of voltage and those currents create across the resistance.
Figure 5 : Shunt Resistor
4.2.2 Oscilloscope probe
Oscilloscope is essential tool for fault finding for electronics development, diagnostics work or repair. The oscilloscope enables the waveforms for various parts of the circuit to be shown in a graphical format. Oscilloscope probes are required to enable the oscilloscope to connect to the required points.
In forming a simple oscilloscope probe, it is possible to use a signal line and earth return connection. This connection does not achieve the optimum performance in term of both electrical and mechanical aspects. It is necessary to consider both aspects to meet requirements.There are a variety of scope probes can be bought and used. Luckily, there is a high degree of inter-changeability between scopes and scope probes. It is necessary to understand which types to use, and what the scope probe specifications may be when choosing the correct type to use in particular application.
Figure 6 : Oscilloscope Probe
4.2.3 Rogowski coil
Rogowski coil is an electrical device that can be used to measure alternating current or high-speed current pulses. It consists of a helical coil of wire with the lead from one end returning through the centre of the coil to the other end. Both terminals are at the same end of the coil. The whole assembly is then wrapped around the straight conductor whose current is to be measured. There is no metal (iron) core. The winding density, the diameter of the coil and the rigidity of the winding are critical for preserving immunity to external fields and low sensitivity to the positioning of the measured conductor.
The voltage that is induced in the coil is proportional to the rate of change (derivative) of current in the straight conductor. The output of the Rogowski coil is usually connected to an electrical (or electronic) integrator circuit to provide an output signal that is proportional to the current. Single-chip signal processors with built-in analog to digital converters are often used for this purpose.
Figure 7 : Rogowski coil.
4.3 Wear Debris Sensor.
Wear debris and jetsam incorporates ferrous, non-ferrous, and clay trash. Among wear flotsam and jetsam, ferrous trash is the biggest segment in light of the fact that numerous machine parts are made of steel. In this way, it is a vital data bearer for machine’s wear condition. Gill Sensors has built up a ferrous garbage sensor, which has been used in checking the wellbeing of gearbox and transmission frameworks of land-based vehicles, to distinguish the ferrous garbage in greasing up oil. The sensor’s structure is appeared in Fig. 8. It comprises of two indistinguishable sensor units; every unit has an inductance detecting loop and a changeless magnet for gathering ferrous garbage to the sensor tip. The two units, one for distinguishing fine ferrous flotsam and jetsam develop (Sensor 1 in Fig. 8) and the other for distinguishing substantial ferrous flotsam and jetsam (Sensor 2 in Fig. 8), can decide the measure of ferrous garbage develop in light of the detecting loop’s inductance change. Moreover, as a result of the lasting magnet, this sensor additionally keeps flotsam and jetsam from proceeding to travel through the oil flow framework, diminishing further harm. Be that as it may, this sensor depends on the mass estimation of a lot of ferrous trash pulled in to the sensor tip over a timeframe. Along these lines, it can’t give continuous data about individual ferrous garbage particles’ size and focus. Also, it can’t distinguish the non-ferrous flotsam and jetsam, which is likewise basic to judge the state of machines with the nonferrous covering.
Figure 8 : Schematic of the debris sensors
4.4 Sensor monitoring system
Sensor monitoring of WEDM processes was performed during an experimental testing campaign of WEDM surfacing processes carried out on steel plate workpieces with a height of 20 mm. The WEDM tests were performed on a GF Agie Charmilles FI 440 ccS CNC wire EDM machine. A brass wire electrode (AC Brass 900) with a diameter of 0.25 mm and a resistance of 900 N/mm2 was employed.
The sensor signal acquisition was carried out only during the surfacing phase, i.e. the last phase of the workpiece machining process which in total involves three phases: roughing, trimming and surfacing.
The sensor monitoring system adopted for signal acquisition during WEDM surfacing consists of three sensor: two current sensors (Pearson Current Monitor Model 6585) to acquire the upper and lower head current signals and one voltage probe to acquire the voltage signal.
High frequency (up to 200MHz) is needed so current sensors were chosen since the considered WEDM surfacing process is characterized by high frequency (71 kHz). In addition to the voltage and current signals, the instantaneous position of the wire during WEDM was acquired. The CNC machine control was adapted to adjust to the bit of the machine position and send it to a 12 bit D/A Converter (Texas Instruments TLV5638) to obtain an analog signal of the wire position. As regards the data acquisition system, in order to achieve a high resolution in the sparks characterisation, a National Instruments board (NI PXIe-1082) with 4 BNC inputs was employed to acquire the data with a very high sampling rate of 50 MHz. The acquisition of the wire position signal together with the currents and voltage signals was carried out to support the search for correlations between workpiece surface defects and WEDM conditions.
While defects such as lines and marks can be clearly localized on the workpiece surface, their localization within the voltage and current signals is hard to achieve, since the machining speed is not perfectly constant due to the initial acceleration. Therefore, once the position of a defect on the workpiece surface is defined, it is possible to go back to the corresponding signals of voltage and currents by using the synchronized wire position signal.
Figure 9 : Circuit Block Diagram of sensor monitoring system.
4.5 Sensor signal data processing
The function of this sensor is to inspect the sensory data of interest, the parts of the currents and voltage signals corresponding to a defect need to be identified and segmented.
Figure 10 : Sensor signal data processing.
4.6 Safety sensor
This safety sensor is used to stop the machinery immediately when detecting an entry or presence of a person during machine operation. It is an important sensor to prevent hazard to the operator while the machining process is running. Once the door is detected to be open, the machining process will be stop.
4.7 Presence Sensor
Presence sensor is use to detects the presence of a person and can stops the machine until the person escapes from the hazardous area
5.0 Principle of sensor operations.
5.1 Tool Wear Sensor.
To obtain a sensitive tool wear sensor, a classification of voltage signal is recorded in between the gap of tool and workpiece by observing the normal sparks, arcs, short circuits and open circuits. The voltage drop time is well coupled with the initial discharged current and the tool wear sensor can be measure based on the voltage fall time on each of the discharged process. Besides, the workpiece wear faster when the frequency of the sparks occurs to breakdown. The tool wear sensor functions are based on the voltage fall time which relate to the electrical current density. Although the tool wear rates are at relatively slow speed, the sensor still can be function.
5.2 Sensor signal data processing.
ALICONA instrument is used to identify the physical position of the defect, however due to the both error which are ALICONA measurement errors and the wire position signal error, a further analyse on a wider portion of the wire position signal is decided to carry out in order to identify a signal feature well related to the presence of defect. By observing the ALICONA topography of the surfaces with defects with the observation of the position signal plot, each defect corresponds to a modification of the wire position signal. By analysing the modified voltage signal portion corresponding to the position signal, it is noted that the voltage drop in this portion indicates that a short circuit occurs at the processing time. Thus, sensor signal data processing able to precisely identify the portions of current and voltage signals of interest for surface defects analysis. However, the correlation between the occurrence of a short circuit and the presence of a defect on the final surface is not so simple: as a matter of fact, the number of short circuits in each test was higher than the number of defects measured on the final surface. This means that the sole presence of short circuit does not determine a defect on the surface.
Figure 11 : ALICONA measurement error.
5.3 Safety Sensor.
Safety Light Curtain is used to detects if operators entering hazard zone by light beams and immediately stops the machine before they are harmed. Safety area sensors use a combination of hardware and software to check constantly for internal faults to ensure safe operation.
The following section describes the faults and malfunctions the safety light curtain detects to ensure safety.
Figure 12 : Safety light curtain to ensure the safety of operator.
5.4 Presence Sensor
Basic safety is broadly classified into the two categories which are machines and equipment will not start until it is safe to do so and will be stopped whenever a hazardous condition is detected. In order to maintain the safe environment and to detect operators entering or present in a hazardous area, measures must be employed on one level. A device that will detect operators must be installed in a protected area if an operator can pass through an opening and enter that protected area to perform his job. Safety distance refers to the minimum calculated distance that the protective device must be installed from the hazard of the machine so when an operator enters a hazardous area, the machine in the area must come to a complete stop before that operator reaches the hazard of the machine.
Figure 13 : Presence Sensor of safety light will be turn on.
5.5 Current Sensor.
Figure 14(A) : Often the shunt is placed in the grounded side to eliminate the common mode voltage. However, other disadvantages exist.
Figure 15 (B) : In this configuration, the common mode voltage could be too high for the ammeter.
The position of the shunt resistor is important in the circuit. When the circuit shares a common ground with the measurement device, often is chosen to place the shunt as close to the ground as possible in order to protect the ammeter from the common mode voltage that might be too high and damage the device or give erroneous results. From this set up, a disadvantage is that leakages that bypass the shunt might not be detected. It must be isolated from the ground or include a voltage divider or an isolation amplifier to protect the instrument in case the shunt is placed in the ungrounded leg. Other ways are possible to not connect the measurement instrument directly with the high voltage circuit, such as using the Hall Effect.
6.0 Wiring Diagram
Figure 15: Wiring Diagram of EDM
6.1 Functional Safety Requirements
(1) The muting function is a temporary interruption of the safety (protection) function that must be actuated and de-initiated consequently.
(2) The safety category of the device, or of the part of the circuit that implements the muting function, must be the same as that of the safety function that disables so as not to compromise the protection of the entire system.
(3) Activation and subsequent de-activation of the muting function must be achieved using two or more hard-wired and independent signals activated by a proper time or space sequence.
(4) It must not be possible to actuated the muting function when the Electro-Sensitive Protection Equipment (ESPE) has the safety outputs de-actuated.
6.2 Circuit Status
The light curtain is configured with the factory default settings and it is based on the machine (EDM) and is unobstructed. The output of the safety relay are open. The motor is off and always ready to run.
6.3 Operating Principle
Starting : Press the Reset button to close the output of the MSR127. Press the Start button to energize contactor K1 and K2. The motor start with the two normally open contacts of K1 and K2 holding the circuit energized.
STOPPING : Obstructing the light curtain de-energizes the safety outputs of the MSR127, which in turn drops out K1 and K2. The contactors disconnect the motor from its power source, and the motor coasts to a stop. Clearing the obstruction in the light curtain does not cause the motor to energize (the Reset and Start buttons must be pressed). The motor can also be turned off by pressing the stop button.
7.0 Troubleshooting
7.1 Tool wear sensor
Monitoring of the tool wear involves in a wide set of physical magnitude of physical nature and a large number of studies have related them to the tool wear process. Sensor that contains time series are usually processed by computer using specific software. Feature extraction is carried out by selecting the appropriate functional transformation and quatifying parameters. The another approach is based on heuristic rules generated by expert observer. The generation and selection of variables and rules requires an expert. For multi-sensor weaar extimation, an analytical model may be too complex and diffcult to establish. A rule-based empirical inference method requires an expert familiar with relational mechanisms and also including the ability to translate those into simple inference rules.
7.2 Current sensor
7.2.1 Oscilloscope probe
It can usually be modeled as a Thevenin equivalent voltage source with capacitance and internal source resistance, and the oscilloscope’s input circuits and interconnections can be modeled as a load resistance with shunt capacitance. The loading effects of an oscilloscope reduce the voltage measured while the a source connected to the instrument.
This loss depends on the ratios of the resistor values at low frequencies. The capacitive resistance and the source resistance are the primary determinant of loss at higher frequencies.Reducting of system bandwidth is another issues resulting from the capacitive loading of the oscilloscope, which affects dynamic timing measurements like pulse risetime.
There are several ways to mitigate these effects. Using high impedance oscilloscope probes that use both active and passive circuits which minimize the effects of loading by way of compensated attenuators or low-capacitance field effect transistor (FET) buffer amplifiers. Another way by utilizing input circuits with fifty ohm internal terminations for direct connections in high frequency situations. In this condition, the majority of circuits are designed for constant load impedance. Low capacitance oscilloscope probes mitigate capacitive loading effects.
7.3 Safety sensor
7.3.1 Checking Safety Sensor Alignment
If there’s nothing blocking the safety sensors, check their alignment. You may have bumped a sensor out of position. Each safety sensor has an indicator light. The sending sensor, which has the yellow light, transmits the infrared beam to the receiving sensor, which has a green light. The yellow sending sensor light should always be lit. But you will only see the receiving sensor’s green light when the sensors are aligned and unobstructed. Make sure that the yellow light is on and then check the green light on the receiving sensor.
7.4 Find Error Code on Motor Unit
If the green light is off, realign the safety sensors until the green light turns on. If one or both sensor lights won’t come on, check the LED troubleshooting light on the motor unit for an error code. The control inside the motor unit flashes the troubleshooting LED a number of times to indicate the cause of a failure.
7.5 Repair Broken or Damaged Wiring
If you see one of the first two error codes, check the wiring between your motor unit and the sensors for visible damage. You won’t be able to check all the wiring if it’s routed through the walls to the sensors. Check the wires that you can see and repair any broken or damaged wiring.
8.0 Conclusion
In this assignment, we have learn the process and the function of the sensor of Electrical Discharge Machining(EDM). There is a few of process in EDM which include sinker EDM process, wire EDM process and fluid EDM process. We had know that Electrical Discharge Machining (EDM) is the thermal or spark erosion process that start disintegration with electrical short that consumes a little opening in a bit of metal it contacts. This basic process is fast and simple. In the process of Electrical Discharge Machining, both the workpiece material and the anode material must be in channels of power which means they must be can conduct electricity. Besides that, there is few sensor which include wear debris sensor, sensor monitoring system, sensor signal data processing, safety sensor, presence sensor, tool wear sensors and current sensor. The sensor will make the EDM works smoothly and without malfunction. The wiring diagram shows the function and how the sensor works.
