Re: [CAD_CAM_EDM_DRO] EDM Plan

millfire2003 wrote:

> Please excuse this post if this is old hat to this forum. I am new
> to this group and have been following the EDM thread with interest.
> I have been slowly but surely building a sinker EDM for light home
> use. The following is what I understand and how I am going about
> building a machine in terms of the electrical aspect.
>
> Basic Concepts
>
> Erosion of metal by electrical discharge. An electrode is placed in
> close proximity to the workpiece, close enough such that the voltage
> withstand capability of the dielectric fluid is exceeded resulting in
> a abrupt, self-sustaining discharge. The dielectric fluid has ideally
> zero (no) conductivity when the electric field strength is less than
> that required to "rip free" electrons and start the discharge. Thus
> the
> use of deionized water or various organic fluid combinations.
>
> The resulting erosion products need to be flushed from the gap and
> removed
> from the dielectric such that the gap is not "shorted" out.
>
> First Order Considerations
>
> One first order practical implementation approach is to charge a
> capacitor
> from a voltage source/power supply and then to apply the charged cap
> across the electrode-workpiece boundary. If the voltage exceeds the
> required breakdown, an arc occurs and the cap is discharged. If the
> gap distance is too great, the electrode is lowered by a control
> mechanism
> and the attempt to discharge repeated.
>
> First Order Implementation
>
> Need one power supply to charge the cap. Research indicates that
> something like 80VDC is in the range.
>
> Need a switch element to temporarily connect the cap to the power
> supply
> to charge the cap. Once charged, this switch element is too be turned
> off, disconnected the "spark" cap from the power supply.
>
> Need a switch element to then connect the cap to the electrode-
> workpiece
> gap.
>
> Need a controller/servo system to control the gap distance as the
> erosion
> takes place.
>
> Practical Implementation
>
> Design an 80VDC power supply (switcher or linear) capable of
> providing sufficient
> *average* current; decouple output of this supply with sufficient
> capacitance with
> low enough series resistance/impedance (ESR) to charge sparking cap
> quickly during
> charge interval.
>
> Use a power FET (or ganged FETs) to serve as the charging switch
> element.
> Use another power FET(s) to serve as switch element to connect cap to
> gap.
>
> Probably the trickiest part: design the circuitry to control the
> FETs. What makes
> this less than easy is the gate drive requirements both in terms of
> speed and the
> fact that the gate drives are not easily ground referenced due to the
> requirements
> of driving the FET gate. There are several design approaches
> including coupling the
> control signals to the gates via pulse transformers or the use of
> several ground
> referenced power supplies.
>
> My approach is to build a small switching power supply (about 40KHZ)
> with low primary to secondary capacitance and the relatively small
> power required to power
> the gate drive circuitry (with separate isolated outputs for both the
> charge FET
> and the discharge FET. The plan
> is to then couple the switch control timing signals into the gate
> drivers via
> high speed optocouplers (6N137 is a candidate). These optocouplers
> can sustain
> 15 kilovolts/microsecond risetimes without suffering transient
> induced false
> logic transitions.
>
> For the actual gate drivers, I
> will be using Maxim (www.maximic.com) parts designed to drive power
> FETs with their attendant
> high (2000pf+) gate capacitance. I could build the drives out of
> discretes
> but why? The resulting gate drive topology is separate, opto
> isolated gate drive
> circuits with isolated, low coupling capacitance transformer powered
> supplies.
> This (hopefully) will minimize coupling of arcing noise into the
> control micros.
>
> The ESR (Equivanlent Series Resistance) of the spark cap should be as
> low
> as is practical. If is is high, it limit both the magnitude and the
> rise time of the
> spark. I will use film caps rather than electrolytics; small valued
> caps in parallel
> with mechanical
> switches that allow selection of the cap value...probably something
> like
> 1 to 100 microfarards. Caps designed for filtering high frequency
> switching
> supplies are ideal for this application.
>
> I will be using two separate PIC 16F877 micros for control; one for
> the cap charge/discharge
> timing cycle (charge, short off period, discharge, short off period,
> charge, etc..),
> the other for the electrode positioning servo (stepper motor). The
> positioning
> servo software will monitor the voltage between the electrode and
> workpiece...if
> high, move closer...if low/zero, too close, back off. These tasks
> are probably
> within the capacity of one 16F877 but at about $8 each, why
> complicate the software?
>

You are monitoring the sparkgap voltage with PIC on board A/D converter?
How you coupling the sparkgap voltage level to controlpanel voltage level?

>
> The pulse timing PIC will use the on-board A/D converter to read
> potientometers
> and use those A/D'd values to adjust the pulse timing. One pot for
> charge
> value, one for charge to discharge off time value, one pot for
> discharge
> value, one for discharge to charge delay. Based on my research,
> something
> like 10 to 40 khz cycle rate should be in the ball park. This
> architecture will allow
> me to tweak the timing while the system is running rather than having
> to
> re-burn the flash code in the PIC. The dead times between
> charge/discharge
> cycles are there to prevent both the charge and discharge switches to
> be on
> simultaoeusly.
>
> Worry about:
>
> 80VDC can cause bodily harm. Minimize wiring distances/worry about
> stray inductance in the
> discharge/electrode wiring to give the faster pulse rise times.
> Careful consideration
> to "grounding" to earth given that the polarity o the arc between the
> electrode
> and workpiece should be reversable as a function of the material
> being cut.
> Arcing creates high risetime current spikes that result in magnetic
> and EMI
> that can disrupt the processor...shielding and careful signal/ground
> layout
> may be required. The servoing software is hoped to be a simple bang-
> bang approach
> but it may need more sophistication due to phase delays in the gap
> sense circuitry.
>
> Or at least that is the plan as it now stands...
>
> Steve S.
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
> Moderator: pentam@... indigo_red@... [Moderators]
> URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
> List Owner
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/