THC-A2 Automatic Torch Height Control for Mechanized Plasma ...

THC-A2 Automatic Torch Height Control for Mechanized Plasma Cutting
Introduction
Torch to Work Distance or standoff during the cutting process is adjusted by monitoring
and controlling the arc voltage. The arc voltage is the same as the power supply output
voltage. Plasma power supplies are a current source-they generate a very steady
operating current when the torch is cutting. Voltage, on the other hand varies depending
on the distance between the cathode (the electrode in the torch) and the anode (the
material being cut). Voltage is directly proportional to resistance (Ohms law V=I*R) the
resistance in the arc is a function of the distance. When the torch to work piece distance
increases the voltage goes up, and when the torch to work piece distance decreases
the arc voltage goes down.
The automatic torch height control uses arc voltage to maintain a consistent distance
from the plate while the torch is cutting. This allows the system to maintain proper torch
height regardless of variations in the material, or flatness of the cutting bed. The
operator sets the arc voltage on the remote control according to the cut charts in the
plasma systems operation manual. This voltage setting is usually between 100 and 200
VDC.
After the torch height control has completed initial height sensing (IHS ¹) and the torch
has pierced the plate, motion of the cutting machine is enabled and the torch begins to
move. Once the cutting begins, the THC starts sampling arc voltage from the power
supply and comparing it to the target voltage set by the operator. It adjust the torch up
or down to maintain the target voltage.
Each voltage setting corresponds to a specific height that optimizes the arc
characteristics for a clean, straight cut. Torch height has the greatest affect on bevel
angle of the plasma cut piece. Setting the voltage to high level, that results in more
material being removed from the top of the kerf than the bottom. This causes excessive
top rounding and positive bevel. Setting the voltage too low results in too much material
being removed from the bottom of plate. This causes undercutting or negative bevel.
Torch height controls have many features and capabilities. The most important is to
maintain a consistent distance from the plate while the torch is cutting. Moving the torch
up, for a safe traveling is also important. Piercing at the right height and collision control
will save you money on torch tips and torch damages.
IHS ¹ is an adjustable parameter that determines the height at which the torch will fire and begin piercing
through the plate.
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Elements of the THC-A2
1. Stepper motor driven torch lifter can be adapted to most machines fairly easily. This
device moves the torch up and down in response to signals from the control console.
Belt driven mechanism are used to translate motion. It has a robust, rigid and dustproof
construction.
Picture 1. Torch lifter.
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2. Control console and remote control box, receives input commands, sends output
commands to the torch lift station and monitors position feedback, voltage signals and
other messages. This small box, could be mounted on the table or on the wall.
ATENTTION! Don’t cover fan on the bottom!
Picture 2a. Control console and remote control box.
button 1 –Manually up;
button 2 –Manually down and set start mode;
button 3 –Pierce height set and manually “freeze” torch;
knob 5 – arc voltage set knob.
3. Interface card (voltage divider, 1 to 50), „divides“ and filter the arc voltage signal and
sends a smaller signal voltage to the control console. All connections are made trough
screw terminal blocks also in interface card. Best place to install interface card is inside
of the plasma power supply.
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Picture 3. Interface card, voltage divider.
4. Connector cables. 12m and 3m long cable to connect elements of THC-A2
Picture 5. Cables
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Features of the THC-A2
- Arc Voltage Control, 50-280 volts, fully automatic, continuously making fine
adjustments throughout the cutting process.
- Max. 200 mm active stroke (vertical torch travel).
- Speed of retract exteeds 150 mm/sec
- Max. adjustment speed 30 mm/sec.
- Max. load 3,5 kg.
- Weight 2 kg.
- Supply voltage 24 VDC (Max. 2A)
- Built-in Ohmic touch-retract initial height sensing (IHS ¹) feature ensures accurate
repeatable initial piercing height settings.
- Pierce height setting from 1 to 25 mm, in 1 mm increments.
- Built-in stall IHS feature is always active and serves as a back-up for the Ohmic IHS.
When cutting non-conductive material surfaces, such as painted plate or underwater
cutting.
- At the beginning of cutting start make arc voltage controlled adjustments after first
“freeze” (first X-Y acceleration) or after delay 0,7 sec (in case when CNC control don’t
have “edge freeze output).
-Collision control. Breakaway is designed to reduce or eliminate damage to the torch
due to accidental collision.
- Upper and lower travel-limit stops. Upper could be set (O-ring on the belt).
- Compatible with most plasma power supplies, with steady operating current and witch
all machine torches. Also compatible with fine plasma.
- Independent of the systems electronics, and only require a computer controlled output
in minimal hook up..
- Manually up and down buttons,
- HV/HF ignition resistant. Controller is 100% galvanic isolated (including analogue arc
voltage signal).
- Dustproof, simple, robust and rigid mechanic allows for easy maintenance and repair.
- THC-A2 units are easy to install.
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Installation and setup of ROBOTORCH THC
CAUTION: Portion of this install may include opening your plasma power supply and
attaching wires. MAKE SURE THE UNIT IS UNPLUGGED prior to removing any cower
or making any connections. Plasma units have HIGH VOLTAGES present that can be
dangerous or lethal. IF YOU ARE NOT EXPERIENCED WORKING WITH HIGH
VOLTAGES DO NOT ATTEMPT TO INSTALL THIS OR ANY OTHER DEWICE
INSIDE YOUR PLASMA UNIT YOURSELF. SEEK PROFFESSIONAL HELP.
1. Install Interface card (voltage divider). Best place to install interface card is inside of
the plasma power supply. The connections are made trough the screw terminal blocks.
Picture 6. Terminal block
Block number Will be connected
1 and 2 (CNC) input to computer controlled output of CNC controller, such
as the one that turns plasma torch ON and OFF.
3 and 4 (PL) output to plasma trigger switch
5 and 6 and 7 (COL) output to CNC controller. 5 and 6 are normally open, 6 and 7
normally closed. Will be switch if horizontally collision
happens.
8 (OS) to torch tip (ohmic touch-retract initial height sensing). NOT
tip voltage! Ohmic touch-retract initial height sensing
functioning if 12 is connected to.
9 and 10 (FR ) edge “freeze” input. Stops making adjustments to avoid drop
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the torch if cutting speed will be slow down. (Need edge
“freeze” output in CNC controller).
11 (-) to ( - ) The Tip Voltage (Arc voltage). About 300 VDC open
circuit.
12 (+) to ( + ) The plasma machine chassis.
DB9 to THC-A2 control console
2. Install the control console and remote control box. It could be in the table or mounted
in the wall. Don’t cover cooling holes on the bottom! The control console had 2 DB9
connectors. One of them must be connected to the interface card (voltage divider) and
other to the torch lift station.
Connect power supply. It works with 100-240 VAC and transform it to 24 VDC.
3. Mount the torch lift station with 4 screws and connect (DB9 connector) it with cable
(shipped with) to the control console. Mount the torch. If torch is mounted like in picture
13:
- set upper limit (move small rubber O-ring in the belt up or down).
-collision control switch must by push down, otherwise THC don’t work and in remote
control you will see “COLLISION ”
Final setup and testing
1. Switch power ON. (Don’t switch plasma power supply ON )
2. Torch must move up to the limit and stop.
3. In LCD you will see Picture 7.
4. Now you could set arc voltage and pierce height.
5. If CNC controller relay connect screws 1 and 2 in terminal block, torch move
down and little bit up to the pierce height. Check, haw heavy it pushes down by
sensing blade surface. If ohmic sensor is connected properly it will sense it very
soft. This test is reasonable to do with thin plate
6. In LCD you will see Picture 10. Torch will move up to the limit, and down again.
7. Switch plasma power supply ON.
8. If you start cut now, you will see arc voltage picture 8 in the LCD. If you will push
pierce height set button now, torch will be freeze and no torch height adjustments
occurs.
9. If a vertically collision happens, the torch will jump up to the pierce height and
continuing cutting. In LCD you will see Picture 11.
10. If a horizontally collision happens, plasma and CNC controller will be switch OFF
and all stops. In LCD you will see Picture 12.
11. Place torch in a right place, turn THC power OFF, wait a second and turn it ON.
Everything is OK again.
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Picture 7a.Power ON and torch up. Start torch height
adjustments after first “freeze” (AFF). Pierce height 8
mm.
Picture 7b.Power ON and torch up. Start torch height
adjustments after delay 0,7 sek (DEL). Pierce height
8 mm.
Picture 7c.Power ON and torch up. After delay
0,7 sec (DEL) stepper motor will be disconnected and
cutting with mechanical surface follower is possible.
Picture 8a. Cutting
Picture 8b.Cutting with mechanical surface follower
Picture 9. Cutting and no torch height adjustment
(edge “freeze feature”).
Picture 10. Cutting, no plasma arc
Picture 11. Vertical collision during cutting
Picture 12. Horizontal collision, THC and motion stop.
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