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|Sources of commercial controllers|
The purpose of a controller is to regulate the temperature
of a heating device either at a fixed point or as a controlled ramping change from one
temperature to another over minutes or hours. Most glass studios
have several controllers for the heat supplied to annealers,
color kilns, and furnaces and these may be combined in one electronic unit or be
separated. A controller may include safety
features required by either the electrical code (NEC) or one of several fire codes
and thus cost more.
Choices (and history) in Temperature Controllers for Glass By Mike Firth
Rev. 3/13/97, 2/18/98, 2000-8-20
Controlling temperature for melting glass and annealing it has used a number of devices historically and has seen great improvement in the last few years. Electric melt and annealing control is relatively easy, but limited in quantity and maximum temperature; while adding control to gas furnaces jumps the cost. One ongoing problem is the need to accurately control the fall of temperature in the annealer over many hours.
Temperature control can take two forms, open loop and closed loop. In an open loop, a device controls the heater, thus raising or lowering the temperature, but the temperature does not affect the device. In a closed loop, the measurement of the temperature modifies the behavior of the device in controlling the heat. A resistance control, dimmer or Variac (variable transformer) is open loop; modern controllers are closed loop.
To quickly summarize the following, open loop devices were first used, then as microcomputer technology began in the 70's, fairly expensive controllers that used closed loops were available. Then the cost started coming down and a series of multi-step keypad programmed controllers, such as Digitry, came on the market. Most recently, controllers that "learn" the characteristics of the furnace or annealer and adapt for better control have come on the market. Some of these have computer ports so that control can be maintained on a computer screen. And now we are seeing multiple ramping units at relatively low cost (under $200.)
In glass working, the chief open loop devices have been the food cooking oven controller, variations on dimmers, and the variable transformer (Variac, a brand name.) In each case, if a roughly constant temperature is wanted, the human adjusts a knob until it is about right. Most retail dimmers do not have anything like enough capacity to control an annealer; but a dimmer can control a heavier triac or it is not difficult to build a 25 amp dimmer (or bigger.)* Oven controls used are variable resistors, like on home ovens, which are easily available and have the capacity but waste energy and are not very sensitive.
The Variac has a venerable history in modern glass blowing because at the time the modern art movement started, there were a lot of large capacity Variacs available as military surplus and triacs and electronic dimmers did not yet exist. A Variac is two coils of wire, looking somewhat like a motor with a knob on the shaft, which is adjusted to change the voltage from 0 to maximum available. They are efficient. They are also heavy and if not available surplus they are also expensive and have a limited capacity. Variacs are important because early in the history of modern glassblowing, they allowed annealing to take place over many hours when hand control would be impossible because they were built into a open loop automatic control as follows.
The Variac was mounted on a board with a 24 hour timer whose purpose was to provide a very slow motor. On the shafts of the timer and transformer were mounted disks or segments around which string was run from one to the other. As the timer dial turned, the Variac dial was dragged along, slowly lowering the voltage. Some people used cam shaped disks, so the Variac moved more slowly during the first hours and faster during the later ones. Some of these are still in use, I saw one at John Littleton and Kate Vogel's studio in 1995. As the voltage fell, the power fell as the square of the change and the temperature fell accordingly. But the temperature curve was only monitored by a human and did not affect the timing of the process. (Image shows Harvey Littleton's Variac controller from his book.)
Recently, exactly the same thing can and has been done for a few dollars and it is one better than Variac. Peet Robison in New Mexico built a controller in which comparator chip compared the amplified voltage from a thermocouple to the output from an digital to analog converter (DAC) turning on a triac controlling the heating element when the thermo voltage was lower than the DAC. The DAC setting was controlled by a small counter that changed every few minutes. The total parts cost is about $8 for building this. Note that it is closed loop, because the temperature reading influences the comparator, but what is being controlled is some arbitrary number not a precise temperature. Also the temperature curve is straight line, which is an improvement, but not the best. Today, for $12.50, a chip is available which corrects for oddities of a K-type thermocouple, so the output is (within a small percentage) a voltage the same as the temperature (6.55 volts = 655.C) and thus finer control is possible.
DIGITRY - The most widely used controller in glass
melting and annealing is probably the Digitry
GB4 [lower in image] which has now been succeeded by the GB5 in 2005.
These control up to 5 units which costs $1465 for the basic unit plus about
$55 per kiln or annealer for thermocouple and additional solid state relay or contactor.
The GB1 controls one device and costs about $550 + kiln add-ons. The Digitry is
convenient to use, providing specific keys on a keypad to select the device and
numeric keys to enter device numbers, temperatures and hours. It allows
multi-step programs, so an annealer can be set to hold a temperature for 2
hours, then take an hour to drop 50 degrees, then take an hour to drop 150
degrees, and so forth. The cost per unit controlled is not as high as a
dedicated commercial controller (which may run $1500-2000), but is still
irritatingly high, especially if less than 5 units are controlled. And there is
the problem that if a single controller goes down, the shop is down. Similar
units are made by other companies, including Paragon
Two new technologies are available for control. One is a self contained unit that will provide a single or multiple ramp and learn the characteristics to the controlled unit. [These can look like the two upper units in the image.] The other is using a computer to control the controller. Actually, the first units can be had with an option that allows computer control of changing the ramp and then letting the unit run.
Several companies have come to market with small (2" x 2" x 4") controllers for about $200 that avoid a complication that has plagued controllers for years. PID (Proportional-Integral-Derivative) is a method that allows programming controllers so they do not overshoot their goal or over-control, wasting energy. Unfortunately, PID requires careful selection of parameters to match the furnace/annealer to the heat source. This is eliminated in the new units because they "learn" the characteristics of the equipment, adapting to different building temperatures, etc. I have an annealer with a low wattage light bulb across the elements that goes on and off with the elements. It is easy and fun to watch as the controller turns on the element and, after the temperature has risen, turns it off to see how much coasting goes on. After testing, the unit finally runs the temperature right up to the setting.
A $300 solution involves buying a CN76000 Auto-Tune Controller (dc pulse output with alarm option, CN76120, $195 or adding remote setpoint option, CN76120-SP, $234, page P-95) from Omega (1-888-TC- OMEGA, ask for the Temperature Handbook, www.tcomega.com ), along with a K-type thermocouple, DH-1-8-K-12, $19, (page A-13), 25 feet of K type thermocouple wire, PR-K-24, $15, male and female K-type miniature plug connectors (SMP-K-MF $4, page G-16) and a Solid State Relay SSR240DC25 (DC control voltage, 25 amps, up to 280 Volt) $26 + Heat Sink FHS-2, $17. There are only 3 pairs of wires to hook up - AC power, thermocouple, output. [Controller Comparison]
AD595 - There is a set of Analog Devices chips that will take a K-type thermocouple and produce a 10 mv reading per degree C or produce a setpoint controller. With a fairly simple switch, a single chip can do either. I paid $12.50 a chip (buying 2 to meet $25 minimum from Newark Electronics, current suggested retail for quantity 100 is $7.28. Data Sheet PDF on 2012-10-18
The two chips shown above are designed to have a K-type connected to the pins at one end (pins 1 and 14/8). For the reasons given in the discussion of thermocouple connections, AD recommends making heavy copper connections to these two pins showing wide foil connections on their PCB layout. I used a wire wrap socket and bent the pins over to make the solder connection rather than use smaller wire. All this is to keep the connections as close to the chip temperature (which is measured by ICE POINT COMP) as possible. I solder a short length of thermocouple connection wire to the PCB and put a mini-connector on the end of that.
Circuit to be tested and posted soon. (he said on 9/20/2000)
This is the circuit for using the AD595. As shown in the diagram, a DPDT switch allows choosing between measuring temperature and checking the set point. A digital volt meter connected to TP (Test Point) and ground (GND) will show 0.001 volts per degree Centigrade (1 mv/C) I use a multi-turn potentiometer. A limitation of the chip is that it will only drive 5 ma. There are opto-isolators that will work with that drive level, but only just barely. So gates are added and the easiest to get are inverting. Although hard to see, the three gates are Schmidt Trigger inverting gates (3 of 6 in a 74H14) because the signal changes slowly and CMOS usually doesn't like slowly changing signals. [2002-10-26 I had kind of let this circuit lay around for a while, but got bugged and today realized I might need a pull-up resistor. When I began looking for specs, I was dismayed to find that the 74HCT14 from Radio Shack are very limited on voltage range because they are to work with TTL. So my whole reason for choosing CMOS - voltage variability - is voided because I didn't know/look at the specs. Now to find one cheap/quick.]
2002-11-04 I got the thing working, after redrawing the schematic from the bottom and tracing all the wires. There was not a good connection at the measuring point. The core problem was that the measuring points were not connected reliably. I tore off the wires and removed the Molex. Finding nothing smaller, I used two terminals of a European style terminal strip. Once firmly connected, it seems to work well at light bulb temps. Will test with one of my boxes when it isn't raining and I am awake.
|This is the layout of my small unit. The
AD595 chip is in the highlighted area upper left corner, the
multi-turn pot is the green box to the upper right and the CMOS
chip is below that. The switch to the left is the 9 volt power on/off
and the thermocouple wires are below that. The switch above
chooses between measuring the temp or the set point. The short
white Molex strip on the right edge is for plugging in meter
probes to measure set point or temp.
The AD595 chip is in a wire wrap socket so the heavy pins make the connection the same temp which the chip compensates for.
|The is the PCB (printed circuit board) layout suggested by Analog Devices - note the heavier wiring from 1 & 14 to the thermocouple wires for temp compensation.|
Power control - Relays
The three basic ways of controlling a bunch of power (30-100 amps, 1 or 3 phase power) are a contactor, a MDR or an SSR. All of these are relays and a relay is a device for switching on and off power in one circuit with another circuit, which may be of a different voltage, for convenience.
A mechanical relay consists of a coil (a winding of copper wire) that creates a magnetic field in a iron core that attracts the iron plate that carries the contacts that switch the power. While relays may take dozens of forms, contactors are a special form that has only normally open contacts that are pulled shut with the magnetic field. The disadvantages of contactors are that they are normally noisy, clanking with each open/shut cycle, may produce electrical noise from the arc, and they cannot be switched more often than every couple of seconds. A major advantage of contactors is that power is completely off when they are open, no leakage, so they are used to kill circuits ahead of SSR's, etc.. A contact may throw an arc on closing or opening. Contactor
"Mercury Displacement Relay (MDR) - An electromechanical switching device having one or more poles that contain metallic mercury and a plunger which, when energized by a magnetic field, moves into a pool of mercury, displacing the mercury sufficiently to create a closed electrical circuit." For this definition and safety steps go to Disposal of Mercury Displacement Relays - Watlow Electric Manufacturing Company
The main advantage of an MDR over a contactor is silence although it can also be cycled faster than a contactor. A big disadvantage is disposal and risk if mercury escapes, which is unlikely but critical because mercury vapor is poisonous and the spill must be treated as hazardous. A nuisance is that the MDR must be mounted in a specific position to work, which may complicate a portable rig (which must then not be operated on its back, only upright.)
SSR is a solid state relay, discussed more below. The advantage of an SSR is that is can be cycled very rapidly (60-120 times per second if needed). The disadvantages are that a small amount of current leaks through the SSR to the element, even when it is off, and when it fails, it is more likely to fail on (short) than off (open.) Also, an SSR, SCR, or Triac must be provided with a serious heat sink and good air flow. It should not be enclosed inside a box.
: I'm not an electronic engineer or even close, but my
understanding is that
"An SCR (Silicon Control Rectifier) is (without going into
great detail) an electronic switch. It is either off or on, but
you can delay when it is turned on in relation to the alternating
current's cycle. If you turn it on early in the cycle, you get
near full power. If you turn it on near the end of the cycle, you
get very little power. Once turned on, it will stay on for the
rest of the cycle. You can turn it on with vary little power, i.e.
a few milliamps to the gate (control element) will allow you to
"switch on" many amps (usually the full 15Amp circuit).
Since I have built the things from scratch (but not melted the silicon) I suppose I am qualified to offer a few definitions.
An SCR (Silicon Control Rectifier) is a device that rectifies current (changes it from AC to DC) which can be turned on with a small current. If used alone with AC, it produces pulses of 1/2 the AC power when on. Therefore, when SCR's are used, they are almost always used in pairs to pass both halves of the AC power.
A triac is effectively two SCR's built on one small piece of silicon with one wire controlling them. The two can't used separately. It is a triac that is used in dimmers and small motor speed controls.
| The charts at the right show how phase control works. The
sweeping curve (mostly in red in the upper image) is the voltage (or
amperage) sine wave for 120 volt AC power. It goes from maximum thru
zero to the minimum back thru zero to maximum 60 times a second in the
U.S. Each hump from zero crossing to zero crossing is a half cycle - 120
of them a second.
Phase control involves turning on an SCR pair or a triac some time later in the half cycle than at zero. [It turns off at each zero crossing.] This has good points and bad points. The biggest bad point is that the sharp rise of the voltage from zero, shown by the vertical green lines in both images, will produce Radio Frequency Interference (RFI) with other electronic devices.
|The best good point is that phase allows very fine control of the
power. While a zero crossing turn on signal allows cutting the power to
one half cycle now and another half cycle later, this means the element
has time to cool before reheating, which may cause strain on the element.
With phase control power can be cut back very evenly and minutely.
Even if a simple relationship is desired, say 1/120th power, instead of
one half cycle each second, the power could be turned on late in every
half cycle so that every cycle produces 1/120th of maximum power - much
In the upper image, power jumps from zero up to the curve (the green line) late in the half cycle and well after the maximum and the area between the green line and the zero line is small - not much power. In the lower image, the jump occurs shortly after the zero crossing, the area between the green curve and zero line is much greater - lots of power.
|How is phase control carried out? In a manual
circuit a variable resister controls charging of a capacitor and the
voltage on the capacitor triggers the SCR/triac a specific time after zero
each time. In a pure microcomputer circuit, it is possible to
literally sense the zero crossing, count microseconds and pulse the
SCR/triac at the right time.
A more common control method is to apply a voltage that produces the same result as the resister/capacitor delay, a voltage produced by a digital to analog converter or a 4ma-20ma current converter.
|Because of the sharpness of the point of the voltage change late in the half cycle, rising then immediately falling, the RFI is typically greater then.|
An SSR (Solid State Relay) is a combination of a Zero Crossing Switch and a Triac/SCR. One of the problems of phase control it that it can produce electronic noise and some devices will overheat or otherwise be damaged by the different shape of the power (from a smooth sine wave.) If the SCR turn-on signal is applied at the time of the zero power crossing, then the output will be an AC half-cycle. Then instead of phase controlling each cycle, power is on or off for complete half cycles and power may be controlled to the nearest 120th of a second.
Why then use SCR's instead of Triacs or SSR's? Well, for higher voltage, higher current, and 3 phase power. Triacs will break down at higher voltages, have limited amperage available and don't offer the needed control. And for motors a curious thing happens: a motor shifts the phase relationship of current and voltage so the Triac thinks the power has never shut off, so it stays on. SCR's allow specific circuitry to deal with 3 phase power and the phase shifts.
|The image at right shows the way I currently control power. A 12 gauge wire has been cut open and the power wire (black) cut neatly and the ends stripped. These were attached to the load screws of a commercial SSR and the area sealed with 100% silicone sealant. A snap on plastic cover is provided with the relay. The connectors in this case are 15 amp standard plugs of a type I like to use because the wire goes straight in to the internal screws. The control is thin stranded speaker wire connected to a phono plug common to my various controllers. The black area inside the loop of wire is the heat sink, not well shown in this image. 2005-04-21|
the right is a second power control cable built the same way as the one above,
except that the SSR is controlled by 120 VAC like the one below instead of
5 VDC. So the cord from the control terminals ends in a common 120 plug
instead of phono plug. I use this to control a second element in my
annealer when I want to take it up to 1450F or so instead of no higher than
1100F. The main element is on the lid and the second is the original hung
around the top of the sides, which is actually broken and just hooked together.
There is a low wattage standard light bulb indicating power on for the main
element. Unscrewing this bulb and adding a common socket adaptor lets me
plug in this control to a second power source (an extension cord when I need it)
rather than rewiring to handle 20+ amps that both circuits would pull together.
montage at right shows the construction of a 220* volt SSR
mounted on a 30 amp dryer cord and outlet. The upper image shows the side
view of the outlet, cord, and heat sink
as well as the edge view of the adaptor
mounting plate. The lower right image shows the face of the surface mount
outlet and a different view of the plate, while the lower left shows the two
solid state relays and the wiring coming out of the outlet. I use RCA phono plugs to plug in the 5 vdc controls and this is shown in the middle with
the wire leading to the relay.
|The images at right show a solid state relay that I built at a time when more time than money was available and when SSR's cost more than they do now. Almost all the part came from a hardware store or Radio Shack. At one end of the metal box is a snap in grounded outlet. At the other end is bolted the triac with a heat sink outside. This proved inadequate in the Texas heat, so sheet aluminum was pop riveted to the top and bottom, with heat sink compound on all. The cord from an air conditioner extension enters though a cable clamp. Inside the box, a terminal strip holds capacitor and resistor and a small circuit board holds the zero crossing opto-isolator Triac driver chip.|
the schematic at right, taken from Teccor
notes* is likely the one I used for building the box above. A low value
resister (22 ohms) limits current flow between the triac and chip and 100 Ohm
resister and 0.1 microfarad capacitor provide snubbing protection against small
Cautions about SSR's, Triacs, and SCR's? None of these turn off completely, there is a small leakage current. For safe work, there must be a mechanical switch, manual, relay or contactor to completely break the circuit. Further, if these devices get overheated, they can fail ON and the only way to break the circuit is mechanically - a circuit breaker switch or contactor. In an annealer, failing ON means the glass gets overheated and sags.
So if a contactor is needed anyway, why not just use that for power control? Well, contactors produce sparks and interference of their own and mechanical contactors are audibly noisy (while mercury contactors are not) and the life of contactor is shortened considerably if it is turned on and off as often as an SSR commonly is - a controller must be told to operate less often with a contactor.
* [Previously, Teccor had a neat file with individual notes, now there is a large file http://www.littelfuse.com/data/Product_Catalogs/PowerThyristorApplicationNotes.pdf ]
The small fractional DIN controllers have screw terminals on
the back that require a minimum of 3 pair of connections: AC
power, thermocouple, and power control. Power control depends on
choices made when buying the controller. I buy solid state relays
(SSR) and buy the matching DC output option. On a solid state
relay, the terminals are Control and Load.
The drawing below shows the relationships of a simple
connection to a kiln or annealer. The controller has three pairs
of connections: 1) its own power, usually 120 volts AC, 2) A
thermocouple, inserted in the kiln, usually K-type for glass
work, connected by proper wires if extended, 3) Signal (low
voltage) connections to the SSR.
----- Original Message ----- From: "Trask
Sent: Saturday, March 23, 2002 8:13 AM Subject: controller
Controller Comparison [Under Construction as I try to make sense of units not owned.]
Love Controls 76000 --- The unit I have actually been
using for several years.
Fuji Micro-Controller X - Model: PXR
The annealer is shipped with the following program.
Sv-1 - 950 (This is your first set value or the temp. that you want
the kiln to go to.)
This segment of the program will do the following:
Watlow Series 96
Digitry GB1 & GB5-
Electrical Heat Elements
So heating is normally done with an element. This is a length of material that gets hot when electricity is run through it. The natural resistance of materials that conduct electricity causes heating any time electricity flows. In the normal use of wiring, every effort is made to reduce the heating by selecting materials with low resistance, most commonly copper and aluminum because of their cost, gold and silver being lower resistance but too costly for anything but contacts on switches.
The element material must first of all stand up to the heat. While a thin aluminum film can be a heater in a warming tray, it will simply melt if taken to red heat. A material that has been developed that remains strong enough when heated, Nichrome which is a nickel-chrome alloy with iron and aluminum added to some variations. Wire: Nichrome (tm) & Other Resistance Alloys - Tech Data The first problem with Nichrome and glass is that it gets very unhappy (ready to melt) at the temps that glass melts and thus produces a short life.
The nice thing about Nichrome and most other materials that might be used for heating is that their resistance goes up at they get hotter. This means that if a certain voltage is applied, a certain amperage will flow when cold and as the element gets hotter, the amperage drops so at some point the heat output is limited and the element naturally stops getting hotter.
The smaller the wire diameter the higher the resistance and the longer the wire the higher the resistance. So the design of a heating element becomes a matter of varying the diameter of the wire against the length of the wire so the temperature of the wire gets hot enough, but not too hot, while putting enough power into the insulated box to raise the box temperature to the desired point. Tables give the temperature of a straight wire of a certain size with certain amps. Commonly, elements are coiled to get more wire into a given space and to increase the temperature because one coil raises the temperature of the next one. A 1000 watt 120 volt heating element might be 3 feet long when coiled and pulled out to working length.
And exception to the resistance curve of the pure resistance elements are silicon carbide heating elements, which drop in resistance up to 1960F and molybdenum disilicide heating elements, which have very low resistance and require a hefty transformer to drop the voltage and increase the amps also the resistance changes considerably with age, requiring adjustment to deal with loss of heating if not adjusted. Both are rigid cast ceramic material and must be bought in the shape they will be used, a U shape being most common. They are used because of the high temp limits. "Molybdenum disilicide MD-31 for element temperatures up to 1700ºC (3100ºF) and MD-33 for element temperatures up to 1800ºC (3272ºF)." 2005-07-01
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