Computers and Firing Control

by Beverly Howard
Originally Published in Dec 1986
The Studio Potter Magazine

(Author's Note... Remember that this was written in 1986 and has not been revised or updated for this posting! Please keep that in mind when reading about the state of computers and tech equipment two decades ago)

Electricity is LETHAL!  The following information is provided for those who are knowledgeable, experienced and comfortable working with and around 240 volt ac electrical systems and high temperature ceramic kilns.  If you are not, please do not attempt to implement any of the following suggestions without the help and assistance of a licensed electrician.

In addition to the electrical dangers, electrical and firing problems can easily lead to accidental fires that can destroy structures not to mention the possibility of killing or injuring people.  While the author has executed the following modifications and those modifications are currently in use in a full time pottery studio at the time of this writing, the author does not accept any responsibility for any injury, damage or other losses that may result from attempting to implement any of the suggestions and designs shown in the following page.  The use and implementation of these suggestions are at your own risk.


It's time to take another look at how the computer can help in the studio, especially for controlling firings. With a computer, this control can supplement and augment the artists input by reducing the time required to tend a kiln. Manual switching of a kiln can require the potter's presence for a sixteen-hour day. In addition to increasing the heat at a controlled rate, some firings may require hold cycles or a controlled reduction of the heat after achieving the peak temperature. These demands on a potter's time are difficult, and the crudeness of the heat controls standard on most kilns makes for inaccuracy.

As for costs, the $5 pocket calculator of today once sold for over $600. The same scale of price reduction exists now in the computer industry. Also, you may already have a computer, or the cost could be justified by the other tasks the computer can accomplish when not controlling firing.

After the time and costs associated with initially transferring control to a computer, the continuing requirements in both drop dramatically. A computer has an additional advantage over a dedicated controller in that it has the flexibility to modify programs to achieve exact results when the artists requirements change radically.

Several interfaces have recently appeared on the market which can control devices outside of the normal computer environment. Their low prices are aimed at the masses rather than a few specialized users.

The Radio Shack Model 100 we use is a notebook-sized portable computer that is being closed out for $299 at the time of this writing. The principles used here relate to most computers and Microsoft BASIC is available for virtually all of them. Functional copies of the original IBM Personal Computer, which sold for over $6000, are now available for less than $600, and a major Japanese firm has begun marketing a version to be distributed and sold in discount stores. If you are not in a position to buy one, you may have a friend or customer who is willing to trade for one.

The Model 100 portable was purchased two years ago for letter-writing. Its use has expanded to include mailing-list entrees, inventory, and billing. The limited storage capacity of the portable has been overcome by transferring the data to my Kaypro IV for storage and processing. The mail list alone now contains over 500 names, and the time savings there are considerable. The portable is also ideal for fairs and shows where new entries can be keyed in directly, and it can be used for recording sales and keeping the tax men happy.

All of these jobs can be handled without a computer, but they are time consuming and tedious and, as a result, may not get done very often or very well. Computers are tools, nothing more. They seldom do anything that humans can't do, but they do things humans avoid if they can. If you would rather not reset the heat on a kiln four times per firing, a PC will reset it every minute without complaint. It will also wake up at awful hours to turn it on or off while you sleep or save you a trip across town if you don't live where you work.

Rebecca Roberts is my life-partner and the potter in my life, and I am the technician in her studio. Prior to Rebecca's use of the computer for kiln control, she began to have a problem with breakage after switching to a new clay body. Two simple and short BASIC programs were written for the Model 100. The first allowed quick and easy entry of temperature readings from a pyrometer that were automatically recorded with the time of the entry. The second read the file and printed out a time/temperature graph that immediately pinpointed the problem. The solution was a more precise control of the heat increases coupled with a ramp-down cycle. The computer was not only the best way to handle the solution but also the least expensive.

Rebecca wanted to retain control of the peak temperature with the assistance of the guard cones. The computer is now set to turn on the power and begin ramping shortly after midnight so that peak temperatures are reached shortly after she arrives at the studio. When she is satisfied with the heat level, hitting one key on the portable starts a hold at a given heat for the selected time period, followed by a gradual heat reduction that will continue for four hours. The cost of the computer has been returned many-fold in the requirements on her time alone.

By Time or Temperature

There are two ways to approach the problem of controlling a kiln. The first is to manipulate the heating process based on time only. The second is the control of the heating process based on both time and direct temperature sensing by the computer. The first is easier to implement with a minimum of technical knowledge and can easily replace a manual ramping routine. Setting up a computer to sense outside readings is a newer and less standardized field that, until very recently, was available only in a first-class research lab.

Ironically, the older and cheaper computers are easier to convert for simple control of the outside world. This direct-power control is made possible by using the small contacts built into most low-cost computers for the purpose of controlling a cassette recorder when loading and saving programs. This built-in relay can be actuated by the BASIC commands MOTOR ON and MOTOR OFF. The contacts are not large enough to handle even the current needed to trip a high-amperage con-tacter and, even if they were, it would not be a good idea to route 110V close to the computer's circuit board. The relay can, however, be used to trip another small low-voltage relay that in turn controls the heavy contacter (see figure 1).

The high-amperage contacter shown is a 220 V, 65 A mercury contacter (available through the W. W. Granger catalog). Remember to mount the contacter vertically only. A small plug-in dc power supply is used to provide the voltage for powering the low-voltage relay, which in turn actuates the mercury contacter. The dc circuit has a 200 mA fuse to protect the computer circuitry. A toggle switch is placed across the control side of the. relay to provide power control in the absence of the computer. There is a surge suppressor across the contacter coil poles. The device shown controls only one. leg of the power supply and should only be used with kilns having double-pole switches to disconnect the coils from the power supply.

The simplest example would be to program the computer to switch the kiln exactly as you have been doing manually, thereby freeing you to do other things or starting it when you would normally not be available. Although the computer can't actually twist the knobs to set low heat, it can be easily programmed to turn the current on and off to achieve, say, 20% heating (ON 10 seconds, OFF 40 seconds) using timed program loops. A better approach would be to eliminate the steps associated with switching from LOW to MEDIUM to HIGH and slowly ramp the current from 0 to 100% over the time desired.

The BASIC listing included with this article shows a program to ramp the heat in a straight line between any desired heat "settings. I recommend starting at 10% and ending at 90% to extend the life of the heavy-duty contacter by avoiding short time cycles that may overlap and cause arcing. The timing loops are based on the Model 100 loop cycle time of 330 loops per second and need to be modified for other computers and clock rates. Running the same program on a Macintosh without modification would reduce a six-hour ramp to under one hour.

While the timing loops may appear simple at first glance, it was a real task to keep them straight line while changing the time cycles. When you start building your own program, pay attention and pretest the ramps before you risk a pottery load. In addition, the Model 100 has an automatic power-off feature that opens the contacts six minutes after the program ends. Ending the ramp cycles with an input request or loop keeps the computer awake.

As skill and needs increase, the complexity of the controlling program can be increased to control holds, ramp-down, drying, etc. From hard-learned experience, I can assure you that it is best to make only one change in your program or firing procedure at a time. The listing is simple and included to serve only as a guide for building your own programs. Keying in complex programs from a listing is a task best avoided. I am now beta-testing several different programs and will make them available on disk or cassette to any interested potters at a nominal cost.

With the addition of a controller board with multiple relays, more than one kiln can be controlled simultaneously. Several of these boards have just entered the market for prices ranging from $50 to $600. These generally operate through the serial port of the computer or plug into expansion slits.

If you are serious about control and sensing, I strongly recommend the board from SIAS Engineering (Rt. 1, Sa-lina, Kansas 67401). This is an eight-relay serial interface board with the capability of nine analog inputs for $199. This base, coupled with a good program, has the potential to control up to eight kilns at a time. In addition, it has built-in capabilities for converting voltage readings to serial data and for sensing the positions of eight Kiln Sitter switches.

Another low-cost approach that requires a higher level of skill is to use the parallel printer port. The magazine Micro Cornucopia (Box 223, Bend, Oregon 97709), issues No. 32 and No. 33, has an excellent article on this approach. Proteus Electronics,
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Inc. (Box 693, Bellville, Ohio 44813), has just announced a $47 board for the Apple II for controlling external devices. The press release indicated that additional interfaces were needed to control devices but did not describe them or list prices.

Finally, the X-10 system that is used to remote-control lights and appliances in the home through the house wiring offers a serial controller compatible with most computers. The modules used with this system are available at Radio Shack, Sears, and many department stores. The computer interface was a flop, and the $130 serial controller is now available from DAK (1-800-325-0800) for $20. The principal advantage of this controller is the ability to use a centrally located PC to control kilns through the building's wiring. If you have a back-yard studio, a computer in the living room, isolated from heat and corrosion, could control several kilns. Bonuses include the isolation of the circuitry from high voltages and simple control wiring. Some minuses are the low level of component reliability and limited service options. The package includes softward development documentation that is obtuse but very complete.

The next level of control is to sample the temperature inside the kiln and to control the heat based directly on that information. I have just started experimentation on this. While it is fairly easy to get readings from the thermocouple to the eye via a needle gauge, the standards for communicating the same information to computers are just now being set. For those with circuit-design capabilities (or friends with same) there is a chip (#AD595) available from Analog Devices (Rt. 1, Industrial Park, Norwood, Massachusetts 02062) that converts the voltage from a thermocouple to a straight-line millivolt-per-degree output.

With the inclusion of sensors, even more options are possible. Besides monitoring temperature in the kiln, they can check the room temperature as well. The Kiln Sitter switch can be rewired as a sensor rather than a cutoff. The standard Model 100 can make phone calls to report the

status of the firing to your home. Using the ramping information together with the peak current and cost per kilowatt hour, an automatic calculation of the electricity cost can be made every firing and stored in a file together with the complete history of the firing for your reference, enabling you to repeat an excellent glaze firing or avoid breakage with a new clay. Within the last month, I have begun to be aware of inexpensive interfaces that make the measurement and recording of temperatures by computer within the reach of the small studio, and my next article will deal with this area.

Most of the options open to artists are not yet visible on the horizon. Like the power wheel, air sprayers, gas and electric firings, and pocket calculators, this technology is not for everyone. For those who can embrace it, many of the less appealing chores around the studio can be turned over to this tool.

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