Programmable controllers

The hallmark of a good programmable controller is simplicity of operation.

• It should be simple to store a pattern and to run a firing
• The operator should be able to monitor at a glance the progress of the firing.

Programmable controllers will store a number of complex firing patterns and control the firing of any of those patterns with reasonable accuracy and without intervention or supervision by the operator. They do this by a combination of hardware and embedded and customer settable software.

Mitigating against controlling with precise accuracy are two main factors;

• The ability of the operator to enter rates of temperature change of which the kiln is not capable, and
• The inability of the controller to regulate the rate of heat loss from the kiln.

Pattern entry methods

Over time, two main methods of pattern entry have evolved;

• Step entry, and
• Segment entry.

Step entry

Fig 1

This is where the three operations of heating holding or cooling are entered as separate and discreet functions which can be linked together in any combination.

This is the method of choice of most instrument manufacturers around the world, as it provides great flexibility and allows the devices to be easily adapted to many industries and operations.

Through the years, the origin of instruments on offer by Australian based Instrumentation specialists have included  Britain, France, Germany, Italy, Japan, Spain, Taiwan; all with this method of data entry. Kiln builders could source their instruments from any of these suppliers, sometimes working closely with a particular maker to improve overall performance of both instrument and kiln.

These instruments are usually stand-alone; are fully enclosed and are intended to be mounted directly into a panel cutout. They invariably bear the name of the instrument maker.

An example of this is the Shinko PCD-33A controller used for years on Australian made Riley glass kilns and Miley pottery kilns. It, like others, will accept up to nine patterns each of nine steps.

Segment entry

Fig 2

This is where the operations of heating or cooling are each coupled with a hold period, as shown above. With the possible exception of a New Zealand maker, this method of data entry appears to be confined to devices originating from North America. It is unlikely to be used on instruments supplied by Australian based instrumentation specialists, so must be sourced directly from their overseas maker.

It originated to meet the need of pottery firing, where a heat step was always followed by a hold of many hours. Thus, heating to around 120°C in a short time may be followed by a hold of 3 to 6 hours; this to finish the drying out of the clay.

These instruments can be ‘stand alone’ but are more generally supplied as a face plate and a printed circuit board to be mounted howsoever the kiln builder chooses.

They will sometimes identify the maker but will more often bear names associated with the kiln builder, such as KilnMaster, GlassMaster, Rampmaster.

Digital displays

One of the significant developments of recent times has been digital displays.

They have become ever cheaper, to the stage that two, three, or even four separate displays may be used on one instrument.

A display can consist of one or a number of segments, depending on what is to be shown.  The segments will usually be one of four colours; red, orange, blue or green and can vary in size and brightness. Each segment is made up of seven small strip lamps which can be turned on individually to produce stylised alpha and numeric symbols.

Controller with multi displays.

Fig. 3

The Shinko PCD-33A controller mentioned previously and shown above has four displays, and is indicative of the level of development that good design and attention to operator need can produce.

Process Value (PV) the kiln temperature, is displayed on a large four digit display.

Set Value (SV) the moment-to-moment temperature called for by the running pattern, is displayed on another four digit display.

Single digit displays show the pattern number selected and the step which is currently running.

The instruments can be configured by the kiln builder or user so that any one or none of the small dots in the PV and SV displays will appear; giving full scale from 10 degrees to 2000 degrees.

The operator can monitor at a glance the progress of the firing.

The MODE button provides foolproof entry of steps in a pattern; and also access to make changes .whilst the pattern is running.

Controller with single display

Fig 68-04 shot of American 20 button controller

Here, all of the data is shown on a single display. The display will normally show the kiln (process) temperature. If other data is required it is accessed by pressing buttons; the desired data will be displayed by flashing alternatively with its coded identifier. Lots of information can be accessed but it is usually necessary to refer to the instruction manual for the correct code.

This type controller is widely used on American kilns. Coincidentally it uses segment pattern entry

Pattern storage

The number of steps or segments in a pattern, as well as the number of patterns which can be stored, will generally not exceed nine. Thus, storage capacity may be;

• four patterns each of seven steps, or
• seven patterns each of eight steps, or
• nine patterns each of nine steps, or whatever.

Both the patterns, and the steps or segments in the pattern, will be numbered from 1 through to 9.

In the rare instance where the capacity is 10, then the numbering will start at zero; proceeding from 0 to 9. Zero comes first, not last. This allows a one digit display to be used.

Instruments with larger capacity in both the number of patterns and steps per pattern will usually also have more optional capabilities and so become much more expensive. Their extra cost is not justified for glass or pottery applications. Click here for more information on firing patterns.

Linking patterns

To allow longer firing patterns to be used, consecutive patterns can sometimes be linked together to double the number of steps or segments. Thus, two nine steps patterns can be linked to give one pattern of 18 steps.

In one instrument, the Shinko PCD-33A/TO370, the whole nine of the nine step patterns can be linked together, making a pattern of up to 81 steps.

The Shimaden FP93 can also have its four patterns each of 10 steps linked together.

Whether it be steps or segments of data, more than nine is rarely needed by kilnformers. Many manage very well with as little as six or seven.

An exception is in the firing of large castings, where firing times can extend into many weeks. In those cases, having the ability to link patterns can be a blessing and allow precise management of temperature change.

Wait capability

When the glass is in a fluid state the glassie wants the kiln to go as fast as it can; often done by inputting step times faster than the kiln can achieve.

This means that the controller will reach the end of the step before the kiln does.

‘Wait’ capability is where the controller halts the running of the pattern at the end of a step until the kiln catches up. Makers may use other terms to describe it, but wait says it most succinctly.

It is an ability possessed by most programmable controllers but rarely by cheaper instruments. It is essential for unattended firing.  

Some instruments allow it to be turned ON or OFF. For kilnforming it should always be turned ON.

Programming & running (Separate or combined?)

A kilnformer will perform a limited number of firing types, each pattern of which can be stored in the controller. Once having them stored they need to revisit them only to make the occasional minor change.

For most firings they wish only to select a pattern number and start the firing.

The programming should be separate from the running.

Most stand-alone instruments allow for this. Some also allow the patterns to be locked so that normal users such as students or plant operators can only select a pattern and press a RUN button; they cannot alter a pattern. 

In other cases the programming and running are linked. Even though no change is contemplated, each step in an entered pattern must be re-visited before a firing can be initiated. This is undesirable and can only increase the possibility of a satisfactory pattern being mucked up.

The way data is entered

This is chosen by the instrument manufacturer and will usually be consistent right through their range of products.

• Controllers can usually be programmed to process temperatures in degrees C or F.
• Temperature entered is the end temperature for the step or a segment.
• The first step or segment must be a heating one. If starting at room temperature to cook the glass, there's no point going anywhere but up.
• Rate of temperature change can be entered either as
  • degrees change per hour, or
  • degrees change per minute, or as
  • time to complete the step, (this is usually in hours and minutes).

Most controllers fitted to American pottery and glass kilns require the rate of temperature change to be entered in degrees per hour. This suits pottery firing, because they heat so slowly.

Because steps in kilnforming are of short duration, it is more convenient for Glassies to think of heating rates in degrees per minute.

This requires a bit of mathematics, but is not all that complicated. Thus, 3 degrees per minute multiplied by 60 minutes is 180 degrees per hour, etc.

With controllers using ‘time for the step’, the mathematics is of a different kind.

Consider a heating step for float glass from ambient to 550°C.

10°C per minute is OK for 4mm or 6mm float, so it will take 55 minutes. Enter 00:55. Easy.

For more delicate glass, or for the more conservative kilnformer where 3°C per minute may be preferred, divide 550 by 3 =183 minutes. Entering 03:00 is near enough. May need pencil and paper for that one.

Entering time-for-the-step allows extremely slow rates of change to be entered. Consider cooling a massive casting. A small change of temperature in a step, with a step time up to 99 hours 59 minutes (99:59), can give minuscule rates of change

Whatever the method chosen by the maker of the instrument, it won’t take long for the user to become expert.

Security

As many instruments are used in areas prone to unauthorised fiddling, numerous methods have been devised to minimise risk.

• It has been found that most people who fiddle also poke; but don't hold a button depressed for any length of time. In the Shinko controller mentioned above a one second delay has been built in to certain button actions to take advantage of this behaviour.

• Other makers may require the pressing of buttons in a set sequence before a change becomes effective.

• Still others may have a secret button which is not marked on the instrument face,

Input signal

As discussed in 50.Thermocouples and pyrometers, there are numerous devices which can be used to sense the temperature and provide that information to the controller. Most modern instruments provide ‘universal sensor input’. This means they can be configured by the user to accept data from any one of up to 14 or more different devices.

These include type K thermocouples used for glass temperatures and type R or the cheaper type N for pottery and glass crucible furnaces. 

Type K thermocouples are the most widely used in industry and hence the cheapest. They are recommended for use up to about 1100°C (2010°F) but are often taken higher; although their life will be shortened.

Instruments on most American pottery and glass kilns use type K only, so don't have universal sensor input.

Output signal

This is the signal sent from the controller to whatever device is used for switching the power to the elements; either Solid State Relay (SSR) or Contactor.

Contactors can be switched by either a 240Volt or a 24Volt a.c. signal, whilst SSR’s are best switched by a low voltage d.c. signal. Instruments must be ordered with the appropriate output. See 55. Power switching devices for more.

Cycle time

Able to be set on most modern instruments, this is the time in seconds for one temperature monitoring and controlling cycle performed by the controller. It can usually be set anywhere between one and 60 seconds.

In the case of kilns with SSR power switching, cycle time will often be set at three seconds. During this time period the controller will check as to whether and by how much the measured temperature is above or below that called-for by the pattern being run. The result will be an estimation of the proportion of the next three seconds during which the power should be turned on to keep the kiln most accurately following the pattern. (this is called 'burst firing' and will be discussed in more detail later)

For kilns using contactor power switching, the cycle time is usually set at around 15 seconds, as this provides more time for the contacts to cool between operations.

Auto Tune

This is a capability built into most modern controllers.

Using ‘fuzzy logic’, it harmonises the controller with the kiln and enables it to more closely follow the pattern; without overshooting.

 If an option, it is best done when the kiln is first received but during the hottest normal firing. It can significantly improve the preciseness with which the kiln follows the pattern.

Safe ambient temperature for instruments

Most electronic devices can be damaged if they are operated or even stored at temperatures above about 50°C (122°F). This is rarely a problem when the controller is mounted in a housing which is either remote from the kiln shell or is adequately insulated from radiant heat.

In one instance an American kiln builder has incorporated an alarm to indicate when the instrument is getting too hot. How much more professional to install it so that overheating won’t occur. As there is little variation in American kiln designs, is this a problem not recognised by other builders? 

Built-in capabilities of instruments

Developers of instruments have included capabilities to meet the needs of a diverse market. Sometimes they may be included simply because they can, or maybe the instrument designer thinks it is a good idea and may come in handy, sometime.

Often the capabilities are inherent in the logic chips sourced from a common supplier.

This is akin to the developers of computer software such as Microsoft, where more and more features are piled in but most of which are rarely if ever used. 

The kiln builder needs to resist the temptation to use more of these capabilities than necessary.

Due to lack of understanding either of the way the instrument works, or even of the entire kilnforming process itself, they could well be giving wrong or misleading advice.

Glass artists may not always be the best people to give them this advice.

One of the many instrument capabilities is called ‘Sensor Offset’.

Skutt Kilns have chosen to activate this capability in their GlassMaster controller, and explain its use in their Operating Manual as follows; “this feature allows you to calibrate the thermocouple when it is reading consistently and predictably incorrect”.

As explained elsewhere, the thermocouple reads the temperature at the end of the probe. Unless made of shoddy material it will rarely be more than a couple of degrees in error.

Even the most experienced professional kilnformer would be loath to bet on their ability to look into a spyhole and tell the temperature to within a few degrees; nor to look at a piece of fired glass and say that a kiln controller was reading a few degrees in error.

It is safe to say that this is not one of the intended applications of this capability.

For more on this, see 48. Sensor Correction.

Harco controller

My experience of the Harco controller is somewhat dated, so I can report only on my knowledge from an earlier time.

Australian made pottery kilns were rarely fitted with a programmable controller. The exception was the locally made Harco controller. When introduced it was revolutionary but was also much more expensive than other non programmable temperature controllers. Thus, it was usually offered as an optional extra which could be plugged in using a unique four pin plug & socket connection arrangement.

Unique among controllers, it incorporated a ‘kiln minding’ capability which turned off the kiln if the PV lagged too far behind the SV. This on the assumption that the pattern entered was achievable and, if not being closely followed, then the kiln may be malfunctioning and should be turned off.

As kilnformers often want their kiln to heat as fast as possible above transition, they enter excessively short times for the final heating step, and this was often sensed  as a fault; much to the user’s frustration. The Harco also had the disadvantage of having the programming and running sequences combined, so each repeat firing had to be approached as if re-programming.

On the plus side it had a built-in delay timer, allowing a firing to be set for turning on at some future time. 

The operating instructions supplied by the maker were so rudimentary as to be almost useless. Tuition was passed on from potter to potter. Some years ago Graham Stone, author of ‘The kiln companion’ wrote a much more comprehensive set of instructions which are included in that publication. It is still a good instrument if received as part of a pre-owned kiln purchase but, unless substantial re-design has taken place it is not the best new purchase for kilnforming applications.

Getting local advice and service

There are a large number of instrument companies in Australia offering a wide range of cased instruments suitable for panel mounting, with local service and advice if necessary. Whilst highly trained in instrumentation, they may not be familiar with the peculiarities of the kilnforming process. They may also be tempted to offer more sophisticated control than is necessary. Other makes of instrument can be purchased over the internet but usually lack any local back-up or service; and again their vendors may not be familiar with kilnforming processes. Beware.