Introduction - making a diy computerised DIY dobby weaving system for a floor loom.
I purchased my Louet Octado 8 shaft dobby loom second hand around 6 years ago. I consider myself incredibly lucky to have the opportunity to weave on this loom, it is a beautiful loom. When I agreed to buy my loom I had little understanding of the loom or what a dobby system was but it is easy to work with despite the steep learning curve. Whilst we could have purchased a computer dobby for my loom my husband, who likes a challenge decided to have a go at making a computer to be used instead of the mechanical dobby. His plan was to give this to me for my birthday in June. In January I was given my present which whilst a little late is definitely worth the wait. This blog discusses some of the trials and tribulations of making a mechanical dobby replacement computer.
What is a mechanical dobby loom?
On a dobby loom each warp thread is attached to a single shaft which is threaded through a heddle. The shaft controls each thread via this heddle and the raising or lowering of the shafts gives rise to a large number of possible sheds through which the shuttle containing the weft is passed. My Louet Octado loom has 8 shafts and has a manual dobby. The dobby consists of a chain of bars which has pegs inserted which selects which shafts are lifted. With an 8 shaft loom there are 254 different sheds which can be created.
What are the limitations of a mechanical dobby loom?
The problem with my dobby loom is that I have been restricted with respect to the repeat length of the pattern. My Octado 8-shaft loom has 50 dobby bars, so that is my maximum pattern repeat length. I could buy a few more bars but the dobby chain would start to drag along the floor and I have noticed that with 50 bars the chain can break during weaving. The computer dobby has the advantage in allowing the user to programme which shafts are to be lifted. This replaces the need for the dobby bars and enables a longer pattern repeat length.
How hard was it going to be to replace the dobby bars with electronics?
To replace the dobby bars and pegs on an Octado loom with a computer, a mechanism is needed to push in the sliding bars at the side of the loom. It was decided that this could probably be done with solenoids (basically electromagnets that push out a metal bar when electricity is passed through them), controlled by a microcontroller called an Arduino. An Arduino is a little computer board that you can programme with lots of inputs and outputs which can be connected to things. For example, the Arduino can be used to switch on lights, get input from switches, connect sensors amongst other things. The initial thinking was that it wasn't going to be too hard to write a computer programme to programme the Arduino. It turns out this project was way harder to get working than was anticipated, hence the delay to the birthday gift!
Lots of Challenges
As the project got underway lots of issues which hadn't been considered were encountered. These issues led to delays, complete rethinks, the burning out of electronics boards, the purchase of other electronics boards, overheating and quite a lot of frustration. Programming the Arduino, was quite a bit more complicated than initially expected. The thinking was that a simple bit of code was needed which would switch the solenoids on and off. As the project got underway lots of complexities and questions arose. Some of the questions that needed to be addressed were:
How do you see where you are in your weaving pattern?
How do you load your pattern into the system?
What happens if you make a mistake and need to go backwards?
How do you switch patterns (for example, from an 8-shaft pattern to tabby)?
How do you safely power the system from the mains?
How much power do you actually need for everything to work?
How do you align the solenoids with the sliding bars?
How do you make the whole setup look good when it's attached to the loom (a key "user" requirement)?
Addressing all of these questions and others took a long time. Small sections of the programme needed to be coded. These sections were checked to see if they worked with problems addressed before moving on. Later, wider whole system issues occurred that needed fixing. My husband says that most coding problems were caused by his failure to not fully understand the logic of what was going on. I personally think it is amazing that he can programme it at all! The most difficult part was getting the screen to function and display nicely.
Making it work (and look nice)
The following steps were undertaken:
installation of a touch screen for the Arduino;
addition of an SD card reader for pattern storage;
purchase of an external power supply (then another one when the power level from the first one was so high that the heat from the solenoids was going to cook everything);
addition of an internal power converter (to change voltages);
inclusion of eight relays (electrical switches) because the power output from an Arduino is too low to run a solenoid,
use of diodes to protect the system from voltage spikes generated by the solenoids when they are switched off.
The creation off a ‘pretty’ wooden box.
A computer-controlled router was used to cut out the wooden box. The nice thing about this approach is that you can cut wood really accurately and change things if you get it a bit wrong. This router is used lots to make the looms for TabbyandTweed.
The computerised dobby box has the advantage that it has been designed small enough so that both the computer or the mechanical dobby can be used interchangeably. This does feel reassuring because the dual function dobby will allow weaving even in a power cut.
Programming the computer with the weaving pattern
Programming of the weaving draft was done in a simple text file with the binary system used, using a 1 when the shaft should be lifted and a 0 if not. The programme is transferred onto an SD card which is slotted into the front of the computer dobby box. The image of the computer display below shows part of a weaving draft and highlights that the weaving is on row 43.
Trial and Error - the Pedal Dilemma
As an example of one of the many stages of trial and error and tweaking that was needed surrounded the switch that is put on the foot pedal to raise the shafts. The idea was that the pedal would trigger a switch when it reached its highest point, which would tell the computer to move the weaving pattern to the next row and activate the solenoids to select the correct shafts. But the problem was that the pedal would bounce off the switch and cause the pattern to progress forward too many rows. This meant that I had to be really careful and release the pedal slowly to try to avoid messing up the pattern which was not very easy. Several solutions were tried including moving the switch to different positions, modifying the computer code to try to account for the bouncing, and even trying an accelerometer sensor (the same kind used in smartphones to track steps). None of these potential solutions seemed to work. So the final tweak was to include two magnetic (reed) switches on a vertical piece of wood next to the pedal, with a magnet on the pedal triggering them. The top switch now fires the solenoids, and the bottom one progresses the pattern and resets the solenoids. This solution has made the system much more reliable without any of the issues of the pattern jumping ahead.
The project is finished (maybe)
The project combined coding, electronics, woodworking, and an understanding of looms and weaving. It was the best feeling when it all worked for the first time. Unfortunately, that feeling was short-lived, as it became apparent that the voltage levels were wrong and the electronics were rapidly burned out. The damaged components were replaced and a new power supply used.
The computer dobby is now working really well and the birthday present is one of the best ever. The first project is shown below which is an Overshot table runner having being woven with a 97 row repeat. Of course, like any project there are things that can be tweaked and improved but that is the nature of prototyping! The cost of the project including all the replacement components needed was around £200.
If you are interested in knowing what is on the loom, off the loom and workshop dates then do sign up to receive my weaving news emails by clicking here.
If you are looking for a unique luxury handwoven scarf or perhaps a handwoven table runner then do visit my website shop by clicking here.