Dot Matrix Printheads

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Print Heads - Mechanics

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The key to dot matrix printer design is the behaviour of the pins (or needles, or wires).
 
Nobody has ever given a convincing and universally accepted reason to use "pins" or "needles" so both terms get used. Take the metal elements out of most printheads and they look like needles - just a few designs have the short stubby shape that would be a pin. 

On the other hand "pin" is a shorter word. The active element of a dot-matrix printhead is a thin metal wire with a tip less than a 50th of an inch across - smaller than the size of a full-stop. Ideally the pixel-dots produced by a printhead should be much smaller than a full-stop, otherwise the printers version of a coma will look unconvincing. Whatever description is applied to the active elements in a dot matrix printhead the working tip needs to be small.

Crude full stop
_
Rather unconvincing comma shape

There are three critical aspects in printhead design: speed, size and cost.

Speed. Dot Matrix Index & OverviewDot Matrix General Text on Printhead Speed
 
Pin speed is a critical factor in printhead design. It might be better to say "momentum" because the issue is how much impact the pin can make on a small area of the print-ribbon. The impact on the ribbon has to overcome surface tension and adhesion between the ink and fibres of the ribbon. An adequate quantity of ink must move onto the paper.

The issue is more than just speed. In some ways an ideal printhead might be piezoelectric crystal which can respond very quickly. In ultrasonic devices the crystal oscillates hundreds of thousands of times per second - which would give a very fast printer. Unfortunately the most esponsive piezoelelectric materials like PZT crystals only change dimension by about 0.1%. Whilst in bulk this is enough to eject liquid from an inkjet it doesn't seem a sufficient change to transfer ink from ribbon to paper.
 
The print pin mechanism normally used in dot matrix heads is electromagnetic. A coil changes a magnetic field and this moves the pin. Naturally there are lots of different designs on this theme - different shapes and arrangements of coil, pin and any intermediate components like armatures, pawls and springs. Laminated soft iron cores like those in transformers tend to respond more quickly. Ferrites might be better still, but perhaps too brittle?
Print-head pins need to accelerate and then return to their resting position as fast as possible. The travel for each pin is from less than 0.5 to as much as 2 millimetres, depending on the printer design. The critical factor is how fast the pins can complete an action. Pin action is partly a matter of mechanical engineering, the acceleration and recoil of the pin. Pin firing is partly a matter of electronics, the build up and collapse of current in the coil and of its electromagnetic field.

Print Arithmetic. Dot Matrix Index & OverviewDot Matrix General Text on Printhead Pin Action

Most printhead designs achieve their pin action somewhere between 1,000 and 3,500 times per second. To print one 14" line per second at 250 dots per inch the print pin needs to be able to achieve 3,500 actions per second. Since a green-line form is 60 lines this would deliver one page per minute if the page were entirely inked. If only 20 lines are partly inked the carriage needn't scan those parts and speeds can be three times higher. In business transactions there are often just a few lines on the page so the printer can accelerate over non print areas.

If a pin is acting 3,500 times per second over a 1mm travel then in effect it is travelling 7metres per second - 25 km per hour. Look at a printhead and it seems static, look in detail and it's travelling as fast as city traffic. The pins have to do this in a fairly unfavourable environment next to a paper surface that sheds wood pulp and kaolin dust with ink for lubricant.

Raising Speeds.  Dot Matrix Index & OverviewDot Matrix General Text on Limitations

Raising pin speeds is mainly a matter of stronger magnetic fields. In turn this can be:

  • Bigger solenoids and return springs compared to pins. Within the confines of a head the room for larger components is limited.
  • Applying (and perhaps reversing) the magnetic field rapidly.
  • More efficient tuning of the electronic and mechanical circuits.


Pin forward speed is mainly a matter of more drive current to the coils in the printhead. The limit on this is the point where heat generated by coil resistance can't be dissipated adequately and component temperature ises to a breakdown point. Recoil is dealt with below. About Recoil (This Page)

Lots of machines perform actions very rapidly

  • The valves in a car engine do several hundred and move several millimetres into the bargain. To do this the valves take a few percent of the 60 killowatt power of the engine.
  • The cone in a loudspeaker does so and its thrust of a millimetres or so is similar to a printhead. Loudspeakers typically need several tens of watts to do this.


The problem for the printhead designer is to get 9 or more active elements all needing several watts of power to achieve high frequency movement focussed down onto a column that will be part of a written character.

Size & Shape.  Dot Matrix Index & OverviewDot Matrix General Text on Printheads
 
Printhead mechanisms are usually oughly conical in side view and circular from the front. The shape comes from the way individual pin mechanisms are bundled together. 

At the back are the drive coils which need to be some size to generate the required power. At the front the pins nearly meet as they face into the paper. 

The pins themselves are hard wire shafts - usually between 1 and 3 inches long. Older heads have longer pins. Shorter pins are lighter and can move more quickly for a given power input. However the design has to pack some fairly powerful actuator coils into a space so the pins cannot be very short (or rather the pins might be short but if so there is some sort of actuator behind them).

Epson 9 Pin Head
At the working face of the print-head the pins are gripped together in a row by a print-jewel. Since standard line spacing is 1/6th inch high the pins need to fit into less than that 4.1mm space. Typically the characters wanted are a bit more than 3 mm high. Designs can differ a bit about the "swath" they print - nine pin heads tend to print a slightly narrower band than 24 pin heads.

The print-head itself is being rapidly moved back and forth across the paper, whilst a collection of between 9 and 48 pins change position at a rate of up to 3,500 times per second. For a small object printheads are usually built quite solidly to cope with the mechanical strain and to absorb then dissipate the heat generated in the drive coils.

Newbury Data 18 Pin Head
To fit 9 pins into a 3.5 mm or smaller swath space the pin metalwork has to be 0.38 mm or less in dimension. Usually pins are a fine wire at the working end. 

Alloys of tungsten and titanium improve wire strength and wear.

Alloys of cobalt an samarium improve the magnetic properties.

So the mechanical path in most printheads has two or three stages - a magnetic actuator passing energy through a pawl into a hard - wearing springy wire supported in a print jewel. 

The prinhead illustrated here gets small dimensions by having two layers of coils. 

Facit 27 Pin head


Print Jewel.  Dot Matrix Index & Overview

The print jewel is the front bearing for the pins. The jewel may literally be a ruby - an industrially made berylium or zirconium dioxide block shaped to carry the needles. Alternatives are saphire and garnet.
 
The print-jewel has to hold the pins so that they are free to travel forward and back to impact the ribbon. The pins must not be free to travel in any other direction or the print will look a mess.

The jewels role is much the same as in a mechanical watch, it has to cope with hundreds of millions of repeated actions without changing it's dimensions. Ideally it shouldn't wear away at the shaft of the pin either. 

Not every printhead uses a traditional jewel. Low cost printheads quite frequently use nylon-like plastics.  Heads built this way seem to have a significantly shorter life.

Lubrication is available from slightly unusual sources.

Lubrication.  Dot Matrix Index & Overview
 
The print-jewel may have two sources of lubrication. On older printers there are often pads of felt around the pins immediately behind it, and these may carry a very light oil - perhaps as much to prevent rust as to lubricate the jewel. The ink on the print-ribbon is usually an important source of lubricant. Printer users commonly run print-ribbons dry as they try to economise. If the ink is acting as a lubricant unning with a dry ribbon causes premature print-head failure, and print-heads are very expensive. Running with dry ribbons could be an utterly false economy.

Ink is a dubious lubricant. A printer ribbon typically runs in a dirty atmosphere, full of gritty dust and sulphur compounds. Some people eckon that dirt in the ink builds up in the jewel guide channel and erodes the pin tips.  On the other hand graphite is a major component of black ink, and an excellent lubricant. Obviously coloured inks don't have the same components.

ND640 printhead - lubricant pads in nose

Armatures.  Dot Matrix Index & Overview

The bulk of the printhead is made up of the coils to drive the pins.
 
Early heads just had a magnet core that would either center itself in the coil or eject from it when current flowed. Protruding from the core was a wire that ran into the jewel. This possibly didn't make very good use of the drive current.When coil is on armature moves to center coil
Most modern heads seem to face the solenoid backwards and then use a pawl to press on the pin and push it forward.When power is on Pwl closes on coil and forces pin forward
A hybrid approach is used in some Epson heads. The pawl carries a plunger which may help complete the magnetic circuit more rapidly.Epson Type Mechanism - Pawl has a plunger that dives into coil

The pin and any armature then have to return to position rapidly for the next firing. Older print-heads have wound steel springs to do this, but later designs commonly used rubber pads. Adjusting a screw at the ear of the print-head changes the pressure in each pad and adjusts the pin flight-time.

Residual Strip.  Dot Matrix Index & Overview

Armatures tend to stick to a coil once they are attracted to it. To prevent this a thin sheet of mylar or rubber is placed between them, this is called the "anti magnetic residual strip". In the long term repeated striking wears a hole in the residual strip and flight-timing goes adrift on an otherwise perfect print-head. Since the pin now sticks out of the head more than it should it will probably get bent, it will then rip the ibbon, jam the carriage and put the printer out of action.

Pin wear Patterns.  Dot Matrix Index & Overview

Pins wear away from two or three angles:

  • at the front from ribbon contact.
  • at the side from contact with the print jewel.
  • possible at the rear from rubbing against the pawl.


Pins are continually being dragged back and forth across the ribbon. This gradually wears away at the ribbon, but that doesn't matter much because it is a relatively low-cost consumable. The continual contact also wears away the pins, making them fractionally shorter. Pin wear is usually uneven because some pins are used more than others - in a 9 pin head the bottom two are used to produce descenders on lower-case letters, so if the printer is only used for upper case these needles are preserved at their full length. The users compensate for the wear by adjusting the platen-gap, but of course if the bottom pins do fire they are now far to long and at risk of being bent. The effect is gradual and only usually noticeable after about 5 years service.

There isn't any reliable cure for this but a bit or carborundum grining followed by a thorough wash of the pins with isopropyl seems to work. (At isk of driving manufacturers into a fit)

Wear inside the print jewel tends to turn the tips of the pins oval. Pins are forced against the left and right hand side of the jewel as the carriage moves.

Recoil.  Dot Matrix Index & Overview

In most cases the pin is returned by a spring, although rubber pads are another possibility. Recoil timing is important - a pin can't fire again until it has regained it's home position. Firing the pin out against a spring means compressing the spring and losing energy that could have gone to moving ink.

A pin could be magnetically driven both outward to impact the paper and back in to it's home position. Active drive in both directions might imply doubling the electrical power and hence the heat dissipated. Once a pin has fired, however, the coil produces a back EMF. Low cost printers just seem to dump this into a diode. Some printers have more complicated electronic resonant circuits. Click for more on electronics (This Page)

Ideally the print component moves easily and naturally at the desired times so it might have a resonance at a frequency that would give the dot pitch if it were sustained. Resonance is probably of limited use as with textual material any one pixel position on the page only has about a 1 in 20 probability of being printed.
 


MultiLayer Structure.  Dot Matrix Index & Overview

Printheads with 9 pins usually arrange them in a circle around a shaft pointing into the jewel. Heads with 24 pins usually use two layers of coils. More layers of coils would probably get inconvenient because the momentum of the pins would be different.


Printhead Cost.  Dot Matrix Index & OverviewDot Matrix General Text on Printhead Pin Action

Printheads tend to be compartively expensive. A new printhead might be anywhere between £80 and £350. Typically heads might be a third the price of a new printer. Whilst heads aren't usually difficult to fit it does sometimes need a couple of screws shifting so it might also need an engineer call-out. Dot matrix printers might be rather more popular and competitive with inkjets if printheads were less expensive. Set against this most printheads last a long time.

The long-lasting nature of heads might be part of the problem. It is sometimes said to be a consumable, but isn't held in stock and sold as one. Printheads are handled as spares. Manufacturers often take the attitude that providing a spare is an opportunity for them to profit, so they price this sort of part to give a fitted cost just under half the cost of a new printer. They justify this on the grounds that the administrative costs of their logistics systems are high.

Basic printhead mechanisms are not particularly complicated. Individual coils and pawls are very similar to the structure of a small relay costing less than a dollar. The obvious problem with making a printhead for nine dollars is that there is a huge market for little relays allowing manufacturers to set up automated production lines. The market for printheads is less substantial and quite a few are effectively handmade.

Some early matrix printers had exchangeable pins that screwed into place. (Centronics did) One problem with this approach is bulk- the resulting head is large and perhaps not as fast as it might be.

Manufacturers seem reluctant to develop and use generic heads. If generic printhead designs were more common then scale economies would be better. Unfortunately manufacturers seem to prefer printer-specific design.
 
 


Platen Gap.  Dot Matrix Index & OverviewDot Matrix General Text on ControlsPlaten Gap and Carriage Mechanism

The print pins can need a certain momentum to give the ideal impact onto the page. To do this the pin needs some freedom to accelerate and this is given by the platen gap - usually something below 2mm. If the platen gap isn't adequate:

  • Pressure on the ribbon by the head will give ink streaking on the page.
  • Printing may be fainter than it should be.
  • The probability of a pin catching in the ribbon fabric or paper and bending is greatly increased.
Printheads also get destroyed when the platen-gap is set too wide. The pins have to shoot out to far to hit the paper, so the spring stores too much energy, they recoil too far and the tip falls out of the print-jewel.

As a rough guide set the platen gap by hand so that the printhead is leaving very faint ribbon traces on the paper, then back it away from the paper one notch. Unfortunately a lot of printheads will print for hours set too far away from the paper, but then a needle recoils too far and the printhead is detroyed.

Dot matrix printheads are complex pieces of mechanics so they are expensive, buying a spare can be half the price of the printer.


Print-Head Electronics.  Dot Matrix Index & OverviewDot Matrix General Text on Printhead Electronics

Pin drive circuits may be more complex than just turning on a single voltage across a coil. Coils themselves naturally absorb electrical energy when first connected as the magnetic field is established. Coils then provide a potentially high reverse voltage known as a back-EMF when the drive voltage cuts out and the field collapses. The waveforms of voltage, current and armature movement can be quite complex. Only simple old printers operate by simply turning a single voltage on and off, faster and more recent printers aim to move the print pins faster and to use less energy by exploiting the mechanical resonance of the pins and their springs and the electrical esonances of the drive coils.

A common technique is to have a set of darlington transistor arrays, one for each pin in the printhead, connected to a power supply rail providing +40Volts. At the other side all the coils are connect to a common rail controlled by one or two darlington transistors that can connect either to 0Volts or to -40Volts. When maximum acceleration is needed to start the pins in motion the transistors put the full 80Volts over any coil. When the pins are flying and just need enough energy to hit the ribbon the common transistors switch to ground and there is just 40Volts across a coil.

Momentary currents are quite high, but not as high as the DC voltages and the typical DC resistance of 4 - 8 ohms imply (ie the peak current is more like 1 amp than 10 amps).

More elaborate pin-drive circuits use tuned circuits - coils and capacitors - to reflect the back-EMF from the printhead coil and use it to drive the pin back home. The problem with this approach is size and complexity. Back EMF is a high voltage and high-voltage capacitors are large. High voltage semiconductors are more complex, expensive and failure-prone. Some older printers just shunt the back-EMF into a diode.

Printhead coils need to generate brief but intense magnetic fields. The electric power used turns to heat and has to be dissipated. Since the print-head is continually moving there is some air-flow, and by adding aluminium fins onto the head it can shed more heat. Some printers have fans built into the carriage to cool the head.


Thermistor control.  Dot Matrix Index & Overview

Most complex heads have a thermistor built into the head so that the processor can monitor the heat level. This allows the processor to extract the maximum performnce available from the head, running it as hard as possible when its hot, then cutting back when it cools down.  When things get too hot the printer reduces it's speed or even stops until the head temperature eaches an acceptable level again. In some cases the printer stops for a while, but in most it goes on printing, just very much more slowly. The continued printing does continue to heat the head, but not as much as the continued movement cools it. After a while when the printhead cools down speed picks up again.

Users are sometimes confused by this behaviour and think something must be wrong. It wouldn't be a bad idea to put a light on the front panel to tell them.

© Graham Huskinson 2010