THE THERMAL PRINTER APPLICATION MANUAL

Transcription

1 THE THERMAL PRINTER APPLICATION MANUAL 1. INTRODUCTION PURPOSE THERMAL AND DIRECT THERMAL PRINTING TECHNOLOGY THERMAL SERIAL AND THERMAL LINE PRINTING TECHNOLOGY THERMAL LINE PRINTER MECHANISM DESCRIPTION MOTION BLOCK HEAD BLOCK The thermal printhead Electrical driving of the TPH SENSORS Head-up sensor End of paper sensor THERMAL LINE PRINTER DRIVING SUGGESTED ARCHITECTURE FOR DRIVING A THERMAL PRINTER FROM A MICRO-CONTROLLER MICRO-CONTROLLER SIZING PERIPHERALS REQUIRED HARDWARE STRUCTURE thermal printhead signals sensors signals Stepper motor signals Rev. Date Page Revision item A 25/01/97 - First issue B 20/02/ Microcontroller driving Thermal Printer application manual - Rev B 1

2 1. Introduction 1.1. Purpose The purpose of this document is to provide general information about direct line thermal printing, in order to get familiar with this technology and to optmize customer s application Thermal and direct thermal printing technology There is basically four types of popular ways to print on paper. The first one and oldest is impact derivated from type writing system, that is using standard paper with a carbon ribbon. The second one is laser printing using LED and toner, also printing on standard paper. The third one is inkjet, where ink bubbles are projected against the media, also standard paper. The fourth one is thermal printing where heat is used to blacken a heat-sensitive paper. From this technology is derivated thermal transfer, to print on standard media, where a ribbon, inserted between the thermal head and the media is able when heated to tranfer its ink onto the media. This also allows to make colour printing. The direct thermal printing technology (by opposition with thermal tranfer), is a technology where the paper is directly in contact with the print head. This paper is pre-treated so that when heated, it gets black Thermal serial and thermal line printing technology There are two main types of direct printing technology. The first one is serial line, where a mobile head with a few of dots installed vertically, are moving horizontally printing a few lines after the shuttle head finishes to cross the entire paper width. The print cycle is then to make the shuttle head move along the paper width and activate the dots simultaneously, and then, make the paper advance for the height of the head. Since the movement of the shuttle head requires time, and because the mechanical organization of this movement is relatively complex, the printing speed is low and the reliability of the product is not so good. In the other hand because the thermal head has only few dots, the cost of such a mechanism is relatively low. The second one is thermal line printing technology (subject of this document), where a complete horizontal line of dots can be activated in one shot. Then the print cycle is only to activate the dotline and make the paper advance for one dotline. Because of this simple driving, the print speed is at least 3 times faster (minimum 50 mm/s) than with shuttle head and reliability is very high (up to 100 kms of printing). Cost is higher than shuttle head mechanisms, but in the recent past years it is becoming closer and closer because the fax market is driving this technology to very low cost applications. Thermal Printer application manual - Rev B 2

3 The following chart summurizes the various differences between these two technologies: Serial Line Printing Speed Reliability - + Cost + - Print quality, Resolution Applications Calculator, EFT,... Fax, ECR,EFT, Label printer, Thermal line printer mechanism description Basically a thermal printer mechanism is made of three different blocks: motion block, head block, and sensors block Motion block The function of this block is to advance the paper in printing direction. The motion is achieved in most of cases by a stepper motor, because it is very easy to control the position of paper only by sending pulses to this motor. A stepper motor is made of magnets and 2 windings, and it is driven by logical level. The number of combination is four (2 possible levels on 2 windings). Each of this logical level corresponds to a step, because each time the configuration of these logical signals changes, the stepper rotor turns of a predetermined angle, making very easy to control its position. Repeting this combination of logical signals along the time, makes the stepper turns continuously, and the speed of the stepper motor is then directly related to the frequency of incoming logical signals. So, it is possible to change the stepper speed by changing the logic signals frequency, making this motion block very flexible. Finally, reversing the stepper direction is made by inverting the logical sequence on one of the two winding. The only point to pay attention to, is the mechanical resonnance that may happen at certain speeds, depending on many parameter such as voltage supply, paper roll inertia, pulling force on the paper (in paper rewinder application). Then the logical levels are organized as follows: First step Second step Third step Fourth step First Winding Second Winding After the fourth step, going back to first step will keep the stepper going on. Thermal Printer application manual - Rev B 3

4 The current injected into the stepper motor is generally between 200 and 400 ma per winding. Then there are from an electrical point of view three main types of stepper motors: Monopolar stepper motor: this kind of stepper is the easiest to drive but the poorest in performance (torque, speed and volume). It is made electrically by four windings (the two basic windings are divided into two windings), all first terminal of them are connected to the positive voltage of power supply and the other ones (4) are connected to one output of the stepper driver. Since the second terminal of all windings are already connected to the power voltage the electronic drivers to be used are open collector transistors circuits plus clamp diode, like ULN2XXX series, or discrete components,. Bipolar stepper motor, voltage mode: the two stepper s winding current direction can be reversed, because all windings terminal (4) are accessible. In this case, the bandwidth is increased giving higher speed, lower volume that is especially usefull in low voltage application (5 volts). The kind of electronic drivers to be used are dual bridges ICs like L293D (SGS, TI,...) or for low voltage applications, LB1831/1631 (Sanyo), HA13421 (Hitachi). Bipolar stepper motor, current mode: When very high performance are required, is is possible to adjust the current inside the stepper motor in order to achieve high speed or high torque application. Because the stepper s coil is primarily in inductance, it is possible to adjust the current by using chopper driver IC s. In the two previous cases, the current inside the windings was adjusted by the coil resistance itself. In the current mode, the coil resistance is very low (about ten times less than the value of the monopolar and bipolar voltage mode value), and the IC adjusts the current into the coil by measuring it with a sense resistance, and injecting high frequency square waves (several Khz) in order to limit the average current value. The kind of electronic drivers to be used are dual bridges chopper ICs like PBL3717 (SGS, Ericsson,...) or L6219 (SGS). The stepper motor gear is then mechanically conected to gears which makes the adequate demultiplication and drive the rubber roller Head block The thermal printhead The thermal head is made of an hybrid substrate on which are deposited gold lines, a resistance dotline and IC s to control the driving of the dotline. On the surface of the printhead is deposited a layer of glass to protect the dotline from abrasion. The dotline is divided in heat elements depending on the resolution of the head and the printing width. For instance a typical head of 8 dots/mm, in a 2 inches thermal printer application (54 mm effective printing width) is divided in 54x8 = 432 dots. By applying the power voltage on one dot of the thermal line through the IC s, the temperature of the dot increases and blacken the paper that is between the rubber roller and the thermal printhead (TPH). Thermal Printer application manual - Rev B 4

5 Dotline Driver IC Connectors Substrate Paper Rubber roller IC The energy applied depends on voltage, resistance dotline, time of activation (Joule effect), substrate temperature. APS thermal line printer mechanisms are featuring outstanding characteristics such as: Very long abrasion life (100kms) due to its thick film technology Low spread in resistance value (+/-3%) allowing a precise control in dot energy and activation Wide range of power supply (from 18 to 26 volts for 24 volts TPH) Compacity thanks to the direct connection of connector on substrate Very good resistance against energy overload (about 50% margin) High resolution: standard is 8 dots/mm up to 16 dots/mm Electrical driving of the TPH The driver IC are usually powered under a logic voltage (5v +/- 5%), so that they can be directly connected to I/O of a microcontroller. Then, the power voltage is applied to the dotline, and each selected dot is activated by the IC driver after its value is being loaded into the TPH. Each driver IC is able to drive 64 or 144 bits depending on the head. The electric circuitry of the TPH is divided into two stages of synchronous shift registers plus open collector transistor in order to drive each dot. Then, a thermistor is also included in the electronics in order to monitor the heating time depending on the temperature of the substrate. Thermal Printer application manual - Rev B 5

6 In the following example of a 2 inches TPH, 3 drivers Ics of 144 bits each are used to drive the 432 = 144*3 dots of the TPH. Head activation is shared among 3 strobes (1st to 144th dot, 145th to 288th dot, and 289th to 432th dot). V Supply dot 1 dot 2 dot 432 Strobe 1 Strobe 2 Strobe 3 Latch Clock Data in AND AND AND LATCH REGISTER (432 bits) 3 Driver IC s 144 bits Each The dots activation can be shared among several strobes signal in order not to have a too important current flowing from the power supply into the TPH. The head activation sequence can be divided into three steps: 432 dots values loading with the synchronous communication (Clock & serial data lines): Tv CLK Clock Tsetup Din Thold Din Data In Datas transfer into the latch register (Latch line): Latch Tw LAT Dot activation (Strobe lines) can be done one after other or all together, depending on application : Thermal Printer application manual - Rev B 6

7 Strobe Driver Out Td0 Td0 So, after each sequence of load and activation, the stepper is to be activated in order to make the paper advance of one dotline. Paper advance depends on head s resolution and is equals to the dot height (8 dots/mm gives 1/8=0.125 mm paper feed pitch). Generally, the number of corresponding steps is one step to make the paper advance of one dotline, but in some applications (mainly low voltage), it can go up to 3. The big advantage of having these two stages (shift register + latch register) is that during dot activation, a new dotline can be loaded into the shift register, allowing to parallelize step 3 (dot heat) with step 1(datas loading). The only point to pay attention to, is to keep the logical signals (clock, datas and latch) at a stable state, during 10 us before and after any strobe line change in order to prevent any perturbation or error in dot value loading, this because strobe is driving a high current Sensors Head-up sensor The printer mechanism is usually provided with a lever in order to be able to separate the TPH from the rubber roller. This is especially usefull to clear paper jam and to load the paper. A sensor (generally a mechanical switch) gives the status of this lever. The signal of this sensor can be directly interfaced with a I/O of a microcontroller End of paper sensor In order to detect the presence of paper, one additionnal sensor (generally a reflective opto-sensor) is also provided. This prevents the head from heating directly the rubber roller, and this is very important to handle this signal carefully because heating directly the rubber roller may affect its geometrical characteristics and also the print quality. It is advised to check after each dotline printed the status of these two sensors, to secure the use of the printer. Thermal Printer application manual - Rev B 7

8 3. THERMAL LINE PRINTER DRIVING This chapter explains how to coordinate the three blocks of thermal printer: motion block, TPH and sensors. Basically, the print principle can be described for one dotline, and then repeated as long as printing is needed. the following timing gives the general timing to drive a printer as described in the previous section: 2 inches thermal line printer mechanism one step per dotline 432 dots + thermistor opto-sensor (end of paper) + lever (switch) Data in 432 dots loading Clock Latch Strobe 1 Strobe 2 Strobe 3 OR Stobe 1/2/3 Stepper ONE STEP CYCLE TIME = ONE DOTLINE Next Step After every cycle time, check is needed for paper end status, lever position and thermistor. The thermistor will allow to stop printing if temperature exceeds maximum temperature (usually 65 C), and adjust heating time for the next dotline. By overlapping loading of dots into the TPH and strobe, printing speed can be increased (when current dotline is heated, next dotline is loaded into the shift register). Thermal Printer application manual - Rev B 8

9 4. Suggested architecture for driving a thermal printer from a microcontroller 4.1. Micro-controller sizing Most of the thermal printer mechanisms are driven from a microcontroller. For the most common applications the power of a 8 bits microcontroller running from 8 Mhz external clock is enough. Typical microcontroller is 8051 (Intel,..), 68HC05 or 68HC11 (Motorola), ST9 (ST), H300 (Hitachi). For high speed and/or high resolution application or when a large graphics capability (so memory adressing, and speed) is required, some applications may require 16 bits micro-controller like 68HC12 or 68HC16. the first category mentionned is covering 90% of the applications Peripherals required No specific integrated peripherals are required to drive a thermal mechanism. The most common and popular way is to use only a Serial Synchronous Communication Interface to load the dotline into the termal printhead. the reason is that it is required for each dotline to load the thermal printhead with the datas, and this is to be done in a reasonnable time (about 1ms). So, reasonnable communication speed must be about 500Kbits (448 dots, 2 inches thermal printhead loading speed is about 1ms/448#2.2us about 450 Kbits). This kind of peripherals at this speed are present on almost all versions of standard 8 bits micro-controller on the market. Usually, and because the micro-controller needs to be interfaced on a communicaion port (to load characters or graphics to be printed), the Asynchronous Communication Interface is used to interface a RS232 (or equivalent) incoming signal. Thid kind of interface is also present on most of the above mentionned type of microcontroller. When parallel loading is required (high speed and/or graphics applications) the expanded bus can be used or standard I/O as long as handshake is available or emulated by external hardware (centronics). So, having a synchronous communication port on the micro-controller is the only requirement to drive a thermal printer mechnaism at a standard speed of 50 mm/s Hardware structure The signals to behandled on the thermal mechanisms can be divided into 3 main types: Thermal printhead signals Sensors signals Stepper motors signal thermal printhead signals Clock and Data in signal are to be handled by the synchronous communication integrated peripheral as described above Latch signal is a standard I/O line (one pulse from time to time) Thermal Printer application manual - Rev B 9

10 Strobes are standard I/O lines (pulse length is about 1ms, so is no problem). Note that the software driving of this signal can be handled by software interrupts, using internal timer. This is the best way to drive this signal because while heating, the microcontroller can do others tasks, like reloading the next dotline, or handling communication with host. One usual precaution is to use an internal or external watchdog system in order to generate a reset on the microcontroller in case the heating time exceeds a pre-determined value, in order not to destroy the thermal head in case of software or hardware problem (due to EMC or any other external perturbation). Data out signal is generally used to make historical control in case of high speed application (above 60mm/S for 24 volts printer, and 40 mm/s for 5v printer). Basically, to make historical control is to apply two different levels of energy (for one level of historical control) depending on the previous stage of the dot currently heated. If the current dot was heated the dotline before, then a shorter heating is applied to this dot, than the one applied to a dot than was not previously activated. This requires a double load of the dotline for every dotline to be printed. One common way to make this function is to: 1/ first load the complete dotline with the combination of the previous line. this is the historical heat. If a dot needs to be black on this line and if this dot was previously activated, then a 0 is loaded. If this dot was previously white a logic 1 is loaded. Of course if the dot is white a 0 is loaded. There are two ways to achieve this selection between dot previously activated or not. The first one is do the job by software, that is no so difficult, but requires some time and memory for the CPU. The second way is to use the dataout signal in order to comibine with an exclusive OR (XOR), the incoming datas with the outcoming datas from the thermal head and AND the result with the incoming datas (to prevent loading 1 if previous dot was 1 and current dot is 0 ). Then if a dot was previously heated and needs to be so again, a 0 will be loaded into the head,making the job very easy. 2/ After this special data loading is achieved, a strobe shorter than the standard one (usually about 35% of the nominal heating time) is to be done. During this short heat, it is needed to load again the complete dotline, with all dots to be blacken (heating time generation under interrupt control is then useful). This is easy to do by software (as above mentionned) or by loading again the same dotline than the first one and disabling the XOR function (one logic signal, I/O of microcontroller is fine). 3/ Then, when both, short heat and new loading and data latch are done, dots are activated for a longer pulse (about 65% of nominal heating time). There are several ways to handle and optimize these signals, plesa contact APS for any further application sensors signals There are generally two sensors signal: head-up sensor (for head-up state detection) and opto-detector sensor (for paper end detection). Head-up sensor: this signal can be directly interfaced to an input (I/O) of the microcontroller via a pull-up or pull-down resistor (refer to appropriate thermal printer mechanism technical reference) Thermal Printer application manual - Rev B 10

11 Opto-detector: the opto-detector is made of one infra-red LED that is to be crossed by a DC current (usually about 20mA; refer to technical reference). This is done by connecting a pull-up resistor (som hundreds of Ohms to the anode of the photo-diode, because the ground is usually common between emitter an receiver of the sensor. Then the collector of the photo-transistor can be connected either to a pull-up resistor and then to a analog or standard input (I/O) of microcontoller (after level compatibility is checked), or a one transistor circuit (see thermal printer technical reference) sets the level to TTL or HCMOS levels, and is then connected to the same kind of I/O port Stepper motor signals These signal are three or four logical signals depending on the circuits used to drive the stepper motors. These logical signals are usually standard I/O of microcontroller since the pulses length to be issued are the length of the stepper step not less than1ms. These signals need to be synchronised with the dot activation as previously described in this document. Then, they are usually driven by software interruption, like the strobes signals. There are usually three logical signals to drive a stepper motor integrated circuit controller (when using Ics like L292D, PL3717,...). the first two signals are the phase signals (as described in the previous section) andthe last one is a current enable, to disable the current into the winding of the stepper motor, when not used, and to avoid overheating. This overheating is especially to be taken into account when using compact thermal mechanisms, because the stepper s small size is very sensible to overheat (above 15% ON/OFF ratio, the stepper may reach too high temperature). When using PBL3717, four bits may be needed to adjust the current on the stepper s windings (2 bits for 2 phases, and four levels of currents driven by 2 bits). The case of four signals is usually when the driver interface between I/O port of microcontroller and the stepper motor is very basic, like open collector circuits (discrete transistor plus clamp diodes or ULN2XXX series IC). Then, the four I/O ports are directly interfacing the stepper motor phases. Thermal Printer application manual - Rev B 11