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X‑Y TABLE WITH STEPPER MOTOR CONTROL From the number of enquiries we receive it is obvious that there is a great amount of interest in machine control. With this in mind, we have produced a practical demonstration X‑Y table project using stepper motor control. It could be expanded to control a variety of processes and machines. First of all, though, perhaps we should explain what an X-Y table is because many readers may not have come across such a device before. Casting your mind back to school days, you will recall that a graph has two axes, the “X” axis, which is the horizontal direction, and the “Y” axis – not surprisingly, the vertical direction. Within the confines of the graph, any point can be located from the origin by giving its coordinates in terms of plus or minus X units, and plus or minus Y units. The origin, or reference point, is normally called (0,0), meaning X=0 and Y=0. The same logic – no pun intended (or was it?) – can be applied to locate positions away from an origin for just about anything, as long as you know the units being used. Map co-ordinates are just one example. Suppose we want to locate a position on a solid (flat) object? Exactly the same system applies. And this is the basis for the X-Y table. We lock the object – a piece of paper for drawing on, a PC board to be drilled, a piece of metal to be engraved – in position, and by either moving the object with respect to a fixed point, or moving something else with respect to the fixed object, we can move a pen, a drill, an engraving head, you name it, to an exact spot by giving it the X-Y coordinates. In this case, we cheat a little and place our origin (0,0) in the bottom Have you been wondering how to use the stepper motor driver cards we featured in the latter stages of 1997? We had this project in mind then and though it has taken a while, it has finally come to fruition. This series of articles will show you how to assemble the hardware and software to drive an X‑Y table. PART 1: INTRODUCTION left hand corner, so all points on the object are positive numbers (it just makes life easier to do it that way). X-Y tables are commonly used in a huge variety of applications from industry through to medicine and virtually everything in between. Our X-Y table is reasonably small by industry standards but it will be capable of doing quite large and sophisticated jobs. In the months to come, it will be extended so that it can be used to plot and drill PC boards which have been laid out using Protel. For the moment though, let us now describe the basic X‑Y table with stepper motor drive. An IBM-style computer is used as the interface between the operator and the table. It doesn’t have to be the latest whizz‑bang Pentium. A 486 or even a 386 will work quite well as long as it has a VGA graphics card fitted. While the slower processors Mechanical Design & Construction by Ken Ferguson Electronics by Rick Walters 24  Silicon Chip To be fully described next month, here is the complete X-Y table with a blank piece of PC board mounted in its clamps. Construction should be well within the capabilities of most hobbyists with basic metalworking and welding skills. will take a little longer to run a task, the stepping speed of the motors will be the limiting factor. Most programs, although written in GW Basic, are supplied as an EXE as well as a BAS file. The BAS file will allow you to readily make any changes to the software that you may deem necessary. The computer controls the dual stepper motor driver card, featured in the September 1997 issue of SILICON CHIP. The +5V and +12V supplies for this card can be picked up from an internal disc drive power connector or from an external power supply. The stepper motors we have used are 12V 1.8° types which with the hardware used, make four steps for a table movement of one thousandth of an inch (.001"). In some respects this is too fine, as it takes a while to traverse from zero to maximum but with the limited availability of threaded rods, this proved to be the optimum choice. Although Australia is a metric country, Imperial measurements were chosen as most PC board components are still laid out on an Imperial grid (ie, 100ths and 10ths of an inch). The outline of the table measures 750mm x 700mm and the X and Y axes can each traverse 300mm. Software details We shall describe the software first before we go onto the mechanical side, as this will be your interface while operating the table. The control screen is shown in Fig.1. This is the only screen for XYTABLE.BAS or XYTABLE.EXE and shows the current X and Y position of the table, along with a menu across the bottom of the screen. “Arrow keys X‑Y direction” indicates that the four arrow keys on the keyboard are used to move the table in the X and Y directions. The right arrow and up arrow keys increase the X and Y position, while the left arrow and down arrow keys reduce it. The next menu entry is “I or M ‑ units”. These keys select either an Imperial or Metric screen display of the current X and Y position. The metric display (Fig.2) is just a mathematical conversion of the inch value. If the Metric display is selected, then the X and/or Y co‑ordinate is changed and the table will only move to the closest converted Imperial measurement. If, for example, we commanded the table to move to 25mm it would move to 25.018mm or .985 inches. The next lower imperial step is .984 inches and this converts to 24.994mm which is less than the 25 called up. “X‑Y to set” indicates that by pressing, for example, the X key on the keyboard, you will be asked for the new X position. This message is shown in Fig.3. The value can be entered as a number with a decimal point (ie, 3.2 or 3.186) or without the decimal point (ie, 3200 or 3186). On the metric display, entering 43 will be interpreted as 43mm. You are then asked if you wish to alter the Y position. You may enter a value or by pressing the ENTER key, you can bypass this entry. Similarly, pressing the Y key follows the same MAY 1999  25 sequence in the reverse order. If values larger than the preset maximum values are entered, the table will move to the maximum and then stop. The menu shows another keyboard function key as “S‑Stepinc”. Pressing this key allows you to select Manual or Automatic control of the stepping increment in units, tens or hundreds of thou (U, T or H). After a value is selected, the arrow keys will only step in that increment. Fig.4 shows the screen with the three feed increments after M (for manual) has been pressed. If the “X‑Y to set” mode is used, the program will switch to automatic stepping and leave the feed set to the last automatic stepping increment. The automatic mode always steps in the largest possible increment (hundreds), stepping down to tens and units (if necessary) as it homes to the selected coordinate. Stepping rate All the functional keys described so far are shown in the menu bar at the bottom of the screen. There is one additional key which is not identified on the menu and this is the R key. It is used initially to optimise the stepping Rate. When the R key is pressed, the current stepping delay is shown with an invitation to change it; a bigger delay will slow the rate and vice versa. This value can be tested while running the program by selecting X or Y values an inch larger or smaller than the current position then reducing the value until the motors begin to mis‑step, then increasing it until they run smoothly again. The motors can be stopped at any time by hitting the spacebar (or any key). They will always make one additional step before stopping, as the instruction to look for a keypress is at the beginning of the stepping subroutine. There are two other keys which you may find useful. The HOME key will rapidly move the table to X=0, Y=0 when it is pressed and the END key will move the table to the maximum limits. These limits, along with a few other parameters which will be explained later, are initially written to a disc file (XYPLOT.FIL) using a separate program (XYSETUP.BAS or XYSETUP.EXE). We will not go into the details of the stepper card in this article. If you need more information, refer to the September 1997 issue. The card is allocated an address between 1 and 8 via a jumper on it and this allows the computer to control several cards connected in parallel to the one printer port. The selected address of this card is also written to the disc file. This file allows you to run XYTABLE.EXE but alter the values it uses, as the compiled EXE program runs a great deal faster than the interpreted Basic. The other parameters saved to the file are the X and Y positions each time the program is exited, the motor stepping rate, the selected measurement units and the printer port used to drive the card. Figs. 1-4 (left): these X-Y table control screens are fully described in the text. The difference between the first and second screens is that the first is imperial and the second metric – even though Australia uses the metric system, most engineering specifications are given in imperial units. 26  Silicon Chip When using this interface card, it is important that the program is loaded and run before the 12V is applied to the cards. When power is applied to the card, the outputs of IC2 may be high or low. This is a random function but if the Q0 and Q1 outputs were both high, Q1 and Q4 as well as Q2 and Q3 would be turned on, causing at least one transistor to self‑destruct. The others could also be seriously damaged. Relay modification To overcome this problem, we have produced an add‑on circuit with some extra logic and a relay to switch the +12V supply to the output transistors only after the software has set all IC2's outputs low. The circuit for this modification is shown in Fig.5. At switch‑on, both flipflops are reset by the 1MΩ resistor and the 0.1µF capacitor connected to pins 14 and 15, which means that the Q outputs are low. Thus D1 and D2 will hold the base of Q1 low and the transistor will be turned off. When the software is run, it first sets all the IC2 outputs low then takes IC1‑Y6 low and high then IC1‑Y7 low and high. These outputs are normally high but again, at power‑up any one output could be low. This is why we toggle two outputs (to be sure, to be sure). As each flipflop is clocked the Q output will go high. The 1kΩ resistor will now pull the base of Q1 high, which will energise RLY1 which feeds the 12V supply to the output drivers. If you already have this card you could build the circuit up on a piece of perf board and mount it on the PC board in the vacant area adjacent to IC2. Maximum stepping rate The maximum motor stepping rate will vary, depending on several factors: the applied motor voltage, the motors themselves and the computer’s clock speed. We need to step the motors as fast as possible but there is a problem. If the maximum stepping speed was set to suit a 486, then if the program was run on a Pentium, it would step the motors so quickly that they would not be able to respond and would just sit there chattering. We found values around 190 worked well with a 386 using GW‑Ba- sic and 1950 when using the EXE file. Use these values as the starting point for faster computers. Don’t contemplate running the BASIC program for anything but testing your software modifications as it is FAR TOO SLOW to be useful. While XYTABLE is useful for manoeuvring the table and getting the feel for the system, it is not much use if you wish to move it through a sequence of positions over and over. To this end, we have produced another program called XYREAD.BAS. This is capable of reading a sequence of positions which you have tabulated and saved as a file. It has not been converted to an EXE file as you will obviously wish to modify it to add your particular requirements to it. We have made the table move to the X‑Y position it reads from the file then the computer will beep, waiting for a keypress. It will then move to the next set of co‑ordinates it reads and beep. The opening screen for this program is similar to that of Fig.1 except that instead of a menu across the bottom of the screen, you will be asked for the name of your file. To assist you we have included a Here is a close-up of one of the two stepper motors and drive mechanisms for the X-Y table. The stepper motors themselves are commonly available 12V, 1.8° types which with the hardware used, make four steps for a table movement of one thousandth of an inch. That’s pretty good accuracy by anyone’s standards! MAY 1999  27 file named XYTEST.MOV which has a sequence of X‑Y movements. The file structure is based on that used by NC drills but without tooling information. It consists of an X location followed by a Y location. If either location stays the same on the next step only the new value is printed. A brief extract of a typical file would look like this: X04125Y008 X045 X00825Y0065 X00975 Y039 As you can see, it consists of one X‑Y instruction per line. All dimensions are based on 99.999" being the maximum allowable value, although the decimal point is omitted. Thus X04125Y008 defines X at 4.125" and Y at 0.8". If the X value was to remain the same the next entry (on the next line) could be Y00775. Trailing zeros are omitted. Once the end of the file is reached the table is homed to 0,0. This is just a precaution in case XYPLOT.FIL is corrupted, as this file stores the last X and Y co‑ordinates before the program is exited. A file like this can easily be created with a text editor using non‑document or ASCII mode to save it. We have used the MOV suffix for our file but you may choose whatever you find logical. However, you should always add a suffix, as it helps to identify or Fig 5: this add-on circuit for the stepper motor controller will prevent damage if two outputs are high at the same time. It can be built on a scrap of perforated board or even blank PC board. group files (DIR *.MOV), especially if you don’t create a special subdirectory. The seven files, XYREAD.BAS, XYTEST.MOV, XYTABLE.BAS, XYTABLE.EXE, XYSETUP.BAS, XYSETUP. EXE and XYPLOT.FIL are available free from our web site, or on floppy disc (price is $7.00 including p&p from SILICON CHIP). If you don’t have a subdirectory called BAS on your hard disc, create one (from c:\ type MD BAS then press Enter). Copy the seven files to this directory, and either add C:\BAS; in your path statement or change to the BAS directory (CD \BAS) to run the programs. Naturally, if you edit the BAS programs you can change the file location in line 6030 to suit yourself. Next month, we will give details of construction for the X‑Y table. See SC you then. This photo shows three projects: (right) the power supply for stepper motor cards (December 1997), and left, the controller for two stepper motors (September 1997) mounted in a case, together with the single stepper controller (August 1997) which will give X, Y and Z control. Whoops! Have we let the cat out of the bag? OK, an X-Y-Z table is planned for a future issue! 28  Silicon Chip