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LATH-E-BOY: An Intelligent Touchscreen Lathe Speed Controller This design combines two very popular projects, the Induction Motor Speed Controller and Micromite Plus Explore 100 with 5-inch Touchscreen, then adds some other circuitry, to provide an easy way to control a lathe. It automatically adjusts its speed to suit the material which is being turned and provides a constant display of the lathe’s status and allows its speed and direction to be selected. M ost lathes, apart from small wood-turning lathes, are powered by an induction motor. The problem with using an induction motor is that up till now, the usual ways to control lathe chuck speeds involved belts and stepped pulleys or a gear box. While they are still useful, it is now possible to control chuck speed and direction using our 1.5kW Induction Motor Speed Controller (IMSC), which was originally published in the April and May 2012 issues of Silicon Chip (see siliconchip.com.au/Series/25). But as well as providing those functions, why not provide extra features such as a speed read-out, touch-screen control interface and so on? That’s all doable by building an Explore 100 with the 5-inch touchscreen and then programming it to control the IMSC. As you can see from the screen grabs in this article, the Lathe Controller interface is quite simple to use and saves Design by Peter Bennett 36 Silicon Chip you quite a bit of time and effort since all you need to do is specify the material type and diameter and it will automatically select a suitable motor RPM. You can then adjust this further if necessary. And having selected the material type and/or spindle speed, you can then control the motor direction and fine-tune the speed, while monitoring the actual RPM. This article gives all the details on how to add the extra circuitry required to the IMSC and Explore 100 and hook Words by Nicholas Vinen Celebrating 30 Years siliconchip.com.au them up together, and to the lathe, to achieve this level of control. Circuit description The circuit for this project is shown in Fig.1, overleaf. It is broken up into several blocks, to reflect the physical layout of the system. The large block at centre right represents the Micromite Plus Explore 100 unit, with LCD touchscreen. This is housed in a large Jiffy box, along with a few passive components, an optocoupler and four transistors. These components interface the Explore 100 to the rest of the circuitry required to control the induction motor. Those connections are made via two Cat5 cables which are plugged into 8-pin RJ-45 sockets CON2 and CON3 (note that CON3 only uses six of the eight available wires). Pins 1 & 2, 3 & 4, 5 & 6 and 7 & 8 are connected to the twisted pairs within the cable (but note that not all Cat5/6 cables are wired like this). The connections made over Cat5 use current loops and, in the case of the motor speed control signal, 4kHz pulse-width modulation (PWM). It has been designed this way to allow for relatively long cable runs (of up to 50m). In most cases though, those cables will be a few metres at most. With CON2, all eight connections between the two main modules (the IMSC Interface and the Explore 100) are optoisolated so that ground loops are not an issue, despite the possibility of a large distance between the units. This also prevents ground shifts due to the long wiring from affecting the accuracy of the control signals. CON3 connects the Explore 100 control box to the relay box, which is wired between the outputs of the Induction Motor Speed Controller (shown at left) and the induction motor itself (at bottom right). The relay box switches the two windings of the motor to control start-up and direction of rotation. The three high-current mains relays are driven by NPN transistors Q1-Q3 within the control box, via the 6-wire cable and each relay has a coil backEMF quenching diode. When RLY2 switches on, it energises the motor start winding. When RLY1 is switched on, it reverses the polarity of this winding, so the motor will start spinning in the opposite direction. As its name suggests, the start winding is only energised when the motor is first started, hence the relay. After that, the start winding is disconnected so it doesn’t burn out. The motor keeps spinning in the direction that it started. RLY3 is used to energise the Run winding. You may wonder why this is necessary since the Induction Motor Speed Controller can be switched on and off. When the IMSC is switched off, it will slowly spin the motor down at the programmed ramp rate. By disconnecting the run winding from the IMSC, the lathe motor will spin down more rapidly and naturally, improving safety. Three LEDs are also fitted into the box housing the Explore 100, labelled Reverse (yellow), Start (red) and Run (green). These are effectively wired in parallel with the three relay coils (via CON4), with 560Ω current-limiting resistors in series with each LED. These provide feedback on what the motor is doing. Speed Controller interface Now turning our attention back to the control circuitry around the Explore 100 and the second Cat5 cable, this is wired to a small box attached to the side of the IMSC which provides an Screen1: the setup screen appears when the Controller is first powered on and allows you to set either the material type and diameter or the chuck RPM. siliconchip.com.au The IMSC, interface circuitry, relay box and plugpacks were mounted on the rear of the lathe stand, with the touchscreen controller box up on top. isolated interface to it. A small, separate circuit board labelled “output frequency sense” is fitted inside the IMSC enclosure. Let’s take a look at this first. This is connected across the U and W motor outputs which power the main “run” winding. The differential voltage between these outputs passes through an RC low-pass filter comprising two 5.1kΩ 1W (mains-rated) resistors and a 220nF X2 mains capacitor. This has a -3dB point of around 71Hz so it filters out the IGBT switching edges. The resulting sinewave signal is then applied to the infrared LED within a PS2501 Darlington output optoisolator. D4, a 1N4007 diode Screen2: once setup is complete, it switches to this screen where you can start, stop and reverse the motor, monitor chuck speed and tweak it if necessary. Celebrating 30 Years January 2018  37 Fig.1: circuit diagram of the Lathe Controller, with the Induction Motor Speed Controller (at left) and Explore 100 (centre right) circuits shown as “black boxes”. See the relevant articles (referred to in text) for internal details. The additional circuitry ties these two modules together as well as providing motor speed feedback, safe motor starting and reversing, feedback-based speed control and status indication. connected in inverse parallel with this LED prevents it going into reverse breakdown for one half of the output phase. This means that the output of the optocoupler is switched on to produce one pulse for each AC cycle fed to the motor. The two extra 5.1kΩ resistors limit the LED current to around 17mA [350VDC(peak)÷(4 x 5.1kΩ)], which is well within the 80mA rating of the device. The output pulses from the frequen38 Silicon Chip cy sense circuit are fed right through the IMSC interface box and back to the Explore 100 unit via pins 1 and 2 of the Cat5 cable. One end of this signal is terminated to the Explore 100’s local ground while the other has a 1.2kΩ pull-up resistor to the 3.3V rail, giving a 3.3V square wave signal. This square wave is filtered using a 120kΩ/1µF low-pass filter, before being fed to pin 11 on the Explore 100 I/O header (“read RPM”). The PIC32 (Micromite Plus) in the Explore 100 Celebrating 30 Years can then count the number of pulses on this pin each second to determine the spindle speed. This RC filter has a time constant of 120ms which may seem quite long, with respect to the 50Hz waveform when the motor is running at full nominal speed, with a 50Hz output. However, the filter has to cope with a pulse rate from 50Hz down to about 5Hz, so the 120ms time constant seems to be a reasonable compromise. As well as measuring motor speed, siliconchip.com.au the Micromite also needs to be able to control the speed. This is done using the PWM output on pin 22 of the I/O header (CON1), which drives the base of NPN transistor Q4 via a 1kΩ current-limiting resistor. When Q4 is on, it pulls current through the upper LED in the HCPL2531 dual high-speed optocoupler within the IMSC Interface module (OPTO2). Because the emitters of the two output transmitters are joined together, we’re only using half of this device. siliconchip.com.au The collector of the output transistor at pin 7 is connected to a 3.3V rail output from the Induction Motor Speed Controller while the emitter at pin 5 has a 1kΩ pull-down to the analog ground of the IMSC, resulting in a 3.3V square wave at pin 5 of OPTO2. This passes through an RC low-pass filter of 4.7kΩ and 10µF, having a -3dB point of 3.4Hz. This smoothes the PWM waveform to produce a variable voltage that depends on the PWM duty cycle. The variable voltage is then fed Celebrating 30 Years to the control input (Vin) of the IMSC. The 3.3V and GND rails for this part of the circuit are connected only to CON4 on the IMSC so that digital noise on other pins does not unduly affect the analog control signal. There is a second, Darlington output optocoupler within the IMSC interface (OPTO3) which drives the RUN-bar input at CON5 of the IMSC, enabling or disabling the motor output. A 1kΩ pullup resistor to 3.3V sets the default state to have the motor switched off. January 2018  39 It only switches on when pin 7 on the Explore 100 I/O header goes high, allowing current to flow through the emitter LED within OPTO3. The LED current is set by a 470Ω resistor between this LED cathode and ground. When pin 7 goes high, OPTO3 switches on, pulling RUN-bar low. The OUT terminal on CON6 of the IMSC is pulled low by the speed controller when the motor is up to speed. This is fed through the IMSC interface to arrive at pin 2 of OPTO1, the cathode of its internal emitter LED. The LED anode is connected to the 3.3V supply rail of the IMSC via a pi filter consisting of a 100nF capacitor, a 10nF capacitor and a 110Ω resistor which also acts as a current limiter. Thus, when the motor is up to speed and OUT is low, 30mA will flow through this circuit, switching on OPTO1 and pulling its output pin 4 low. This is normally held high by a 270Ω resistor and this signal is fed to pin 13 of the Explore 100 I/O (“Up To Speed”) so that it can be sensed by the Micromite. Remaining circuitry Earlier, we described how RLY1RLY3 are used to start the motor spinning in either direction and then to allow it to continue to run. The coils of the three relays are driven by NPN transistors Q1-Q3 which are in turn controlled from I/O pins 21, 23 and 25 on the Explore 100. Each has a 1kΩ base current limiting resistor and a backEMF quenching diode connected across the relay coil. Indicator LEDs1-3 are connected in parallel with the relay coils, each with their own 560Ω current-limiting resistor. So these LEDs light up to indicate whether the motor start or run winding is energised and to show which direction the motor is running. The rest of the circuitry comprises the mains power supply and motor wiring. The 230VAC input plug Earth connects to the Earth terminals on the IMSC and the motor housing. Active and Neutral pass through a double-pole power switch and then onto the input terminals of the IMSC and two plugpacks. The 12V plugpack powers the relays while the 5V plugpack powers the Explore 100. The rest of the circuitry draws power either from the regulated supply rails within the Explore 100 or the IMSC. The three IMSC outputs are wired up 40 Silicon Chip to the terminals of relays RLY1-RLY3 and in some cases, directly to the motor terminals. See the panel elsewhere in this article describing how the motor connections are made. As mentioned earlier, two of the three motor drive outputs (U and W) are also connected to the Output Frequency Sense circuitry. Software operation Fig.2: here’s how the designer’s lathe motor was wired up to the speed controller, ignoring the relays which control start-up and reversing, for the moment. The start capacitor is shorted out since it’s no longer required. Note the two possible ways to wire up the one end of the start winding. The main goal of this project was to have a supervisory control for the lathe, into which could be entered the material type and diameter to be turned. The software would then set the required speed and would control the lathe to maintain that speed, making the turning process much simpler. The Explore 100 with 5” touchscreen provides the ideal platform. The set-up screen is shown in Screen1. It provides auto and manual RPM control modes. In auto mode, the user selects material and diameter and the controller does the rest. If manual mode is selected, the user sets the speed regardless of material and diameter. Once the selection has been made, the operation page is displayed, as shown in Screen2. FORWARD, REVERSE and OFF are self-explanatory. The spinbox “Tweak RPM on-the-fly” allows the user to switch to manual mode and adjust the motor speed. Target RPM is the speed we want while Actual RPM is the inferred motor speed, based on the frequency measured at the motor controller output. This is an excellent proxy for the spindle RPM, as verified with a temporary Hall Effect pickup on the tool post and a magnet on the chuck. The material and diameter selections are repeated on the Operation page. The three square “radio” buttons in the lower right corner tell the software which of the three motor belt pulley This shows the wiring between the IMSC and interface box. The speed feedback board is just visible below the main PCB. Note the improved ventilation. Celebrating 30 Years siliconchip.com.au Parts list – Lath-e-Boy Lathe Controller The pre-existing direction control switch box, which was wired to both motor windings. positions is in use, as this affects the maximum and minimum RPM values. The lower speed pulleys are used only if additional torque is required at low speed. (A radio button is like a checkbox except only one in a group can be selected at any given time.) To ensure the software is responsive, pretty much all events are handled in interrupt routines, including the touchscreen interface, which utilises the TOUCH (REF) function. The motor speed is sensed by measuring the intervals between an interrupt triggered by the level change on the READ RPM input. Motor speed control is achieved us- This junction box connects the Controller outputs to the motor. siliconchip.com.au 1 Induction Motor Speed Controller kit [Altronics Cat K6032] 1 Micromite Plus Explore 100 kit [SILICON CHIP Online Shop SC3834 or from www.rictech.nz] 1 5-inch diagonal colour LCD to suit Explore 100 [eg, siliconchip.com.au/link/aaig or siliconchip.com.au/link/aaih] 3 10A 250VAC DPDT relays, 12V DC coil (RLY1-RLY3; [Jaycar SY4065]) 3 DPDT relay cradles (optional, for RLY1-RLY3; [Jaycar SY4064]) 2 10A mains cables, cut in half (for mains input and to connect plugpacks) 1 12V DC 500mA regulated plugpack 1 5V DC 1A regulated plugpack 1 10A 250VAC DPDT toggle switch (S1) 1 10-way connector with matching plug and cable (to connect IMSC interface to Speed Controller) 4 RJ-45 modular connectors 2 Cat5(e)/Cat6 cables with twisted pairs 1&2, 3&4, 5&6, 7&8 1 large solder type protoboard (cut up as required) 1 large jiffy box (for Explore 100 and associated components) 1 medium-sized jiffy box (for IMSC interface) 1 diecast aluminium box (to house RLY1-RLY3; must be earthed) various lengths and colours mains-rated and light-duty hookup wire Semiconductors 3 PS2502-1 Darlington optocouplers (OPTO1,OPTO3,OPTO4) 1 HCPL2531 dual high-speed optocoupler (OPTO2) 4 BC337 NPN transistors (Q1-Q4) 1 yellow 5mm LED (LED1) 1 red 5mm LED (LED2) 1 green 5mm LED (LED3) 3 1N4004 1A 400V diodes (D1-D3) Errata involving incorrect colour 1 1N4007 1A 1000V diode (D4) Capacitors 1 10µF 10V electrolytic 1 1µF multi-layer ceramic 1 100nF MKT or ceramic 1 10nF MKT or ceramic 1 220nF X2 MKP coding for the induction motor has been applied (39, 41 & 43) Resistors (all 0.25W 1% unless otherwise stated) 1 120kΩ 4 5.1kΩ (1W 5%) 1 4.7kΩ 7 1kΩ 3 560Ω 1 240Ω 1 110Ω ing a simple proportional feedback strategy. A closed loop continually measures the error and reduces it. Effective gain of this loop is controlled by selecting the time between corrections and the proportion of error applied to each correction. These numbers are determined by experiment and are quite flexible. Settling time and stability are completely adequate for the purpose. Since the source code is available, the software can be modified by those who would like to adapt it for their own projects. The only niggle is the loading time of the title or “splash” screen. This takes nearly 12 seconds to load from the micro SD card. Perhaps it should be called the “drip screen”! This is due to the way that Celebrating 30 Years 1 470Ω 1 270Ω MMBasic loads data off the SD card. Construction You will need to build and test the Induction Motor Speed Controller and Explore 100 modules separately before you can build the extra circuitry which ties them together. If you’re building the IMSC from a kit, it should come with assembly instructions. Otherwise, refer to our articles in the April and May 2012 issues, plus the additional information and revisions in the December 2012 (siliconchip.com.au/Article/469) and August 2013 (siliconchip.com.au/ Article/4219) issues. For the Explore 100, assembly instructions are in the October 2016 issue; the only tricky aspects are solderJanuary 2018  41 Modifying the motor to allow the speed controller to be connected It is worth reading the April 2012 article so the motor will start forward or reversed be experimentally connected to A, then to on the Induction Motor Speed Controller as required. C. The better of these options is typically to get a background of induction motors Even with the start winding isolation and the one that starts to turn the motor at the and a description of the Controller. While direction taken care of, the subject motor lower voltage. It does not matter whether its main purpose was to vary the speed of would not start, as the Speed Controller the motor starts in the forward or reverse pool pumps, it was also suitable for the tripped out with a fault indication. Over cur- direction. The direction of rotation can be control of machine tools, such as lathes. rent was a prime suspect. Certainly, the in- controlled with the forward/reverse relay Most basic lathes vary the speed of the stantaneous current on starting is enormous or a winding polarity reversal. chuck by changing belts, an inconvenient – at 230VAC 50Hz, it is about 50A. Starting Fig.2 shows the connection of the moand inefficient method of approximating at low speed, which means low voltage as tor to the Controller in this case. the desired speed. As a result, it is likely well, should alleviate this. I found that this ¾ HP motor had to be that many hobbyist lathes remain on the Although the Controller permits a slow accelerated with about a four-second ramp one speed for most of their lives, a far from ramp-up from a low voltage, at slow speeds from 0-50Hz. This is set by trimpot VR2 optimum situation for quality and speed of the winding reactance drops proportionate- (RAMP) in the Controller. As the voltage is operation. Variable speed control is an at- ly to the frequency, so the current does not applied and the armature begins to rotate, tractive modification. necessarily drop as expected. This motor it generates a back-EMF that reduces the Any reasonably sized lathe will use a ca- simply drew too much current for the Con- current and gives room for more voltage to pacitor-start motor. This has a high start- troller to start it. be applied, accelerating the armature furing torque to overcome the load presented There is also a possibility, as yet unveri- ther. The ramp voltage must not increase by the belts, pulleys, close-fit bearings and fied, that with a capacitor in circuit, the Con- too fast for the armature to accelerate and back gears, with a centrifugal switch to troller interprets a leading power factor as a generate the current limiting EMF. take the start winding out of circuit as the short circuit, since in both cases it would see motor comes up to speed. Unfortunately, current increasing without a corresponding Other motor configurations the Induction Motor Speed Controller is voltage increase. But what if both ends of the windings specified as being unsuitable to drive such Fortunately, the Controller itself provides are not brought out, as is typical of a huge a motor. But is it? the solution. It has a three-phase output. We number of small, non-reversible capacitor The main reason given for the unsuit- can split the windings across two phases to start induction motors? Can such a moability is that at a low selected speed, the keep each phase current within the maximum tor still be controlled in the manner decentrifugal switch will cut back in, and the of the Controller, at least up to a certain size of scribed above? current drawn by the start winding may motor. One phase is selected for the main windThe answer in many cases is yes! Not burn the winding out. Almost as an after- ing. Of the other two phase voltages, one leads only can such a motor be speed controlled, thought, a sidebar advises that “there is the main by 120° and the other lags by 120°. it can also be reversed. Fig.3 shows the two also a risk that the over-current protection Either of these should give sufficient quad- most likely motor configurations at left. In in the Speed Controller will simply prevent rature current to the start winding to create both cases, the start capacitor is removed normal operation”. Amen to that! a rotating field but it is necessary to remove and the wire that connects directly to one The subject of this project is a 1970’s the start capacitor and short its connecting of the existing terminals is taped off and era Taiwanese lathe with a 250mm swing. leads together. If the output terminals of the secured. The remaining wire is the new Its motor is a ¾ horsepower (560W) four- Controller are labelled A, B, and C, the main connection point for the start winding. pole capacitor start induction motor. It is winding is connected between A and C and This wire, adequately insulated, is also reversible. one end of the start winding is connected to B. brought out of the capacitor chamber. This At first glance, it appears well within The other end of the start winding can lead and the previously assigned phase the 1500W capacity of and neutral leads connect to the Speed Controller. The the three-phase output of the main and start windings Speed Controller, as shown at are brought out to the onright. Reversal of the direction off switch, which reversof rotation is achieved by swapes one winding to reverse ping any two phases. the rotation of the chuck. Changing a faulty start caHaving access to both pacitor is routine maintenance ends of the start winding on induction motors, hence, reovercomes the problem moving the start capacitor and of the centrifugal switch installing the two-phase wiring re-engaging at low revs. should be well within the caIt is easy to provide a pability of any builder with the relay to isolate the start knowledge and skill to build the winding as the motor Controller. speed is reduced. It is Doing so opens up a greatly also easy to provide a Fig.3: for motors where separate connections are not provided for the increased number of applicarelay to reverse the po- start winding, the start capacitor can be removed and one of its conn- tions for variable speed oplarity of the start winding ections brought out to provide the connection to the start winding. eration. 42 Silicon Chip Celebrating 30 Years siliconchip.com.au Using it with a 3-phase motor While this project was designed to be used with a lathe driven by a single-phase induction motor, the IMSC is capable of driving 3-phase delta-wound motors. Since a 3-phase motor lacks a start winding, start capacitor and centrifugal switch, you don’t need RLY1 or RLY2 and their associated wiring. RLY3 will need to be a four-pole type to allow it to switch all three phases. However, the design as presented here does not drive the “REV” terminal on the IMSC so it has no way of commanding motor reversal for a 3-phase motor. Therefore, you would need to run a connection between the collector of Q1 and the REV terminal on the IMSC so that the Explore 100 can reverse the motor direction. The software should not need any modifications. The relay box, which connects the IMSC to the motor, has an earthed aluminium backplate. If using a 3-phase motor, only two relays are required. ing the few SMDs. After that, it’s pretty much just a matter of soldering the components in place where indicated on the PCB silkscreen. The prototype Speed Controller interface was built into a small Jiffy box which was mounted to the outside of the IMSC, while the Explore 100 interface plugged directly into the Explore 100. As you can see from the photo, the “output frequency sense” section of the circuit was mounted inside the IMSC box itself. The Explore 100 Interface, IMSC Interface and Output Frequency Sense sections of circuitry were built on solder-type prototyping boards using point-to-point wiring, so there are no PCBs or overlay diagrams. The relays were mounted in a separate box with an earthed aluminium backplate, as shown in the photo above. Since each section of the circuit is relatively simple, after soldering the required components to a piece of protoboard, you should be able to use the circuit diagram as a guide to wiring it up. You can use wire wrap wire (Kynar), bell wire or light-duty hookup wire to make the connections between component pins. The Explore 100 and its associated interface components, shown in the shaded box in Fig.1, were housed in a single large jiffy box. You will need to siliconchip.com.au make a rectangular cut-out in the lid for the Explore 100’s LCD plus three holes for the status indicator LEDs and some holes for wires/sockets for the DC power input and RJ-45 (or DB9, as in the prototype) interface sockets. Loading and using the software If you’ve built The Explore 100 kit should come with a pre-programmed microcontroller but you still need to set up the LCD panel and then load the Lathe Controller BASIC code into the Explore 100. You should do this with the IMSC and related circuitry powered down, however, the circuit has been designed so that nothing bad should happen if the unit is powered up without any code running on the Explore 100. In other words, the default state of each output is set up to be safe and not drive anything, including the motor. Instructions for setting up the LCD panel and touchscreen were given in the October 2016 issue (Explore 100 part 2; siliconchip.com.au/Article/ 10303), however, if you don’t have that handy, you can simply enter the following commands over the serial or USB console: OPTION LCDPANEL SSD1963_5,LANDSCAPE,48 OPTION TOUCH 1, 40, 39 GUI CALIBRATE Celebrating 30 Years After typing the final command and pressing enter, you will be presented with a cross-hair target in the corner of the LCD screen. Press on its centre with a stylus-type object (eg, a toothpick) and then repeat for the targets which appear in the other three corners. With any luck, you will get a message on the console which reads “Done. No errors” and that indicates that the touchscreen has been set up correctly. You can then download the Lathe Controller BASIC code from the SILICON CHIP website (free for subscribers) and upload it using MMEdit or similar software (MMEdit is a free download for Windows or Linux; see www.c-com. com.au/MMedit.htm). Once the code has been uploaded, MMChat should automatically launch and you can then issue the “OPTION AUTORUN ON” command, followed by “RUN” and the graphical user interface (GUI) should appear on the LCD screen. You can verify that this appears to be working before disconnecting your PC and you are then ready to power the whole rig up and test it out properly. We suggest you do this initially with nothing in the lathe so that you can verify it’s all working correctly without risking any damage. The operation of the software was explained earlier, although it’s pretty much self-explanatory anyway. SC January 2018  43