Fullscreen HTML5Med Res (??MB)HTML4

For when you need to know... G-FORCE METER Just what are the g forces involved in a balls-to-the-wall lap of Mt Panorama? This little beauty will tell you: instantaneous acceleration, braking, cornering; forwards, backwards, sideways . . . and it’s battery operated and completely portable, so you can swap it from car to car! W hy would you want a g-force meter in your car? Good question. This project comes about because as soon as we published the Digital Spirit Level (August 2011) we had a number of readers contact us to ask “Can this be used as a g-force meter?”. Your wish is our command! We know they are fitted to some high-performance vehicles, such as the Nissan GTR, showing the instantaneous acceleration, braking and cornering forces. These can be used to gauge vehicle and/or driver performance. The faster the car accelerates or corners, the higher the g-force. Ditto for braking – is the driver putting the vehicle under too much stress by braking too late or too hard? And what about the driver him/herself? Throwing the car around a corner might look pretty spectacular but it’s also pretty stressful on the driver (not to mention the car!). Of course, you’re unlikely to do a hot lap around Bathurst. But the g force meter can just as easily be used to save petrol and wear & tear and/or temper youthful exuberance by making your driving as smooth as possible. G-force meters, more properly called accelerometers, measure force in a particular direction. If your car doesn’t have one of these (and let’s face it, you probably don’t think it does), now you can easily add one! Actually you might be surprised to find out that most modern cars do contain one or more accelerometers. For example, air-bag systems use them to detect accident severity and so decide whether to inflate the air-bags and if so, how fast. The engine or body computer may also contain an accelerometer to detect when the vehicle is on a hill, in order to change how the transmission or engine behaves. But in most cases, there’s no display to show you the readings – nor Design by Andrew Levido 28  Silicon Chip is there any way to capture the data. Apart from automotive use, there are many other places where g force measurement would be handy. For example – a powerboat crashing over waves: you know you can really get thrown around . . . but just how much? Or perhaps one of those heart-inyour-mouth thrill rides at theme parks and shows. Just what are the forces involved? (Actually you might be surprised at how low many of them are!) But if you’ve ever tried to buy a commercial g-force meter (or accellerometer) you would know that for most people, they have been basically unaffordable. We’ve changed that with this little beauty! On the level? Yes! So this project is an adaptation of the Digital Spirit Level which was published in our August 2011 issue. In fact, it uses the same MMA8451Q accelerometer IC and shares virtually Words by Nicholas Vinen siliconchip.com.au all of the hardware with that project. But the software has been changed so that, rather than reading out a tilt angle in degrees, it shows the forward/ back or left/right acceleration in units relative to the earth’s gravity (1g = 9.81N = 9.81m/s2). The new software places the MMA8451Q in a mode where it can make rapid readings but the range is limited from -2g to +2g for each axis. Why not a greater range? Simple: if your vehicle experiences forces in excess of 2g, the limited range of the readout will be the least of your worries; such high g forces are usually only experienced during a prang (and a pretty bad one at that!). For instance, the Bugatti Veyron, which is the world’s fastest production car (top speed on the up side of 400km/h), can accelerate from a standing start to 100km/h in 2.4s. Measured maximum g-force? 1.55g. Readings At this point we should explain just what the readings on the Acceleromsiliconchip.com.au eter display mean. Firstly, the reading indicates the experienced force which doesn’t necessarily match the actual vehicle acceleration. Say you are in a car accelerating at full throttle. Depending on how powerful the engine is (and how much the vehicle weighs), you will experience a sensation of being pressed back into the seat. This is due to Newton’s third law of motion: “For every action there is an equal and opposite reaction”. In this case the action is the car accelerating forwards and the reaction is you being pressing back into the seat. Now consider the same car, parked facing up a hill. You will experience a similar sensation. In this case, it is the force of gravity pushing you back into the seat. In these situations, the accelerometer will experience the same forces you do. So in both cases, it will report a “forward acceleration” - despite the fact that in the second example, the car isn’t moving. So why didn’t we make it read zero in the second case? Firstly, the force being reported is real, so you could argue that the unit should respond to it. Consider what happens if you accelerate up a hill; the engine must work harder than it would to accelerate at the same rate on level ground. So the fact that the accelerometer reading will be higher in that situation makes sense. Similarly, it will read lower when accelerating down a hill, which is gravity-assisted. Secondly, to compensate for the effects of gravity would be surprisingly difficult. To disentangle the gravity and acceleration vectors, we would need a digital gyroscope (also available, using MEMS technology). This could be integrated to keep track of the vehicle’s orientation, compute the effect of gravity and thus eliminate it from the readings. But because a gyroscope measures instantaneous rotation (not tilt angle), calculating the gravity vector would still be a bit tricky, requiring an integration function. In the end we decided that including the gravitational component in the November 2011  29 readings was both sensible and easy. If you want to measure pure vehicle acceleration, you will need to do so on a flat surface. represents the acceleration of the vehicle. It is then split In this article, we use the words “force” and “acceleration” interinto forward-back and leftchangeably, even though we know they aren’t the same thing. right components. Newton’s second law of motion says: Force = Mass x Acceleration. If either of these is higher So while the acceleration due to gravity (1g) is more or less constant, than the currently disthe force due to gravity depends on the weight of the object. played acceleration value, Form factor While this is a “g-force” meter, it actually reads acceleration. But it the display is updated with Besides the software, the does so by measuring the force it experiences due to that accelerathe new value and the fiveother change compared to tion. Is that confusing or what? second timer is reset. We decided to risk the wrath of physicists everywhere and use the Digital Spirit Level is the If the timer expires, ie, cheaper and simpler hous- these terms as people are familiar with them, rather than worrying the same peak value has about being technically correct. ing; a UB3 jiffy box. been displayed for five Power is still from two AA seconds, the display is realkaline cells but if you want to run it By measuring the amount of deflec- set and it then shows the acceleration from 12V (eg, from a cigarette lighter tion, the IC can measure the force it value for whichever of the two axes socket), it’s a simple matter to add an experiences. is currently experiencing the highest appropriate voltage regulator. The arms each form a capacitor acceleration. with the surface they are mounted on In a non-accelerating vehicle on a How it works and this capacitance changes as they level surface, the reading will be close The circuit is built around the deflect, because the distance between to zero. 3-axis MEMS accelerometer (IC1), the capacitor “plates” is changing. PIC18LF14K22 microcontroller (IC2) These capacitance shifts change the Circuit description and a four-digit LED 7-segment display frequency of an oscillator in the IC and The full circuit for the accelerometer (DISP1-4). the oscillation frequency is counted. is shown in Fig.1. The only change The g-force experienced is shown on A mathematical formula can then from the digital spirit level is that we the display with the first digit indicat- be used to convert this frequency into have added a power switch, S2. ing the direction (F=forward, b=back, an acceleration reading for that axis. The digital spirit level was switched L=left, r=right) and the three remaining When the calibration button (S1) is on by shaking and it automatically digits showing g-force in the range of pushed, IC2 reads the current measure- turned off with inactivity but since 0.00-2.00g. ment from the accelerometer and stores this unit may be used for long periods, The MEMS accelerometer contains it in its internal flash memory. a power switch was judged the more three micro-machined mechanical From then on, this is used as the sensible approach. arms. They are at right-angles to each gravity reference vector. This is subThe micro, IC2, drives the eight other and they each bend and deflect tracted from subsequent readings, 7-segment display anodes directly in response to force along one axis. forming a difference vector which from its outputs, via 4.7 current- Force vs acceleration +3V DISP1–4: FND500 OR EQUIVALENT 10 F 100nF 10 1 Vdd 10k 4.7k 4.7k 4 POWER 10 F MCLR 100nF Vpp 2x AA CELLS 1 VddIO 2 100nF 7 BYP 14 Vdd 6 SDA 13 4 11 SCL IC1 MMA8451Q 11 SA0 INT1 INT2 GND GND GND 5 10 12 SC 2011 PGC 18 17 9 G-force meter 30  Silicon Chip 1k 10 CAL S1 +3V 16 RC0 15 RC1 14 RC2 7 RC3 6 RC4 5 RC5 8 RC6 9 RC7 8x 4.7 DISP1 10 g 9 f 7 a 5 dp f 1 e 6 e SCK/RB5 INT1/RA1 INT2/RA2 RB5 RA0 RA5 RA4 RB7 Vss 20 DISP3 a b g f DISP4 a b g e c d f a b g e c d f b g e c c d d K IC2 PIC18LF14K22 SDA/RB4 a b 2 d 4 c DISP2 8 8 8 8 D Q1 TN0604 G 12 S 19 G PGD D Q2 TN0604 S D Q3 TN0604 G 2 S 3 Vpp +3V D Q4 TN0604 G S PGC GND 1 (ICSP SKT) 5 Fig.1: with the exception of the power switch, the circuit of the g-force meter is identical to that of the digital spirit level published in our August issue. For a full description of how the accelerometer chip operates, refer to that issue. Note that the software is quite different for this project. TN0604(N3) D G S siliconchip.com.au Fig.2: (above) the PCB component overlay, with the underside of the board at right showing the SMD accelerometer chip. A same-size top-side photo is below (only the SMD chip is on the underside). limiting resistors. The display common cathodes are driven using four Mosfets, Q1-Q4. Their gate voltages are controlled by outputs RA0, RA4-5 and RB5 of IC2. These are special Mosfets which have a low on-resistance even with a low gate drive voltage. This is important since the supply voltage is limited and any voltage losses across the Mosfets will reduce the display brightness. Since the display is multiplexed, with each digit on 25% of the time and since the microcontroller outputs driving the anodes have limited current capability, we need as much brightness as we can get. Communication with the accelerometer, IC1, is via I/O pins RB4-5 and PA1-2. RA1 and RA2 are connected to the interrupt pins of IC2 (INT1 and INT2 respectively) and these are used by IC2 to signal events and that acceleration data is ready to be read out from its internal buffers. Commands and data are sent over the I2C bus with pins SDA (data) and SCL (clock). Each has a 4.7k pull-up to VCC, as I2C utilises open-collector outputs to enable bus sharing. The power supply for IC1 is smoothed with an RC filter consisting of a 10 resistor and 100nF and 10F capacitors in parallel. It also has a separate filter capacitor connected to the BYP (bypass) pin, pin2. For calibration, pushbutton switch S1 is connected between input RB7 of IC2 and ground. IC2 enables its internal weak pull-up current source on that pin so that when the button is pressed, the input state changes from high to low. It is debounced in software. The whole circuit runs off a 3V battery consisting of two AA cells bypassed with a 10F capacitor. IC1 also has a 100nF high-frequency bypass capacitor. Construction First mount is IC1, the accelerometer. This only comes in a small 16-pin QFN (quad-flat no-leads) package. In fact, all MEMS accelerometers and gyroscopes seem to come in similar packages, presumably for compactness (they are frequently found in mobile phones). Because it has no leads, it’s quite tricky to solder. In the Digital Spirit Level article, one particular method was recommended. We have also tried another method which worked quite well and this is described in the panel on p33. Follow those directions there to solder the IC. Then flip the board over and install the resistors. We recommend the use of a digital multimeter to check the value of each before it is installed. Follow with the IC (a socket is optional), ensuring it goes in the right way around. Next, fit the four 7-segment displays, again careful with polarity (the decimal point goes at lower-right in each case) and make sure they are neatly lined up and flat against the PCB before soldering them in place. Solder the capacitors in next. Note that the two electrolytic types should be laid over on their sides. In each case the longer lead goes into the pad towards the bottom of the board. The four Mosfets can then be mounted, with their leads cranked out using small pliers, to suit the pad spacing on the PCB. They are static-sensitive so be careful in handling them. Mount the 5-way pin header next. This provides an in-circuit programming connection, compatible with Microchip’s PICkit3. If your chip is pre-programmed and you don’t plan to re-program it, this header may be omitted. The PCB “hangs” off the front panel by means of four 12mm spacers, as these two photos (one taken from top, one from the bottom) show. The pushbutton switch (SW1) is the only control which emerges through the panel. siliconchip.com.au November 2011  31 Re-flow its pins using flux and solder wick, as described in the panel. Then try again. Here’s the case lid shown without the PCB in place, to reveal the four mounting pillars, the SW1 access hole and the red acrylic “lens” for the 7-segment LED displays. Finally, solder the tactile pushbutton (S1) in place. Ensure it is pushed right down against the PCB before soldering. The button should be orientated so that its leads project out to the left and the right (this is really the only way it will fit as the pads aren’t quite arranged in a square). When you are finished, its actuator shaft should be perpendicular to the PCB surface. Testing it Pass the battery wires up through the hole in the board and solder them to the appropriate pads. Double-check that the polarity is correct (there is no reverse polarity protection!). Tighten a small cable tie around the leads just above the hole that they pass through and trim it. Insert the cells and check that the unit is operating correctly. If it is, the display will light up and read either “Fx.xx”, “bx.xx”, “Lx.xx” or “rx.xx”, where xx.x is a number between 0.00 and 2.00. Hold the PCB vertically and press S1. The number shown should then be closer to zero. You can then change the reading by moving the board in, out and sideto-side. Remember that it has a fivesecond peak hold. Remove the cells from the holder. If it didn’t work, check the orientation of all polarised components (IC1, IC2, Q1-Q4 and the two electrolytic capacitors). Assuming they’re OK, the most likely problem is that IC1 isn’t soldered properly. Finally, the completed g-force meter opened up to show the method of assembly with a 2x AA battery holder glued to the bottom of the case and a power switch (S2) through the case side. 32  Silicon Chip Housing it Remove the lid from the UB3 jiffy box and attach a printout of the front panel (available from siliconchip.com.au) to use as a drilling/cutting template. Use a sharp knife to cut out the display area rectangle from the photocopy before attaching it. You can then use the same knife to (carefully!) etch the outline of the display into the lid. Drill the five holes to 3mm where indicated. Also drill a series of holes around the inside of the outline for the display, then knock the panel out (side-cutters can be used to remove any remaining plastic sections keeping it in place). Use a large, flat file to carefully file the edges flat and to shape the cut-out to the etched outline. You may need to use needle files to finish the corners. Remove any “lip” formed in the process of filing with a sharp knife. De-burr the holes then temporarily attach the PCB to the rear of the lid using the four 12mm tapped spacers and eight M3 machine screws, with the black screws on the outside. Check that the pushbutton is properly lined up with its hole and that it doesn’t “stick” when pressed due to misalignment or the hole being slightly too small. Enlarge it if necessary. Remove the board and place the acrylic sheet behind the cut-out and glue it in place. We used hot melt glue but you can also use neutral cure silicone sealant. Make sure not to get the glue on surface of the “lens” or it Parts list – g-force meter 1 PCB, code 04108111, 100 x 44mm * 1 tactile pushbutton momentary switch with 22mm actuator (S1; Altronics S1119) 1 pushbutton cap to suit S1 (Altronics S1481) 1 SPST (or SPDT) miniature toggle switch (S2) 1 5-way 2.54mm pitch pin header 1 2 x AA battery holder 1 UB3 jiffy box 1 transparent red Acrylic or Perspex sheet, 60 x 25mm 4 M3 x 12mm tapped spacers 4 M3 x 6mm pan-head machine screws 4 M3 x 5/6mm black machine screws 1 small cable tie 1 60mm length foam-cored double-sided tape hot melt glue or neutral cure silicone sealant * see below Semiconductors 1 MMA8451Q 3-axis accelerometer (IC1) * 1 PIC18LF14K22-I/P microcontroller programmed with 0410811C.hex (IC2) * 4 TN0604N3 Mosfets (Q1-Q4) * 4 FND500 7-segment LED displays or equivalent (Jaycar ZD1855, Altronics Z0190) Capacitors 2 10F 16V electrolytic 3 100nF MKT or monolithic ceramic Resistors (0.25W, 1%) 1 10k 2 4.7k 1 10 8 4.7 1 1k SHORT FORM KIT * A short-form kit consisting of: 1 Printed Circuit Board (04108111) 1 MMA8451Q Accelerometer Chip 1 PRE-PROGRAMMED PIC18LF-14K22-I/P microcontroller 4 TN0604N3 Mosfets (IE, ALL THE HARD-TO-GET PARTS!) is now available direct from SILICON CHIP for only $44.50 plus $10 p&p. See the handy order form on P102 siliconchip.com.au Soldering the QFN SMD IC The procedure for soldering the QFN device, as detailed in the Inclinometer article, is (briefly) as follows: tin the pads, place the IC on top, line up its pads, then reflow the solder added earlier to form the joints between the PCB pads and the IC pads. While this method works and doesn’t require any special tools, we tried a different approach this time, which we think might be more reliable. First, place a small amount of solder on one of the PCB pads. We started with the top-right pad; left-handers may prefer to start at the upper left. Then place the IC alongside its final position (but not on this pad) and check its orientation. Be very careful as it’s difficult to remove once it’s in place. Then heat the solder on that pad and, using angled tweezers, slide the IC into position. Remove the heat and check the IC under a illuminated magnifying lamp to see whether its pads are lined up correctly on all sides. They are copper coloured and are just visible around the bottom edge of the chip; the PCB pads should be tinned and therefore look silver. It’s unlikely that the IC will be perfectly positioned on the first attempt, so re-heat the solder on that one pad and very gently will be damaged. Just flow it around the edges, as shown in the photo. Once the glue has set, trim away any areas that interfere with the corner posts, programming header or any other tall components. Then drill a small hole in the box itself, for the toggle switch. You can put it anywhere you like; we opted for the left side. Don’t put it too close to the lid. Cut the red battery wire about 5cm from where it leaves the PCB, strip both ends and solder them across one pole of the chassis-mount toggle switch (ie, one to the centre and one to an adjacent tab). Re-insert the two AA cells and check that the switch works. You can then re-attach the PCB to the rear of the lid and push on the cap for switch S1 (push it hard, so it won’t come off easily). Install the switch in the box and do up the nut tight. Then peel the protective coating off the strip of double-sided foam-core tape, press it onto the back of the battery holder and stick the battery holder in a convenient location in the case (in the middle is best). Make sure it’s stuck down well. siliconchip.com.au nudge the chip in the right direction (again using the tweezers). Check it again under a magnifying glass and repeat this procedure as many times as is necessary, until it is correctly positioned. Note that boards with a solder mask can fool you; the solder mask isn’t necessarily perfectly aligned with the pads themselves. See the photo above. We thought we had lined up the IC with the pads but we had instead lined it up with the holes in the solder mask layer – we fixed this (and the shorted pads!) after taking the photo. Once the pads are properly lined up on all four sides, apply solder to the pad diagonally opposite the one you started with. Unless your soldering iron has a very fine tip, you will need to put a fair bit fo solder on the tip for it to bulge, then use gravity to flow it up against the corner junction formed by the the IC and the PCB. You can then flow solder onto all the remaining pads using the same method (ignoring solder bridges for now). Then apply a thin layer of flux paste all around the edges. Now place some solder wick flat on the PCB, as close to the IC as possible and heat it with your soldering iron. Once the flux starts to smoke, gently push it up against the edge of the IC. After a couple fo seconds, any excess solder will flow into the wick and also under the IC pads, filling the gap between it and the PCB. Wait a few seconds, then remove the wick and the soldering icon. It should leave just the right amount of solder on the pads. You can then clean off any remaining flux with isopropyl alcohol, although if you used “no-clean” flux paste (usually a good idea) this isn’t strictly necessary. Our board worked first time after installing the IC using this method. If yours doesn’t, add some more solder to each pad and re-flow them again to ensure that they are all properly connected. Note that this method is very similar to that described for soldering fine-pitch SMD ICs such as TSSOP and QFP in the October 2009 issue (“How To Hand-Solder Very Small SMD ICs”) and once you get the hang of it, it can be applied to a wide variety of SMDs. It’s then just a matter of screwing the lid on the box and (if provided) pushing the rubber caps over the screw holes to hide them. All that’s left is to figure out how to stick the unit to your dashboard (or wherever you want to put it). You can use double-sided tape or BluTak (which can work surprisingly well, depending on the other surface). But be careful because both are likely to leave residue on the dashboard which may be hard to remove. If you have an obsolete or broken GPS unit, you could re-use its suction cup mount for this purpose. Wherever it is mounted, make sure it doesn’t interfere with your field of vision or block the visibility of any important instruments (eg, speedometer). the accelerometer on and press the calibrate button. The display should then read close to zero. It will then remember the calibration setting even after it is switched off. It only needs to be re-calibrated if the mounting arrangement is changed. To use it, just switch it on and glance at it after a manoeuvre to see the peak acceleration. Don’t be distracted by it and remember to keep your eyes on the road! You can interpret the readings as follows: Calibrating it You will need to be parked on a level surface for proper calibration. If you have access to a poured concrete parking lot or garage floor, that is probably the best option (although it may have shifted since it was poured). Park the car on a flat surface, turn 0.00-0.20g: gentle acceleration/braking/ cornering or gentle slope 0.20-0.35g: moderate acceleration/ braking/cornering or moderate to steep slope 0.35-0.60g: hard acceleration/braking/ cornering or very steep slope 0.60-0.80g: racing 0.80-1.00g: super-car territory 1.00g+: extreme manoeuvres/ collision SC November 2011  33