Fullscreen HTML5Med Res (??MB)HTML4

,..,~ Buildlllis Electronic Cockroach Here's a project that's just for fun. It's a robotic car that behaves just like an electronic cockroach. Put it on the ground, switch it on & it heads for a dark comer. By JOHN CLARKE In the early days of semiconductor electronics, electronically controlled toys were very popular. For the first time, it was possible to build in complex control systems that were either too difficult or impossible using mechanical techniques. Those early models were quite expensive due to the high cost of the 16 SILICON CHIP parts but, of course, this situation no longer applies. Parts are now quite cheap and low-cost motors and wheels are easily obtainable from hobby shops and parts retailers. This electronically controlled car, dubbed the "E,lectronic Cockroach" (because it seeks the dark), is an inexpensive toy that will give you a chance to combine your elec- tronics skills with a few simple mechanical skills. It uses two ICs which cost around $1 each and two motors which are only $3.95 each. What does it do? Basically, the Electronic Cockroach runs along the floor and steers away from the light. It runs straight ahead if there is equal light intensity on each side of the vehicle but if one side is darker than the other, the vehicle steers fowards the dark. A real cockroach has six legs but our electronic version has to rnake do with three wheels - two at the front and one at the back. The two wheels at the front are independently driven by separate motors while the wheel at the rear simply trails behind. This rear wheel is mounted on a swivel from the motor. In our circuit, each motor is driven by a pulse width modulated (PWM) voltage signal rather than by a continuous DC voltage as in the original idea. This technique ensures that the peak vpltage is always applied to the motor, regardless of the speed control setting. Rather than varying the DC level, the speed of the motor is set by varying the pulse width. TRIANGLE WAVEFORM PIN 6, IC2a +6V APPLIED MOTOR VOLTAGE ov LOW BACK EMF TRIANGLE WAVEFORM PIN 6, IC2a 1-+-- -~ - - - , - - + -- Speed regulation --'r +6V1-.----, APPLIED MOTOR VOLTAGE ov Fig.1: the motor speed in the Electronic Cockroach is controlled by comparing the motor's back-EMF with a triangle waveform to derive a voltage pulse train. If the motor slows, the back-EMF falls & the pulse length increases to bring the motor back up to the correct speed. axle and can rotate through a full 360 degrees. At the front of the vehicle are three light detectors (LDRs), one in the centre facing straight down and two at the corners facing to either side. These LDRs measure the light intensity and provide control signals to switch the motors on and off accordingly. Incidentally, the idea for this car comes from Shaun Williams from Alawa, NT. He originally sent in a circuit for a vehicle which used LDRs and a motor gearbox drive for the two front wheels. We thought that the idea was good enough to develop further, while reducing the cost as much as possible. In particular, we wanted to eliminate the motor gearbox drive. Although being the best way to drive the vehicle, it would have added about $40 to the project and this would have reduced its appeal. Eventually, we decided to drive the front wheels from the motors via rubber bands, a technique that's cheap but effective. Because the drive ratio from the motor to the wheel is not as high as that available from a gearbox, the motor drive circuitry was also redesigned to obtain the greatest possible torque Another worthwhile feature of the circuit is speed regulation. This helps the motor to maintain its speed even if the gradient changes or the motor is loaded due to the nature of the "terrain" (eg, thick carpet). Fig.1 shows the basic principle of the feedback control. What happens is that the circuit monitors the backEMF generated by the motor. BackEMF is the DC voltage generated by the motor to oppose the current through it. The faster the motor spins, the greater the back-EMF. This back EMF is compared with a triangle wave generated by an oscillator and the resulting pulse waveform then drives the motor. When the motor is running at high speed, (ie, when it is unloaded), it produces a high back-EMF and so the voltage pulses applied to the motor are quite narrow. However, if the motor is loaded, it slows down and the back-EMF drops. The circuit then automatically increases the width of the pulses (and thus the average voltage) to increase the motor speed. Circuit details Let's now take a look at the circuit details - see Fig.2. Although at first sight it appears to use a lot of op amps, these are all contained in two quad comparator ICs (ICl & IC2) These comparator ICs are LM339 devices which can operate down to 2V. T~eir outputs are open collector which means that you must to use a pull-up resistor to obtain a high output. The advantage of open collector outputs in our circuit is that they can be connected together as OR gates. The circuit is also somewhat simpler than it first appears because the two motor drive circuits are identical. IClc, IClb, ICla and Ql drive one motor (Ml), while ICld, ICZb, ICZc and Q2 drive the other (M2). ICZa is the triangle wave generator PARTS LIST 1 PC· board, code 08310921, 207 x 83mm 2 Johnson 170 motors (available from model shops) 8 2mm screws and nuts 2 42mm diameter plastic wheels (Aristo-craft or equivalent) 1 150mm-length of 1/8-inch brass tubing 1 150mm-length 1/8-inch brass threaded rod 4 brass nuts to suit 1 22mm aluminium knob 2 12mm brass untapped spacers 2 9mm brass untapped spacers 2 6mm brass untapped spacers 4 1/8-inch steel washers 1 4-way AA square battery holder 1 battery clip for holder 4 AA alkaline cells 4 6 x 60mm diameter rubber bands 1 SPDT toggle switch 3 ORP12 or equivalent LDRs (LDR1 -LDR3) 2 10kQ horizontal trimpot Semiconductors 2 LM339 quad comparators (IC1 ,IC2) 2 BD646 PNP Darlington transistors (01 ,02) 1 3.3V 1W zener diode (ZD1) 2 1N4002 1A diodes (D1 ,D2) 2 1N4148 switching diodes (D3,D4) 1 5mm red LED (LED1) Capacitors 1 470µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 3 2.2µF 16VW PC electrolytjc 1 0.1 µF MKT polyester Resistors (0.25W, 1%) 1 68kQ 1 22kQ 6 10kQ 11.2kQ 11 1kQ 1 390Q 1 180Q 1 47Q Miscellaneous Solde·r, tinned copper wire. referred to earlier. This device is wired as a Schmitt trigger oscillator by virtue of the 68kQ feedback resistor conn·ected between pins 1 & 7. It oscillates by the following action. FEBRUARY 1993 17 +6V - - - - - - - - - - - , +3.3V - - - - - - - - - - 111 SPEED VR1 10k 10k +3.3V V+ 2 1k 13 LDR1 LEFT ORP12 03 1N4148 1k 2.2 + 111 BACK EIIF 1k 16VWi +UV 1k 68k 1.2k 10k LDR2 CENTRE ORP12 1 .,. J7.J 10k BCE .,. 22k v'v 2.2 + 16VW+ +6V +3.3V - - - - - - - - - - - V+ 112 SPEED VR2 10k +3.3V 14 1k 04 1N4148 LDR3 RIGHT ORP12 1k .,. 1k 2.2 + 10 + 16VW+ 16VW+ 112 BACK EIIF 1k .,. .,. .,. POWER l° ... S 1-;.:_:.,._ : _ _ _ _ _+---\'147'1J:lr-6V_ - T 6V: + __,.,.__ __,.,.___ +3.3V 1800 V+ ZD1 3.3V 1W 100 + J .,. ···1 ELECTRONIC COCKROACH Fig.2: IClb, I Cl a & Ql drive motor Ml on one side of the vehicle, while IC2b, IC2c & Q2 drive motor M2 on the other. Normally, Ml is controlled by IClb which compares the back-EMF with a triangle waveform generated by IC2a. When IClb's output switches high, pin 1 ofICla goes low & turns on Ql to pulse the motor. However, ifless light falls on LDRl than on LDR2, pin 13 ofIClc switches low & the motor turns off. LDR3, ICld, IC2b & IC2c control M2 in exactly the same manner. 18 SILICON CHIP When power is first applied, the 2.2µF capacitor on pin 6 has no charge and so the output at pin 1 is high. The 2.2µF capacitor now charges via the 22kQ resistor until the voltage at pin 6 exceeds the voltage on pin 7. When that happens, pin 1 switches low and the 2.2µF capacitor discharges via the 22kQ resistor until the voltage on pin 6 drops below the voltage on pin 7 again. Pin 1 of IC2a now switches high again and so the cycle is repeated indefinitely for as long as power is applied. Thus, the 2.2µF timing capacitor is alternately charged and discharged via the 22kQ resistor and the resulting output is a triangle waveform as shown in Fig.1. This waveform has an amplitude of about 200mV (1.541. 76V) and a frequency of about 66Hz. This triangular waveform is applied to the non-inverting inputs of comparators ICla, IClb, IC2b & IC2c. IClb compares the triangle waveform with the voltage on its pin 4 (inverting) input, as set by trimpot VRl and the back-EMF developed by the motor (Ml). This voltage sets the duty cycle of the voltage pulses that appear on IClb's pin 2 output, as shown in Fig, 1. The voltage pulses from IClb are next inverted by comparator stage IC la. This stage uses the triangle waveform at its non-inverting input (pin 7) as a voltage reference. The voltage pulses from IClb swing between DV and 3.3V, whereas the triangle waveform varies between 1.54V and 1.76V. Thus, when the output ofIClb swings low, pin 1 of ICla is pulled high and vice versa. Note that the output of ICla is pulled high to +6V (via a lOkQ resistor), despite the fact that the supply rail to the IC is less than this figure. This is possible because of the open collector output and ensures that PNP transistor Ql fully turns off when pin 1 is pulled high. When pin 1 of ICla swings low, Ql is turned on via a lkQ current-limiting resistor. This transistor is a Darlington type (BD646) with a minimum DC gain of 750. Thus, we only require a small amount of base current to ensure that the transistor is fully turned on (ie, saturated) when driving the motor. Dl protects Ql by quenching any large spikes that are generated by the motor when the transistor turns off. Fig.3: install the parts on the PC board as shown in this wiring diagram. The three LDRs should all be mounted at full lead length (see text) . D3 and its associated components make up the back-EMF monitoring circuit. Note that because we only want to sample the back-EMF developed across the motor, this sampling process must take place when Ql turns off. When Ql is off, the back-EMF developed by the motor is sampled by a voltage divider consisting of two lkQ resistors. D3 will be forward biased during this time and so a sample of the back-EMF also appears across the 10µF filter capacitor. This voltage is further filtered by a lkQ resistor and a 2.2µF capacitor and then applied to pin 4 of IC1b. Thus, if the back-EMF rises, the voltage on pin 4 ofIClb also rises, the output pulses from IC1b narrow, and the motor slows down. Conversely, if the back-EMF falls, the voltage on pin 4 falls and so the output pulses lengthen to bring the motor back up to speed. VR1 adjusts the initial voltage level on pin 4 and thus sets the overall speed of the motor. When Ql turns on, D3 is reverse biased and thus the voltage previously developed across the 1DµF filter capacitor does not change. The second motor, M2, is controlled in exactly the same manner by ICZb, ICZc and Darlington transistor Q2. The back-EMF of this motor is monitored via diode D4, while VR2 sets the overall speed of the motor. LDR control From the foregoing, it might seem that the two motors run continuously but that is not the case. Instead, one or both motors can be switched off, depending on the light falling on the three LDRs (LDR1 , LDR2 & LDR3). LDR2 monitors the ambient light level and, in company with its associated 1.ZkQ resistor, sets the voltage at the non-inverting inputs of comparators IC1c and IC1d (pins 11 & 9). If the light level goes down, the resistance of the LDR increases and the voltage on pins 11 & 9 also increases. In the case of motor Ml, comparator IC1c monitors the voltage across LDR1 and compares this with the voltage across LDR2. If less light falls on LDR1 than on LDRZ, the voltage on pin 10 ofIClc will be greater than that on pin 11. As a result, IC1c's output (pin 13) switches low and pin 1 of IC1a goes high. This turns Ql and motor Ml off and so the vehicle turns towards LDR1 (assuming that MZ is running). Conversely, if more light falls on LDR1 than on LDR2, IC1c's output effectively goes open circuit and so has no effect on IC1a. IC1b thus supplies a PWM waveform to IC1a as described before and so Ml runs at normal speed. LDR3 controls motor MZ in exactly the same manner. If less light falls on LDR3 than on LDR2, motor M2 switches off and so the vehicle steers in the opposite direction. Note that a 1.2kQ resistor is used in series with LDR2, while lkQ resistors are used in series with LDR1 and LDR3. This arrangement ensures that both motors switch off if there is equal light on all three LDRs. So, when the Electronic Cockroach crawls into a dark corner, it automatically switches its motors off to conserve the batteries. Power supply Fig.4: each motor shaft is fitted with a 10mm length of brass tubing as shown in this diagram. A solder mound is then added to the tubing so that the rubber band stays on the shaft. Power for the circuit is derived from a 6V battery pack consisting of four AA cells. Sl switches the power on or off, while LED 1 lights when the power is on. The 6V rail directly powers the Darlington transistors (Ql & QZ), while the ICs are powered via a decoupling circuit consisting of a 180Q resistor and a 470µF capacitor. This decoupling network filters out any supply rail ripple that's caused by the heavy current drawn by the motors. Finally, a regulated 3.3V rail is derived using zener diode ZD1 and a 1D0µF capacitor. This regulated rail FEBRUARY 1993 19 MOTOR SHAFT- Fig.5: this plan view shows how the motor shafts are coupled to the front wheels via the rubber bands. Position the axle so that Jhe rubber bands stretch by about 7mm when they are installed & adjust the spacers so that the wheels clear the PC board by 2mm. MOTOR -SHAFT RUBBER BAND- UNDERSIDE OF PC BO ARD 12mm UNTAPPED BRASS SPACERS SOLDERED 6mm UNTAPPED BRASS SPACERS / WHEEL RUBBER - BAND WASHERS TO PC BOAR~D_\ _ _ _~,-"'-t I - - - -----------1 \ ADJUST FOR RUBBER BAND TENSION 1/8" BRASS TUBING WHEEL I '\ CRIMP END WITH PLIERS 2mm j 130mm supplies the LDR networks and provides the bias for ICl b and IC2b. Construction A PC board coded 08310921 (207 x 83mm) accommodates all the parts see Fig.3. Before installing any of the parts , check the holes sizes for the motor mounts and th e rear wheel pivot. The motor mounts should be drilled to 3mm while a 5mm hole will be required to accept the 9mm-long spacer that pivots the rear wheel. This spacer should be soldered into place so that it protrudes about 3mm above the board surface (see Fig.6). Follow the overlay diagram care- fully when installing the parts on the PC board and don't forget the eight wire links (note: the prototype differs slightly from Fig.3). Make sure that the semiconductors and electrolytic capacitors are all oriented correctly. The two Darlington transistors are mounted with their metal tabs towards the motors. The three LDRs should all be mounted at full lead length. Adjust LDR1 and LDR3 so that they face sideways, as shown in the accompanying photograph. LDRZ should be adjusted so that it faces towards the floor. The two motors can now b e mounted in position using 2.5mm machine screws and nuts and their leads soldered to the PC board. Note that the red wire of motor 1 runs to Dl's cathode, while the red wire of motor 2 runs to D2's anode. This is necessary because the motors must run in opposite directions to each other. The circuit can now be checked for correct operation. Wind both trim pots fully clockwise, then switch on and check for +5V (approx.) on pin 3 of each IC. ZD1 should have a nominal 3.3V across it. Now place some insulation tape over LDR2 and rotate one of the trimpots until its corresponding motor begins to run. When it does, do the same for the other motor. Adjust the RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 20 No. 1 1 6 11 1 1 1 SILICON CHIP Value 68k.Q 22k.Q 10k.Q 1.2k.Q 1kn 390.Q 180.Q 47.Q 4-Band Code (1%) blue grey orange brown red red orange brown· brown black orange brown brown red red brown brown black red brown orange white brown brown brown grey brown brown yellow purple black brown 5-Band Code (1%) blue grey_ black red brown red red black red brown brown black black red brown brown red black brown brown brown black black brown brown orange white black black brown brown grey black black brown yellow purple black gold brown motors for slow running and check that each motor exhibits quite a lot of torque when you try to stop it by grabbing hold of its shaft. If it does, then the back-EMF feedback control is working correctly. Finally, check that each motor stops when you cover its corresponding LDR with your finger. The motor should immediately restart when you remove your finger, If all is OK, switch off and move on to the mechanical assembly. If it doesn't work, go over the board carefully and check for wiring errors. NUT E ,§ .., 9mm UNTAPPED BRASS SPACER SOLDERED IN HOLE IN PCB ..,____ WASHER "NUT \ 118' THREADED BRASS ROD 22mm DIA ALUMINIUM KNOB Mechanical assembly The first step in the mechanical assembly is to fit a 10mm length of 1/8-inch diameter brass tubing over each motor shaft. To do this, cut two 10mm lengths of tubing with a hacksaw and file the ends smooth. This done, crimp each piece lightly at both ends using side cutters, then push them onto the motor shafts (see Fig.4) . To keep the rubber bands running true, a convex mound of solder is applied to the centre of each shaft. This ensures that the rubber bands remain on the shafts and don't wind off when the motors start to run. If the belt begins to wander off centre, it will quickly restore itself. To form this convex mound, run the motor at slow speed by shorting out its LDR, apply the iron and allow the solder to slowly build up on the shaft. When a sufficient mound. has built up , remove the iron and allow the solder to cool with the motor still running. Warning: if the motor is allowed to run too fast during this procedure, you may end up with molten solder flying off the shaft. As a r,.<,.~ ' - - 9mm BRASS SPACER 60mm DRILL HOLE THROUGH KNOB THIS END I Fig.6: the rear wheel assembly is made up using a 22mm-diameter aluminium knob, a 150mm-length of threaded brass rod, two 9mm spacers & several nuts & washers. Make sure that the knob spins freely on its spacer & that the pivot assembly operates correctly before soldering the nuts to the threaded rod. Below: the arrangement of the front & rear wheel assemblies can be gauged from this "under-the-chassis" view. Note that a small piece of black cloth was glued to the rear wheel (ie, to the aluminium knob) so that its appearance matched that of the other wheels. ~ FEBRUARY 1993 21 precaution, we strongly recommend that you wear safety goggles to prevent possible eye injury. Once the solder has cooled, it can be further shaped using a small file. Again, this is best done with the motor running slowly. Wheel assembly The first step in the front wheel assembly is to find the correct location for the axle. To do this, temporarily fit one of the wheels to the axle, position it on the underside of the PC board, and install the rubber band as shown in Fig.5. Now position the axle so that the rubber band stretches slightly (5-8mm should be about right) and mark the position of the axle on the board with a pencil. The axle runs inside two 12mm spacers which are soldered to the underside of the PC board, with additional free-running 6mm spacers fitted to ensure that the inside edges of the wheels just clear the PC board. Fig. 5 shows the details. Initially, the two 12mm spacers should be lightly tack soldered into position. This done, test the assembly by fitting the axle, 6mm spacers and wheels. Adjust the lateral position of the 12mm spacers to provide the correct amount of wheel clearance from the board (about 2mm), then complete the solder joints. The wheels can now be permanently installed by cutting the axle to length and crimping the axle ends with pliers as shown in Fig.5. Note that two small washers are fitted between each wheel and the crimped axle end to ensure that the wheel turns freely. Don't just use one washer here. If you do, it may bind on the crimped end of the axle and make the wheel difficult to turn. The pivoting rear wheel assembly is shown in Fig.6. On the prototype, this wheel was made from an aluminium knob. The normal shaft hole was drilled right through the knob to accept a 1/8-inch threaded brass rod, . while a 9mm brass spacer serves as the wheel bush. This brass spacer is simply fitted into the existing 6mmdiameter shaft hole in the knob. The wheel assembly is fitted to one end of the brass rod and secured with a nut on either side. Make sure that the wheel turns freely but without too much play before permanently soldering the nuts in position. This done, bend the rod into a U-shape around the wheel as shown in Fig.6, taking care to ensure that the rod finishes up at right angles to the axle. The end of the rod is then bent upwards through 90° about 60mm from the axle, so that it fits through the vertical spacer soldered to the PC board. Secure the wheel assembly to the vertical spacer using nuts and washers as shown in Fig.6. The battery holder can be secured to the PC board using two rubber bands (the same size as those used to drive the motors). To improve their appearance, we dyed the rubber bands black using normal fabric dye (just follow the hot pan dying procedure outlined on the packet). In normal operation, LDR2 should face down towards the floor for best results. If you find that the car only runs when LDR2 is covered, increase the value of the 1.2kQ resistor to 1.5kQ Finally, you can easily modify the circuit so that the 22 SILICON CHIP 0 0 0 0 T"'"4 (\J CJ' 0 T"'"4 M CD 0 u Cl) Fig.7: here is the full-size pattern for the PC board (code 08310921). Check your board carefully to ensure there are no etching defects before installing any of the parts. vehicle turns away from the dark rather than towards it. This is achieved simply by connecting motor Ml to motor M2 's pads on the PC board and vice versa. The lead polarities must also be swapped over, so that the motors continue to run in the correct direction. SC