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Twin Engine SpeedMatch Indicator By JOHN CLARKE Avoid unnecessary noise and vibration in twin-engine boats by using this Twin Engine SpeedMatch Indicator. It comprises a meter that is centred when both motors are running at the same speed. When the motors are not matched in revs, the meter shows which motor is running faster and by how much. M OST POWER BOATS over eight metres long have two engines, typically in-line 4-stroke diesels or petrol V8s, each driving its own propellor via a shaft or stern drive. Normally both motors should run at exactly the same speed unless the boat is manoeuvring up to a jetty or mooring, in which case the propellers may run at differing speeds and direction. All boat-owners know how important it is to have the motors running at exactly the same speed. If the motors don’t run at the same speed, there can be excessive noise and vibration and the motors will be far less efficient as one prop tries to pull the boat harder and the other produces more drag. At 38  Silicon Chip the same time, having the motors running at slightly different speeds means that you have to provide correction with the rudder to maintain a straight course and that causes further drag. In fact, a speed difference between motors of as little as 15 RPM can cause lots of vibration that can radiate through the whole boat – most unpleasant. To explain further, with V8 motors a difference of 15 RPM will cause a beat note of 1Hz. This is because V8s have four firing strokes per revolution so 15 RPM is equivalent to 60 pulses per minute or 1Hz. Apart from being most unpleasant to those on board, such low frequency vibration also causes lots of wear in the engines, gearboxes and shafts. So synchronisation of motors is highly desirable. In fact, late model up-market boats often do have a facility for synchronisation while there are also electromechanical synchronisers available for older boats although these can be difficult and expensive to fit. So most boat owners equalise the motor speeds as well as possible by watching the tacho readings and listening for the beat frequency. Trouble is, most boat tachos are not very accurate (typically ±3% or worse at mid scale) and they can also be subject to wavering readings. Furthermore, if you are driving the boat from the flysiliconchip.com.au bridge in bad weather, it can be very difficult to clearly hear the engine exhausts, meaning that it is even more difficult to listen for “beat” notes. And if your hearing is not the best (very common with older drivers), the difficulty is compounded. Clearly, an electronic beat indicator is required. In setting out to produce a suitable design, we thought about an indicator based on a LED bargraph. When it was centred, the motors would be in sync. However, trying to see LEDs on a bright sunny day when driving on the flybridge is next to impossible and that goes for almost any electronic indicator. That is why most boats have conventional analog meters – they are easy to see! Hence we decided to base our design on a good old-fashioned analog meter movement. When the motors are running at the same speed, the meter will be centred and if not, it will show the difference at up to 200 RPM (or whatever you decide to set). It is then easy to adjust the throttles so that the meter is centred. The basic set-up of the Twin Engine SpeedMatch Indicator is shown in Fig.1. It compares the tachometer signals from each motor and the difference in RPM is shown on the panel meter. The panel meter needle is centred when the motor speeds are identical. If the port (left) motor is running faster than the starboard (right) motor, then the needle will move left. Similarly, if the starboard motor is running faster, the needle will move to the right. The meter shows only the difference in RPM and it does not matter if the engines are running at full speed or at idle. The tacho signals will usually be a low-voltage signal from a Hall Effect sensor or reluctor, or they can be obtained from the ignition coils or from another source such as a low-voltage tachometer signal from a sensor. Where these are not available, such as in a diesel motor, a signal from the alternator can be used instead. Fig.2 shows how the two tacho signals are compared. Each tacho signal is fed to a frequency-to-voltage converter (IC1 & IC2). The resulting voltage outputs are then buffered and compared in a differential amplifier, IC3d. This is offset using trimpot VR3 and then buffered by IC3a. The offset voltage centres the meter siliconchip.com.au Fig.1: the basic set-up of the Twin Engine SpeedMatch Indicator. It compares the tachometer signals from each motor and displays the difference in RPM on a centre-zero meter. METER PORT ENGINE STARBOARD ENGINE TWIN ENGINE SPEED MATCH INDICATOR TACHO SIGNAL TACHO SIGNAL +12V METER BUFFER VR3 IC3a (OFFSET) IC3d PORT ENGINE TACHO SIGNAL FREQUENCY TO VOLTAGE CONVERTER (IC2, VR2) IC3c BUFFER (ie, to half scale) when the tachometer signals are the same frequency. Circuit description The full circuit is shown in Fig.3. It comprises two LM2917 frequencyto-voltage converters, a quad op amp package plus associated resistors, capacitors and diodes. Each tacho signal is applied to a filter network consisting of a 10kΩ resistor and 22nF capacitor. This is followed by a 22V zener diode and a 20kΩ resistor to ground. This filtered signal is fed to the non-inverting input of a Schmitt trigger at pin 1 of the LM2917 (IC1 & IC2). The Schmitt trigger threshold (pin 11) is set at about +0.55V by the 10kΩ and 1kΩ voltage divider connected across the 6V supply. The output from the Schmitt trigger drives an internal charge pump which involves capacitors C1 & C2 (see Fig.4). C2 is discharged using the series 100kΩ resistor and a 1MΩ trimpot (VR1 and VR2 for IC1 and IC2, respectively). The LM2917 is a special-purpose chip which has a number of refine- DIFFERENTIAL AMPLIFIER IC3b BUFFER Fig.2: each tacho signal is fed to a frequency-tovoltage converter. The resulting outputs are then buffered and fed to a differential amplifier which drives the meter. FREQUENCY TO VOLTAGE CONVERTER (IC1, VR1) STARBOARD ENGINE TACHO SIGNAL ments to ensure that the frequencyto-voltage conversion is linear. First, capacitor C1 is charged via a current source to a voltage that is ¾ the main supply to the IC. This charge current is duplicated (using a current mirror) for capacitor C2. During discharge, C1 is discharged to ¼ the main supply at a constant current. The specified upper and lower voltage thresholds ensure that the current source and discharge current circuitry operate within their designed voltage range. In addition, charging and discharging is at a rate that is twice the frequency of the tachometer input. This doubling of input frequency reduces Specifications Power Consumption: 12V at 20mA Tacho Input Range: 0-6000 RPM Display Range: typically set to ±200 RPM Tacho Input voltage: 0.83V to 350VAC November 2009  39 REG1 7806 +6V RIGHT (STARBOARD) ENGINE TACHO SIGNAL IN1 10k 10k 1W 1 K 22nF A ZD1 22V 1W 20k 11 9 Vcc +IN OUT 10 µF 16V 100nF Cout IC1 LM2917N Eout –IN Vee C1 CPo –IN +IN 12 2 3 10 4 68Ω IN K GND K ZD3 16V 1W 100 µF 25V A 8 D1 1N4004 A +12V VIA FUSE 0V 5 4 5 IC3b 6 33k 7 TP1 100k 10nF VR1 1k 1 µF 10k IC3: LM324 VR3 1k 1M LEFT (PORT) ENGINE TACHO SIGNAL IN2 3 2 1 IC3a 470k 13 +6V 10k 1W 1 K 22nF A ZD2 22V 1W 20k 11 9 Vcc +IN 12 470k Cout IC2 LM2917N Eout –IN Vee C1 CPo –IN +IN 12 2 3 10 4 8 5 100nF 10 9 IC3c 8 11 33k 4.3k TP2 VR2 1 µF 10k D2 1N4148 1mA METER D1 SC  2009 A – ZD1–ZD3 D2 K 100 µF K 1M A + A 100k 10nF 14 IC3d K A 7806 K TWIN ENGINE SPEED-MATCH INDICATOR GND IN GND OUT Fig.3: the full circuit for the Twin Engine SpeedMatch Indicator. IC1 & IC2 (LM2917N) are the frequency-to-voltage converters, op amps IC3b & IC3c are the buffer stages and IC3d is the differential amplifier. VR3 & IC3a provide an offset voltage for IC3d to centre the meter. the ripple across C2. Fig.4 shows the internal schematic of the LM2917. The charge pump voltage at pin 3 is applied to the non-inverting input of the amplifier internal to the LM2917. The inverting input to this amplifier at pin 10 is connected to the emitter output at pin 5 and this sets the amplifier as a unity gain buffer. A 10kΩ pull down resistor provides the emitter load. Op amps IC3b & IC3c are connected as unity gain amplifiers to buffer the pin 5 outputs of IC1 & IC2. The buffered outputs are then fed to op amp IC3d which functions as the differential amplifier. IC3d works as follows: the output from IC3c is amplified with a gain of -14, as determined by the 470kΩ resistor between pins 13 & 14 and the 33kΩ input resistor. The output from 40  Silicon Chip IC3b is first attenuated by the 33kΩ and 470kΩ voltage divider at pin 12 of IC3d (non-inverting input). The signal at pin 12 is therefore only 14/15 of the output from IC3b. The overall gain for signal at pin 12 is 1+ (470kΩ/33kΩ) or 15. Therefore, the overall gain for the signal from IC3b is 15 x 14/15 or 14, ie, the same gain as for the signal from IC3c except that it is positive (instead of negative). Note that we are using the LM324 right on the limits of its specifications in this circuit. This is because the LM324 op amp only has a 50µA sink current for output voltages less than +0.5V. This is why the resistor values in the circuit are relatively high. However, considering the DC outputs from the LM2917 frequency-to-voltage converters are generally above 0.5V when the engines are idling and more at higher RPMs, this is not really a problem for this application. If you are using this circuit for a different purpose and require a better result especially at low outputs from the frequency-to-voltage converters, we would recommend using an LMC6484AIN CMOS rail-to-rail quad op amp in place of the LM324. Meter offset Op amp IC3a buffers the voltage from VR3 and provides the offset voltage for IC3d. IC3d is offset so the meter sits at half-scale (ie, centred) when there is no difference between the two input frequencies. For this half-scale condition for the 1mA meter, 500µA needs to flow and so VR3 is set for this condition, ie, close to +2.25V. The meter movement is damped with a 100µF capacitor across it. Norsiliconchip.com.au 7.5V INPUT 1 CHARGE PUMP 11 8 SCHMITT TRIGGER AMPLIFIER 2 12 REFERENCE VOLTAGE 3 10 5 OUTPUT 4 100k C1 10nF 1M C2 1 F 10k Fig.4: the LM2917N frequency-to-voltage converter consists of a Schmitt trigger, a charge pump and an amplifier wired here as a unity gain buffer. mal full scale deflection of the meter will occur with +4.5V from IC3d. Note that while a gross difference in engine speeds can result in more than full scale deflection of the meter, the resultant overload is quite modest since IC3d’s output can only go slightly above +4.5V with a 6V supply. We have also included diode D2 across the meter. If a circuit fault applies excessive voltage to the meter, the diode will conduct at about 0.6V restricting the meter current to 0.6V/200Ω or 3mA. Power for the circuit comes from the boat’s 12V battery (ie, one of the engine batteries) via a fuse (ie, a switched accessory supply rail) and is applied through diode D1 for reverse polarity protection. The 68Ω resistor and 16V zener ZD3 protect against transient voltages, while a 100µF capacitor provides supply decoupling. Regulator REG1 then provides the 6V supply and its output is bypassed with a 10µF capacitor. A 100nF capacitor is also connected across the supply near IC1. Construction The Twin Engine SpeedMatch Indicator is constructed on a PC board coded 04111091 and measuring 105 x 63mm. This can clip into the integral mounting clips within a UB3 plastic case if required. Alternatively, four corner mounting points are provided for mounting in a different box or inside the dashboard of the boat. Note siliconchip.com.au Parts List +6V 9 that if you have two helm positions in the boat, you will need two SpeedMatch Indicators. The component layout for the PC board is shown in Fig.5. Begin construction by checking the PC board for breaks in the tracks or shorts between tracks and pads. Repair if necessary. Check that the hole sizes are correct for each component to fit neatly. The screw terminal holes are 1.25mm in diameter compared to the 0.9mm holes for the IC, resistors and diodes. The four corner mounting holes should be 3mm in diameter. Begin by inserting the links, PC pins, diodes and resistors. We used 0Ω resistors in place of wire links but the latter could also be used. The diodes must be mounted with the orientation as shown. When inserting the resistors, use the resistor colour code table to help in reading the resistor values. A digital multimeter should also be used to measure each value. Sockets are used for all three ICs and these must all be oriented in the same direction, with the notches as shown. Once they’re in, fit the 3-terminal regulator (REG1) and the three trimpots, all of which mount with the screw adjustment oriented as shown. The terminal blocks consist of two 2-way sections which are locked together before the are inserted and soldered into the PC board. The capacitors can be mounted next, ensuring that the electrolytics are ori- 1 PC board, code 04111091, 105 x 63mm 1 1mA MU45 moving coil meter (Jaycar QP-5010; Altronics Q-0500A) – see text 4 2-way PC-mount screw terminal blocks (5.08mm pin spacing) 3 DIP14 IC sockets 2 solder eyelet lugs 2 PC stakes 2 1MΩ multiturn top-adjust trimpots (code 105) (VR1, VR2) 1 1kΩ multiturn top-adjust trimpot (code 102) (VR3) 1 75mm length of 0.7mm tinned copper wire (for links) Semiconductors 2 LM2917N frequency-to-voltage converters (IC1,IC2) 1 LM324 quad op amp (IC3) 1 7806 6V regulator (REG1) 2 22V 1W zener diodes (ZD1, ZD2) 1 16V 1W zener diode (ZD3) 1 1N4004 1A diode (D1) 1 1N4148 switching diode (D2) Capacitors 2 100µF 16V electrolytic 1 10µF 16V electrolytic 2 1µF 16V electrolytic 2 100nF MKT polyester 2 22nF MKT polyester 2 10nF MKT polyester Resistors (0.25W 1%) 2 470kΩ 2 10kΩ 1W 2 100kΩ 1 4.3kΩ 2 33kΩ 1 1kΩ 2 20kΩ 1 68Ω 3 10kΩ Miscellaneous Silicone sealant, hook-up wire ented correctly. Finally, the three ICs can be mounted in their sockets, again ensuring each is oriented correctly. Testing The Twin Engine SpeedMatch In­ dicator requires a 12V DC supply or anything from 8-16V DC at about 20mA. Apply power and check that there is +6V between pins 9 & 12 of both IC1 and IC2 and between pins 4 & 11 of IC3. If there is no voltage, check for +6V at the output of REG1. Note that +6V is a nominal value and could range from +5.85 to +6.15V, depending on November 2009  41 R OTA CID NI ESI N OR H C NYS R OT O M NI WT 19011140 100nF 10k 1W 1k +M METER+ V0 V21+ D2 4148 10 µF 100nF 33k 100 µF 10k IC2 LM2917N 10nF 1 µF 22V 100k ZD2 22nF 20k 2 NI 33k CON2 METER– TP2 2 PT 4.3k 1PTTP1 470k 10k 10k VR3 VR1 0V IN2 IC1 LM2917N 10nF 1 NI 100k 22nF 1 µF CON1 IN1 0V 22V 20k ZD1 IC3 LM324 470k 10k 1W 0V +12V D1 4004 68Ω REG1 VR2 16V 100 µF ZD3 Fig.5: install the parts on the PC board as shown on this wiring diagram and the photo at right. In particular, make sure that all polarised parts are correctly installed and that trimpots VR1-VR3 have their screw adjustments positioned as shown. the particular regulator. If there is no voltage from the regulator, D1 may be reversed or there may be a short circuit between the +6V rail and 0V on the PC board. Marine meter movement The meter shown in this article is a standard 1mA FSD (full scale deflection) analog movement which can be obtained from Jaycar or Altronics. However, depending on your application, this may or may not be suitable. For example, it may be OK if used on the helm dashboard inside the cabin. However, it almost certainly won’t be suitable if used on the helm dashboard on the flybridge where it will be exposed to the elements. Most boat owners may want the meter to match the other meters on their dashboard and this approach will no doubt be far more expensive – as is everything associated with boats. On the other hand, taking this approach will mean that the meter will probably include illumination, will be sealed against moisture ingress and condensation and incorporate a lens (eg, in VDO gauges). If you are going to use a matching meter, it will probably need to be adapted from a voltmeter. In that case, you will need to pull the meter apart to change the scale. You will also need to remove the internal series resistor (voltage multiplier). For the purpose of this article, we made up a replacement scale for the specified 1mA meter movement. If you use this particular meter, you can change the scale by carefully prising the plastic cover off the meter, undoing the two securing screws for the original 1mA scale and then attaching the replacement panel. Fig.6 shows our replacement scale, which has maximum readings of ±200 RPM, or rather PORT +200 0 STBD +200. Note that this is a relative indication only and cannot be relied on as having great accuracy. All analog Table 1: Resistor Colour Codes o o o o o o o o o o No.   2   2   2   2   3   2   1   1   1 42  Silicon Chip Value 470kΩ 100kΩ 33kΩ 20kΩ 10kΩ 10kΩ 4.3kΩ 1kΩ 68Ω 4-Band Code (1%) yellow violet yellow brown brown black yellow brown orange orange orange brown red black orange brown brown black orange brown brown black orange brown yellow orange red brown brown black red brown blue grey black brown meter movements have their best accuracy at full-scale deflection of the meter and minimum accuracy at close to zero deflection. In fact, since the SpeedMatch Indicator will be set up by you, it will be quite accurate for the centre speed match indication. Setting Up Connect the unit to the meter’s M+ and M- terminals using leads terminated in solder eyelets. These eyelets are sandwiched between the nuts supplied with the meter. Ensure the meter polarity is correct. That done, apply power to the PC board and adjust trimpot VR3 so that the meter is centred. Further setting up requires either a Table 2: Capacitor Codes Value 100nF 22nF 10nF µF Value IEC Code 0.1µF 100n 0.22µF   22n 0.01µF   10n EIA Code    104    223    103 5-Band Code (1%) yellow violet black orange brown brown black black orange brown orange orange black red brown red black black red brown brown black black red brown NA yellow orange black brown brown brown black black brown brown blue grey black gold brown siliconchip.com.au 0 20 PORT 1 50 100 50 0 50 100 SILICON CHIP SpeedMatch 15 0 20 0 STARBOARD Fig.6: this full-size meter scale can be cut out or downloaded from the SILICON CHIP website. signal generator that can produce at least 1V output or by connecting the unit to the boat motor itself. Tachometer signal As mentioned, the inputs for the Twin Engine SpeedMatch Indicator can come from the ignition coil or from low-voltage tachometer signals. Where these are not available, such as in a diesel motor, signal from a separate sensor or the AC from the alternator can be used instead. The Twin Engine SpeedMatch Indicator will operate without any changes using either the ignition coil or low-voltage signal. If the alternator has to be used then this may provide a higher frequency than from the other tachometer sources. The signal from the alternator is an AC signal and may be marked as AC, AUX, S, R or TACH. An idea of how many pulses from the alternator per engine rotation can be gauged by measuring the diameter of the crankshaft pulley and dividing this by the alternator pulley diameter. The number of poles in the alternator is multiplied by this pulley ratio. The number of poles is usually 4, 6, 8, 10 or 12. The Twin Engine SpeedMatch Indicator was designed for between two and four pulses per engine rotation. If the alternator signal is higher than this, then the 10nF capacitors at pin 2 of IC1 and IC2 will need changing to a different value. The 10nF value is reduced by the ratio of 3/number of alternator pulses per engine revolution. So if the alternator produces 36 pulses per engine revolution, then the capacitor siliconchip.com.au will need to be 10nF x 3/36 or 820pF, using the nearest capacitor value. For a separate tachometer sensor, this may also deliver a higher number of pulses per revolution. The 10nF value is reduced by the ratio of 3/number of sensor pulses per engine revolution. In addition, for this sensor, there may be two leads – one for the signal and one at 0V. The 0V connection is provided on the PC board for this purpose if it is needed. Now connect the tachometer signal from one motor to both IN1 and IN2. Connect a digital multimeter, set to a DC volts range, between test-point TP1 and 0V on the PC board. With the motor running, adjust trimpot VR1 for a reading of 0.8V per 1000 RPM, eg, 1.6V at 2000 RPM. This sets the meter scale to ±200 RPM. If the voltage cannot be set within the range of the trimpot adjustment, then the 10nF capacitor at pin 2 will need changing. If the voltage is too high, use a lower value capacitor and if the voltage is too low, use a larger value. As a guide, reducing the capacitor value by a factor of two will reduce the voltage by the same amount. Having adjusted VR1 so that TP1 is at 1.6V at 2000 RPM, set trimpot VR2 so that the 1mA meter is centred. That is all the set-up requires. Now connect the IN1 and IN2 inputs to the separate motor tachometer signals and test the operation. Note that it is quite possible that you will find that when the SpeedMatch is indicating that the motors are synchronised, the tacho readings may not be exactly the same. This is to be expected with most analog tachometers since they are not particularly accurate, especially those with 270° movements (ie, most tachos). For example, a tachometer with a mid-scale accuracy of ±4% will have an error in the range of ±100 RPM at an engine speed of 2500 RPM. So it is quite possible that the port engine tacho might indicate 2400 RPM while the starboard engine tacho indicates 2600 RPM when the engines are actually doing the same speed. At low engine speeds, the tachos may be much more inaccurate. For example, at 1000 RPM, the accuracy may only be ±10%, which means, again, that the readings can be off by ±100 RPM. Why are analog tachos so bad? It is because their basic accuracy of, say, ±2% only applies at full deflection. So if the tacho reads to 6000 RPM, its reading is actually 6000 RPM ±120 RPM. It does not get any better at lower readings and in fact, the linearity at small deflections for all analog meters is generally not good. Unfortunately, where the tacho signal is derived from the alternator, as in the case of some diesels, the tacho signal itself can be inaccurate because of variable slip in the drive belt. The only cure for this is to install a Hall Effect sensor and an accompanying magnet on the harmonic balancer, flywheel or the prop shaft. Installation The Twin Engine SpeedMatch Indicator is presented as a bare PC board and separate meter. For installation we recommend you seal the meter top cover to the body with silicone sealant. The meter can be mounted in the boat dashboard using a suitable bracket. Standard boat gauges tend to fit into a 33/8-inch (85.73mm) diameter hole and the meter would need to be mounted onto a metal plate. The PC board can mount inside the boat dashboard. If you want to mount it in a box, it will fit into a UB3 box measuring 130 x 68 x 44mm. The +12V supply connection should be run to a fused accessory supply line that’s switched by the ignition, while the wiring to the ignition coil should use mains-rated (230VAC rated) cable. For moisture protection use cable glands for wire entry and seal the box with silicone sealant after calibration. 24V operation Some boats may have 24V batteries. For 24V operation, the 16V zener diode ZD3 should be changed to 33V 1W and the 100µF 16V capacitor at the input to the 3-terminal regulator REG1 should be increased in voltage rating to 35V or 50V. In addition, REG1 should be fitted with a small heatsink such as SC Jaycar HH-8504 or HH-8502. November 2009  43