Here is a typical scenario. You are reversing your flybridge twin-engined cruiser into a berth (doesn’t everyone have one of these?). You must do it at low speed (pretty obvious!) and you can’t use the rudder to steer with since rudders don’t work at low speeds. The only way to steer is to use the motors.
Normally, in a twin-engined boat, you make sure the rudders are centred and then you manoeuvre the boat by nudging the motors into and out of gear and using very judicious (tiny!) amounts of throttle or none at all. For example, if you are going forward, you can steer to port (left, if you’re a land-lubber) by putting the port engine into reverse and the starboard engine into forward gear. Or you might just leave the starboard engine in neutral while nudging the port engine in and out of reverse gear.
Going in reverse is a whole different ball-game. Now you are looking at the rear of the boat while you manoeuvre it into a narrow berth. In this case, if you want to steer to the left going backwards, you put the starboard engine into reverse and the port engine into forward . . . or combinations of those settings. All the while, you have to cope with the effects of currents and wind. It can be a nightmare.
The sender unit (above) uses seven reed switches to detect the rudder position. It transmits data to the receiver unit (below) via a 433MHz wireless link.
It can be even harder in a single-engined boat. The rudder still doesn’t work at low speeds and you don’t have the luxury of two motors to do the steering. In this case, you do have to use the rudder but in order to get the boat to respond to the rudder, you have swing it hard over, in one direction or the other, and give the motor a quick stab of power in forward or reverse gear to push the stern of the boat in the required direction. Sounds tricky, doesn’t it? Well, it is.
Going back to the twin-engined boat for a moment, before you can start these low-speed manoeuvres, you must have the rudder centred. But since typical boats require many turns from lock-to-lock, it is almost impossible to know when the rudder is centred. The practical way to do it, is count the turns from lock-to-lock and then halve it, to centre the rudder. So if it is six turns from lock to lock, you turn the wheel fully to port or starboard and then wind the wheel back by three turns. Trouble is, it’s easy to lose count when you’re winding the wheel back and forth.
How much easier it would be if you had an electronic rudder indicator! Commercial rudder indicators are fitted to some boats but they are very expensive.
So that was the brief. The skipper of SILICON CHIP can’t steer his boat (hope I won’t get into too much trouble for this . . .) and he wanted an electronic indicator. Being the autocratic type that he is, who was I to argue? His justification is that the project would have other applications, so here is the result.
This Rudder Position Indicator consists of two units, each of which mounts in a small sealed box with a transparent lid. The sensor unit monitors the movement of the rudder arm and transmits information to a receiver unit via a UHF radio link at 433MHz.
The receiver display unit is portable so that it can be moved from the flybridge driving position to the helm inside the cabin. It shows the rudder position using an array of high brightness LEDs, with adjustable brightness to suit indoor and outdoor use.
Specifications & Performance
Rudder Position Resolution............seven steps, plus centre indication
Sensor Type &............ magnet and reed switch
Communication Method ............ 433MHz UHF digital wireless transmission (Amplitude Shift Keying)
Range ............ approximately 20m (depending on antenna orientation and obstacles)
Power source ............ 4 x AAA cells or external 12V supply
Battery life (sensor unit)............approximately two years with 4 x AAA cells
Battery life (receiver)........... approximately two years on standby or 2-8 hours in use, depending on LED brightness
Size (each unit) ............105 x 75 x 40mm with a protruding 15cm whip antenna
The rudder display can show one of seven positions: three steps to port, three to starboard and one when it is centred. The port, starboard and centre positions use different LED colours to make the direction more obvious at a glance. For extra precision in setting the rudder straight ahead, the middle LEDs flash when the rudder arm is directly over the central sensor.
Both the sensor and receiver units are fitted with short whip antennas (about 15cm) to provide sufficient range for use on larger boats. In most boats, the hydraulic steering arms are located in a compartment called a “lazarette” and this may or may not be lined with aluminium foil coated insulation, to cut down noise and heat. In this case, it may be necessary to run a coaxial cable from the sensor unit to a whip antenna mounted outside this compartment, to allow the signal to reach the helm position(s).
The same comment applies if the boat has an aluminium or steel hull.
Both the sensor and receiver units can be powered from an internal battery (which can be rechargeable) or from an external 12V power source. An external power source can also be used to trickle charge the internal batteries. The approximate charge state of both batteries is indicated on the display unit.
The sensor unit is always powered, so you don’t have to switch it on and off each time. Even so, its low current drain means that it will run for at least a year on four AAA cells. Just how long depends on how often you use it and the cell type used. If you use good-quality alkaline cells, the transmitter battery could last two years or more.
Many boats have a 12V lead-acid battery in the lazarette and in that case, you can omit the sender unit’s internal battery and use that as a power source instead.
The UHF link makes installation easy; there is no need to run wires from the rudder to the helm which can be a major task in a typical large power boat.
The first aspect we considered was how to sense the rudder position. There are four obvious sensor types to choose from: a rotary switch, a potentiometer, an optical sensor or reed switches. In each case, either the sensor needs to be attached to the rudder shaft or an arm must be attached to the shaft with the sensors arranged in an arc above or below it, so that the arm triggers one at a time.
Fig.1: how the sensor unit is arranged. It’s mounted on a platform and is activated by a magnet on the underside of an arm that’s attached to the rudder shaft.
Rotary switches and potentiometers tend to wear out fairly quickly with continuous use and they can also be fouled by water, grease or dirt in a marine environment, unless they are fully sealed. An optical sensor is a better choice but is the most power-hungry option and it also requires the most complicated wiring, as both the light source(s) and sensor(s) require power.
So we settled on reed switches, with a magnet attached to a cranked arm that is mounted on the rudder shaft. Seven reed switches are arranged in an arc below the arm so that as the arm moves, the magnet passes over them, closing each reed switch in turn. Fig.1 illustrates this arrangement.
While it is possible to design these circuits using discrete logic and special-purpose ICs (in fact, we initially tried to do just that), there are several advantages to a microcontroller-based solution. First, if we use a microcontroller in each unit, fewer parts are required. Since we want to fit the display unit into a small box with an internal battery (so it’s easily portable), this is important.
Also, because the microcontroller in the sensor unit can drive current through the reed switches intermittently, the battery drain can be kept very low.
So for the final design, each unit is based around a microcontroller which does virtually all the work, in combination with a wireless transmitter or receiver module. Most of the time, the micros are in a low-power sleep mode, keeping the battery drain down to about 15µA (including current for the regulator). When active, the micro wakes up and performs the necessary tasks before going back to sleep.
Each unit comprises two PCBs: a lower control board which hosts the battery, micro and most other components, and an upper board which hosts either the reed switches (sender unit) or the display LEDs (receiver unit). All boards are the same shape and size and fit snugly into the sealed boxes, so only the top board is visible through the clear lid.
For an overview of how the two units are configured, refer to Fig.2, the block diagram. The sensor unit (left) contains the reed switches for rudder position sensing and the microcontroller to monitor them. When the switch state changes, the micro powers up the 433MHz transmitter module and sends a data packet containing the new position. This packet is amplitude shift keyed (ASK) and bi-phase encoded.
Fig.2: this block diagram shows how the sensor and receiver units are configured. The reed switch outputs are processed by microcontroller (IC1) which then powers up the 433MHz transmitter module to send a 16-bit data packet on the new rudder position. This signal is picked up by receiver and processed by another microcontroller (IC2). This then drives a LED display (consisting of series LED strings) via decoder/driver IC3.
The receiver/display unit (right) is portable and only listens for packets when it is switched on. When it receives a valid packet, the microcontroller decodes it and extracts the new rudder position. It then displays this position by determining which row of high-brightness LEDs is lit.
The display unit incorporates a boost regulator. This is necessary to drive the series strings of five LEDs that form the main display. With a typical forward voltage of around 2V, at least 10V is required to drive each string (slightly more due to the 100Ω series current limiting resistor they share).
The boost regulator develops roughly 12V at 20mA when the LEDs are lit, from a nominal 6V battery (it can operate down to about 3V). It can also run off an external 12V supply, in which case very little or no boosting is needed. In this
case, a series resistor in the power supply input ensures that the LED voltage doesn’t exceed 12V, even if the supply voltage is up to 14.8V (eg, when a lead-acid battery is on charge).
Note that while the wireless modules are referred to as operating at 433MHz, the actual frequency band used is 433.05-434.79MHz.
Sensor unit details
The micro in the sensor unit is in low-power “sleep” mode almost all the time. Its 32kHz watchdog timer (WDT) is continuously running and this “wakes it up” several times a second (maybe it sleeps quite poorly!) to check the reed switch state. To do so, it turns on an internal pull-up current source for each input and checks the voltage. The current sources are then immediately disabled and remain off until the next time, to conserve power.
Further action is only taken if the switch states differ from the previous reading. Otherwise, the period the micro spends running is very short and the power consumed during these periods is negligible.
When a change in reed switch state is detected, the 433MHz transmitter module is powered up. Several 16-bit packet pairs are transmitted with a short delay between each, in case interference corrupts one or more of the packets. Each packet pair encodes the updated rudder position, battery charge state and a unique identifier number, which is randomly generated when the battery is inserted.
Once five complete packets have been sent, the transmitter is shut down and the device goes back to sleep until another rudder movement occurs.