Scale Model Electronics

What was model making remote control?

No matter whether ship model builders, car model builders, flight model builders or in functional model construction: As soon as a model is to be moved faithfully, the remote control technology comes into play. And it is precisely in this area that development has made enormous progress in recent years.

What was that for some time ago for a drama, when only the frequency ranges 27, 35 or 40 MHz were available for controlling models. In addition to the frequency interference caused by the CB radio devices that were widely used at the time, many models have also been broken by the double assignment of transmitter channels. Many model builders had to prove a very high pain threshold here, in order not to lose the pleasure of the hobby. But this disaster is now finally over to the delight of many modelers.

The new remote control technology with 2.4 GHz

With the release of the 2.4 GHz frequency band and the availability of technically sophisticated transmission and reception technology from the WLAN range, it was only a matter of time until this clever technology was also used for model construction remote controls. And with the introduction of the new 2.4 GHz remote control technology, not only many of the old problems were solved. In addition, some new and very interesting features were made possible:

Higher transmission security

Unlike the old systems, which have permanently transmitted on a fixed channel, the new remote controls jump back and her between the channels and occupy one of the approx. 80 available channels only for fractions of a second. The method known as frequency hopping has the great advantage that only minimal information is lost if a channel is disturbed. In addition, some remote controls support the LBT function (Listen Before Talk). With this function, the transmitter first checks whether the new channel is really free before it sends its information to the receiver on the frequency of this channel.

Better noise suppression

Since the receiver is digitally bound to the transmitter, the receiver only reacts to the signals of "its" transmitter. The parallel operation of several transmitters or models in the narrowest spaces (e.g. on a model airfield or in a model car race) is no longer a problem with 2.4 GHz remote controls.

Correction of transmission errors

By transmitting additional information, the receiver has the possibility to check the received signal for correctness. Thus, the receiver in the model is only able to output correct control information to the servos, controllers and electronic components. The dreaded self-life of the models due to disturbances is thus a thing of the past.

Fast signal transmission

Due to the greater bandwidth of the transmission channels, control information is transmitted at a higher resolution and at a higher speed. In combination with fast servos, this allows for reflex control movements, such as those required for controlling fast model cars, in fractions of a second.

Simple antenna laying

Since the antennas are only about 3 cm long in a 2.4 GHz remote control system, they can be "hidden" in the model more easily. Particularly true to prototype models are clearly upgraded when no long antenna wires discolor the appearance. In addition, the handling of the remote control transmitter is much easier with a short transmitter antenna. There are no more telescopic antennas that can be pulled off or broken.

Use of a reverse channel

High-quality model-making remote controls not only transmit control signals from transmitter to receiver in the model. The return channel is used to transfer information from the model back to the remote control. Together with the corresponding telemetry sensors, the strength and quality of the receiver signal, the voltage of the receiver battery, the motor temperature or also the flight altitude or the flight speed can be displayed in the transmitter display.

Of course, there is also a wide range of 2.4 GHz remote controls, which covers the complete range of possible applications. From the simple 2-channel rotary knob remote control for car models to cost-effective 4-channel hand-held remote control systems, the offer extends to fully equipped hand or desk transmitters, with which helicopters, jets and sinfully expensive large models can be controlled.

Receiver with additional functions

Because several 2.4 GHz receivers can be operated or bound to a 2.4 GHz transmitter, many model makers naturally use the option and build their own receivers in each model.

Thus, for example, on a model airfield, it is possible to switch between the models very quickly. Some of the receivers are equipped with two or even 4 reception levels, each with its own antenna.

This has the advantage that each antenna can be aligned in a different direction and thus the model has a perfect reception in every flight position or in every driving situation.

In addition, the manufacturers "pack" other useful functions into the receivers. Thus, a gyro receiver has an integrated electronics with position and acceleration sensors, for example, to keep a flight model absolutely stable in every flight attitude. Sudden crosswind during landing does not throw the model out of the flight path any more.

What was model-making servos?

With a remote control transmitter, the movements of the control sticks as well as the momentary positions of the switches and rotary/slide controls are converted into electronic signals. These signals are encoded (encrypted) and transmitted to the model by radio. In the model, the electronic control information must be received, decoded and converted back into a mechanical movement.

Modeling servos have been developed for converting a control signal into a movement. In addition to electronics, servos also contain a small motor that drives a lever (cross lever) via a gear. The movement of the lever is proportional to the joystick. If the control stick is in the center position, the servo lever is also in the center position. If the control knotter is deflected, the servo lever moves in the same direction at the same time.

What was technically feasible on the transmitter via potentiometers or Hall sensors is already a much greater challenge in the model. Because the model builders place the most different demands on the servos in their models:

Size and weight

A servo must fit the model. Many model manufacturers therefore already specify the size of the servo to be used when designing the model due to the dimensions of the servo installation slot. However, there are also models in which the choice of servo is optional, because the required servo holders or servo shafts have to be created by the model maker itself. For this reason, servos are available in a wide variety of sizes and designs.

Controlling torque and holding torque

The next important criteria for a servo are the actuating torque and the holding torque. This means how much force a servo generates for the linkage of rudders or steering rods and how much force the deflected elements are held in their position.

Since conventional servos have a drive lever that can be rotated and are therefore subject to lever law, the values are given in Ncm. A servo with a controlling torque of 30 Ncm is able to lift a mass of approx. 3 kg, if the control point is 1 cm away from the pivot point of the servo lever.

Adjusting speed

The setting speed is another important criterion that distinguishes high-quality servos. The less time a servo takes to turn from one end turn to the other, the better the fast control commands are implemented by the transmitter. However, even if electronic stabilization systems (gyros) are used for model helicopters, it is necessary that the connected servos can react quickly and reliably to the control impulses generated by the gyro.

Analogueue or digital

A servo is connected to a receiver via a three-wire cable. In addition to the supply voltage (plus and minus), the control information is transmitted on the third wire. To do this, the receiver outputs a control pulse 50 times per second or every 20 ms (milliseconds).
That sounds quite fast at the first moment. However, you have to take into account that the control information of the transmitter control sticks and switches is transmitted one after the other and that the pulse width, depending on the control stick position, is only 0.9 – 2.1 ms (servo center position = 1.5 ms) (see signal A in switch diagram 1). During the remaining time (19.1 – 17.9 ms), the servo does not receive any adjustment information and thus has no option to drive the servo lever into the required position during this time or to keep the position specified by the transmitter.

The switch diagram 1 shows the schematic setup of a conventional analogueue servo. It can be clearly seen that the servo motor receives only one control pulse over a period of 20 ms (see signal B). The greater the deviation of the servo lever from the actual to the intended position, the wider the motor pulses are. The current position of the servo lever (actual position) is communicated to the control electronics via a potentiometer.

A digital servo is mechanically constructed like an analogueue servo. However, a microprocessor is used instead of the voltage-controlled control stages. After digitalisation and storage of the actuating information from the receiver, the processor can now also control the servo motor during the pulse pauses (19.1 – 17.9 ms) in order to move or hold the servo lever in the required position.
Digital servos thus run faster, more powerfully and have enormous holding forces. And since minimum deviations from the target position are adjusted, digital servos are also much more accurate. However, due to the frequent control of the servo motor, the power requirement of the servo also increases.

The switch diagram 2 shows the schematic setup of a digital servo. You can clearly see that the servo motor receives far more control pulses in the period of 20 ms than with the analogueue servo (see signal B). Here, too, the pulse width depends on the servo lever's setpoint and actual position.

What was model construction controller?

In addition to the mechanical control of rudders or steering linkages, it is also necessary to be able to influence motors with the help of the remote control. For this purpose, there are motor controllers or speed controllers that are controlled in the same way as a servo. This means that the motor speed is changed depending on the position of the control stick on the transmitter.

Speed controller or speed controller

Although speed controllers are often referred to, they are not always speed controllers but mostly speed controllers. This means that if the control stick for the motor function is set to 50% of the max. power at the transmitter, the speed controller supplies the motor in the model with 50% of the battery power. Whether the motor actually works at 50% of its power or turns faster or slower due to changing loads, a speed controller cannot detect or influence.

A speed controller, on the other hand, records the current speed of the motor and adjusts the energy for the motor automatically if the speed increases or decreases as a result of a load change. This control mode (Govener mode) is important for electric model helicopters, for example, since they are to be operated with a constant rotor speed.

Brushed controller or brushless controller

When selecting the appropriate speed controller, the motor must first be taken into account. If the motor is a collector motor in which the current is transferred to the armature via two carbon brushes (brush), a brushed controller must also be used.

However, brushed motors have many disadvantages. The carbon brushes take off and must be maintained or replaced continuously. When the engine is running, the brushes produce the so-called brush fire, which can lead to considerable interference when the remote control signals are received.

For this reason, more and more brushless motors have become established in model construction. These motors function like three-phase motors, thus also have three connection lines and do not require carbon brushes (brushless). As a result, these motors require specially designed brushless controllers.

The following figures show the differences between the two engine types:

Observe technical data

When selecting a speed controller/speed controller, the technical data must always be observed. While the maximum current and the maximum operating voltage must never be exceeded, the specification of the motor windings (turns*) for model controllers is a minimum value that must not be undercut.

Since the controllers partly also protect e.g. Lipo batteries from deep discharge and can be individually adapted to the connected motor, the manufacturer and programming instructions must be observed during installation.

What was model construction electronic components?

If a model is to be equipped with more than just driving and steering functions, electronic components are required. With these modules and a little bit of craft skill it is then quite easy to switch position lights, headlights, relays or other consumers on and off with the help of the remote control. If you also hear a prototypical motor sound under the bonnet, the model construction is in its most beautiful form.

The reliable power supply of the model via a battery switch is also a very important aspect. In order to improve the operational safety of your models, many modelers use two rechargeable batteries to power the receiver and the servo. A battery switch then monitors the voltage level of both rechargeable batteries and always uses the rechargeable battery with the higher voltage level for the power supply of the model. If a battery fails with a technical defect, the second battery can still reliably supply the model with power.

Which rechargeable batteries and charging technology are suitable for model construction?

Model builders need to know not only everything about their model and the built-in electronics. Model builders are also familiar with the subject of rechargeable batteries and charging technology. This is also absolutely necessary, because rechargeable batteries supply the models with vital or vital energy. Therefore, many modelers also attach great importance to a reliable power supply of their models.

NiMH rechargeable batteries and battery packs

Standard NiMH round cell batteries, e.g. in the Mignon format, are primarily used for beginner sets for the transmitter power supply. For the receiver power supply, welded battery packs in the most varied designs are the better choice.

The contacts in a battery compartment can quickly lead to interruptions or loose contacts due to the vibrations to which a model is continuously exposed during operation. 

The consequences of such shaky contacts are interruptions of the receiving system, which can lead to total loss of control. A nightmare for any modeler.

Lithium rechargeable batteries

In the area of the drive batteries, lithium polymer batteries (LiPo) have long expired the rank of NiMH batteries. This is no wonder, because LiPo batteries have a lower weight and a significantly higher energy density than NiMH batteries.

However, these batteries want to be treated correctly. Too deep a discharge is just as harmful as an overload. In extreme cases, the battery can be mechanically destroyed with an acute risk of fire and explosion.
For this reason, it is important to also set the battery type and number of cells when programming speed controllers/controllers. The controllers/regulators then switch off the motor or reduce the power in case of a threatening deep discharge.

In order to be able to monitor each individual cell during the charging process, LiPo batteries have special balancer connections in addition to the high-current connection cables. Since LiPo batteries are partially covered with foil only, it is also important to ensure that tip or sharp objects cannot damage the outer skin.

With correct handling, however, LiPo batteries offer long-lasting driving and flying pleasure.

Lead-acid batteries

But if you now believe that in times of lithium rechargeable batteries and brushless motors, old-proven lead rechargeable batteries are no longer needed, you're wrong. Model ship builders like to use lead-acid batteries because they are both highly capacitive and cost-effective. In addition, you need the high weight of the batteries, so that model ships of the model have the correct water situation.

But also the fans of models with methanol combustion engines like to use lead-acid batteries. Once as power supply for the glow plug or for the power supply of the electric starter. Lead-gel batteries are preferably used because these batteries are leak-proof and can be operated independantly of the position.

In some cases, large lead-acid batteries are also used to quickly recharge driving or flight batteries while traveling in open areas, where no power connection is available. 

Model building chargers

In contrast to conventional round cell chargers, which can be found in every household, model construction chargers are characterized by the fact that they can charge multi-cell battery packs. In addition, the chargers are designed in such a way that they can also charge and discharge rechargeable batteries with different technologies such as NiCd, NiMH, LiPo, LiIon, LiFePO or lead.

But that's not all. Many model makers use replacement batteries to operate their model without long pauses in charging. In this case, it is convenient for the charger to charge two or more batteries at the same time.

And so that the rechargeable batteries can also be recharged quickly, the chargers work with partly very high charging currents. Of course, the charging cable and the plug connectors must fit perfectly to the rechargeable batteries, otherwise contact errors and damage can occur very quickly.

Since model builders use their chargers at home and on the move, many of the devices have a 230 V connection as well as a 12 V connection.
Thus, the car battery can be used as a voltage source while traveling. On the other hand, some club homes that do not require a 230 V power supply have powerful 12 V solar systems to which the chargers can also be connected. Chargers with only a 12 V connection are simply operated at home via powerful 12 V power supplies.

What was FPV?

The abbreviation FPV stands for First Person View and in this context means as much as the view from the cockpit of the model. In principle, FPV enables what model builders have been dreaming of for many years. You can now see exactly how it would be if they could fly with their model or how the feeling is when you look through the windscreen of their model car.

For this purpose, small, light and powerful cameras are installed in the model, which transmit their image signals via radio to a receiver. The images can then be viewed on a smartphone, tablet or monitor.
If the model pilot then wants to control his model exclusively from the cockpit view, monitor glasses have proven themselves best. However, it takes some practice until you get used to the unfamiliar view. In the first place, copters or drones are equipped with cameras, whereby legal regulations must be observed here.

Legal requirements for FPV flying

In general, drones or copters may only be flown at sight. This means that the pilot must see his drone at all times at the transmitter. Consequently, it must not fly too far away, otherwise the flight attitude will not be clearly visible.
The use of a video goggles is only permitted if the flights take place up to a height of 30 meters and the aircraft is not heavier than 0.25 kg or if another person is constantly observing it in sight and is able to alert the controller to dangers. This is then considered to be operating within the control's range of vision.

The premier class in FPV flying

In the meantime FPV flying has also established itself in the racing scene. Powerful racecopter are used here to fly through car parks at a ludicrous pace or to attract tens of thousands of visitors at major racing events in epic locations such as the Drone Champions League (DCL).