SERVO PAGE

This page is maintained by Michael Rodov from Israel.

Without servos you can not control your buggy. So these electronic wonders are important and need to be handled with care.

There are several manufacturers making these things. Some known brands are Futaba, Ko Propo, Sanwa (Airtronics), Hi-Tec, JR (Graupner) and Multiplex. Naturally there are some other far east factories spreading their products on the market. They are not always the best choice for our hobby.

The minimum torque needed for the steering servo is 5 KG. For the throttle a 4 KG servo will do OK but more is always welcome to have sufficient brake power. Ball bearings are advisable and also waterproof housings. Transmission times should be less then +/- 0.22 secs.

!WARNING!
Factories are always giving torque and transmission rates. Some for 4.8 volts and some also for 6 volts. Be aware that some receivers like the popular JR XR2/XR3 have a built-in BEC (Battery Elimination Circuit) that will provide a constant 5,2 volts for the servos even if you use a 6 volt or higher receiver pack.. So if you think that your servo is running at 6 volts, it might be that it only runs at 5,2 volts due to this BEC. Some BEC's voltage may vary between 5,0 and 5,8 volts. This will have effect on your maximum transmission speeds and torque. For 1:8 scale buggy racing we do not need a BEC, so if possible switch off this function or buy a good after market receiver without BEC.

!TIP!
Servo life will be shortened dramatically when the servo is strained to much. If you throw the steering full left or right and you still hear the servo motor rumble, then this is the sign that things are not Ok. At the maximum end points the servo much be quiet. If not, quickly change the end point settings on your transmitter (if possible) to a lower setting. Also check without servo horns attached if all steering links are running free and easy from pressure or strain by hand. If not, solve this first. Heavy linkage may also drain your battery quicker due to the high currents especially when high torque or digital servo's are used. Also be careful with the throttle servo. When the brakes are applied do not overrun the transmitter settings for maximum brakes even when the car is not braking enough. Always use silicone tube or small springs for pressing the brake pad levers. If the brakes are not strong enough check first the status of the brake disks and pads. Fuel on fiber disks can make the stopping power low even when you use a high torque servo.
Click here for full instructions how to set the right amount of travel on the servo's

!TIP!
Some servos like HI-TEC 605 and 615 are designed for working only on 4.8 volts, where other can even function on 6 volts. When a specially 4.8 volt designed servo is connected on a 6 volt (5 cell) battery, things like jittering in neutral position can occur. Specially when used as throttle servo it might happen that the servo is not always following your commands. Due to the over speed the signal coming from the potentiometer inside the servo is not correct. This will give overshoot and things like that. When used on 6 volts as steering servo it might that these problems not occur due to the fact that we use the steering in a more gentle way. No constant pulsing like we do with throttle servos.

!TIP!
Swap after every three races the steering and the throttle servo from place in the radio tray. Why?
The throttle servo has a different way of being used (more pulsing from one point to the end) then the steering servo. By swapping them the potentiometer that regulates neutral position will expand its life as it will not always be used at the same place as neutral point. 

!TIP!
Mount the servo with the original rubber grommets and bushings if possible. If car kits come with thicker mounting screws still try to use the bushings that come with the servo by using smaller screws or other contra mounting plates. Never over tight the screws that secure your servo. If you do that, the vibrations will go through the servo more and life will be reduced dramatically. The purpose of the rubber grommets is to absorb the vibrations! If the bushings are not used, over tightening of the screws might occur! Click here to see how to mount them the right way.

!TIP!
When using high torque or digital servo's it is advisable to use high capacity receiver pack. Due to the higher currents the receiver battery is quicker drained as you might expect. So if you was used to use 500 MAh batteries for normal servo's try to look for a 900 mAh type or the latest generation Ni-Mh batteries with a capacity of minimum 1000 mAh.

DIGITAL SERVO'S
Some factories are offering nowadays digital servos in their program.
Some like them, some not.
These servos are mostly working with higher pulse frequencies as the normal standard servos. The normal servo uses 50 Hz pulses to steer the servomotor where the digital are working around 300 Hz or even higher. This is giving better performance to the motor and saves lifetime (as used with electronics speed controllers for electro powered cars) and enhances the torque. This means that the servo will give sooner his maximum torque.
Where the normal servo reaches his torque peak after10° to 15° rotation of the output arm. The digital ones are giving maximum torque already after +/- 3° rotation as claimed. The use of FET's to power the motor is also giving better performance values. Also the number of steps the motor makes and the accuracy is increased in comparison with normal a servo. But this is something we as buggy drivers won't notice in comparison to a model heli pilot.

As most servo manufacturers are also using servo motors with less windings the current these wonders are drawing can be excessively high. During normal racing conditions a normal 500/600 mAh, Nicad battery pack might be drained before the end of the race. The use of 1000+ mAh Ni-Mh batteries is advised.

Useable servos for buggies are

All values are taken from factory specs. Real figures may be different depending to the loads applied!

The output shafts of some brands are not the same. 
Below the universal fitting of brands.

Click here for correct End Point Adjsutment of your servo

The following chapter is partly copied from TOM'S page and is giving solutions to solve wandering and jittering servos in some cases.

Servo wandering and jitter are common problems which develop in servos with use over time, and this problem can often be repaired without necessitating the replacement of the complete servo.

The servo consists of some control circuitry mounted on the circuit board, a motor which provides movement, a gear train which multiplies the torque of the motor, an output shaft which is the output of the servo, and a feedback potentiometer. The feedback potentiometer is a device which tells the control circuitry where the output shaft is, so that it can tell the motor which way to go to make the output shaft be where it is supposed to be. Servo jitter and servo wander are most often caused because of a dirty or worn potentiometer. The output shaft of the servo is supported by a bearing -- cheap ones by a bushing type bearing and expensive ones by a ball type bearing. A worn output bearing may cause output shaft wobbling which can only be solved by replacement of the bearing. This wobbling can also contribute to servo wandering due to movement of the feedback potentiometer input shaft.

To rectify the problem of jitter, the servo must be disassembled, the potentiometer removed, and taken apart. In order to do this, first the case must be taken apart, and then the motor and circuit board removed. The potentiometer is sometimes affixed directly to the circuit board, in others the potentiometer will be attached with lead wires. It is most often affixed to the case with a small screw. On some servos, removing this screw will enable the potentiometer to be removed, and on other servos this screw holds the back of the potentiometer onto the front of the potentiometer, thus disassembling the pot. Once it is removed, it must be disassembled (if not already done) -- they are normally crimped together with three tabs. Once two of the tabs are bent back, the top of the potentiometer should come off. Note the orientation for ease of future re-assemble.

If you've followed the steps so far, you'll note that the potentiometer is of the carbon type, not the cheaper wire wound type. This is because only the carbon type has the required resolution for precise servo positioning. You will note that there are some small fingers which form the wipers on the potentiometer, and these contact the carbon on the other half of the potentiometer. Often in older servos, these wipers will wear completely through the carbon, making for a poor contact. If they are bent slightly so that they ride on fresh carbon, good contact can be restored. Since everything is apart anyway, it is a good idea to clean the carbon surface with some alcohol, if it shows any sign of crud on it at all.

To re-assemble, just put the potentiometer back together (being careful not to damage the wipers), crimp it or install the fixing screw, and then put everything back together. Everything should be able to fit pretty much only one way.

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