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Setting Up Digital Servos

Don't burn out your expensive digital servos through improper set up

This article instructs you in the proper way to set up the servos in your plane or helicopter to ensure that they are not overloaded, causing them to heat up and burn out key pieces of your airborne system.  Please let us know where you found this article so that we can express our appreciation. 


As digital servos are becoming more popular, modelers occasionally have hot running or burned-out digital servos, wiring or extensions that have melted, and are unnecessarily causing their airplanes to crash due to incorrect digital servo and linkage setups or improper radio programming.


Another old issue is the receiver reboot problem. The causes of receiver reboot issues, which are also called "battery dropout", have remained the same for the last 15 years. A problem with the setup of your plane is indicated if the 6 volt electrical system (7.0-8.4 volts nominal, pre-regulated voltage) that normally powers your model is now running at 3.4 volts.


Many times the builders believe the problem lies in faulty parts or naturally high current flow in the digital servo. If flow damage occurs or there is a lack of power that causes receivers to shut down, there is an indication that the device has been set up improperly.  You may have been advised that use of a digital servo on the throttle would cause it to burn up.  If improperly intalled and set-up you run the risk of burning the servo up; it is not the fault of the digital servo.


In order to accommodate the increased current rates that are the result of setups, modelers frequently think that they should utilize 16 gauge wire and powerpoles, or Deans connectors power isolation systems and other devices. Although using these items isn't wrong, they often hide the nature of the problem. Usually the symptom is what is treated rather than addressing and fixing the problem as a whole.


In cases where a single control surface is being driven by multiple digital servo linkages, modelers should prepare their planes using a current meter. Improper current adjustment can result in erratic performance from servos, receivers, and regulators.  Improper ajustments can cause high idling current, high battery drain, or overheated or burned-out servos and regulators. In the worse case, improper current adjustment can lead to shutdowns from melted wires, burned-out servos, and overheated regulators or receiver reboots. Briefly, modelers are crashing planes from not properly understanding the cause and effect of their poor setup.


These problems usually occur when you are not using a current meter to set up the plane. In cases where the digital servos are improperly set up, the resulting servo preload or linkage binding causes an abnormally high current draw. If you want an accurate reading you must make the travel adjustments with a meter. So what impact does centering accuracy actually have during set up?


If a typical servo is moved from the left fully to the right, the servo makes a move of 1,024 incremental steps from one stop to another. Digital servos take 5,120 steps in order to cover the same exact distance from one stop to another, or 5 times the centering accuracy. The servos will work against each other if they are not properly installed and aligned.


As opposed to regular servos, if a digital servo experiences resistance coming from any source, it responds right away with high torque and holding power, resulting in a high flow of current.


Analog servos need extra travel length to accelerate up to maximum torque and full holding power. In conjunction with the substandard centering accuracy of an analog, as it relates to digital servos, modelers are able to get by with careless setups with analog servos that would make the same plane crash if digital servos were used.


Poorly aligned linkages, binding of tail wheels, poorly adjusted hinges or sub trim, endpoint and poorly aligned midrange adjustments always result in high flow of current.  They will cause burned out or hot running regulators or servos, high battery drain or erratic running servos or receivers.  With the the most current 2.4GHZ receivers, it is possible for a receiver reboot to occur.


Battery dropout is what we used to call it when the receiver reboot problem occurred.


A defective part in the system can also cause this problem, but the main reason is going to be improper setup. You could end up burning the servo so much, if improperly installed, that it has the potential to melt.


10-20 mah is the normal idling current of the digital servo, and should never be exceeded. The size and brand matter, so make sure you check your servo specifications. Your servo should never draw more than 10-20 mah.


To measure the current flow, simply plug the servo into the current in one end, and the receiver on the other end of the servo. If you are finding abnormal meter readings then you can immediately see that there is binding.


The servos current will idle when the aileron surface is in neutral. You will increase the battery draw, which in turn burns up the servo, if the current is higher than idle on the servo.


The three initial setup items that you want to use a current meter to measure are: subtrim, end point, and midrange travel. Travel adjust radio programming should be set to a maximum of 140-150' in order to obtain optimal servo resolution prior to connecting and modifying your arrangement.


*  Subtrim neutral


Turn the radio on, then connect one of the wing servos with the current meter and connect the current meter with the the matchbox or the receiver. Make sure the meter is showing a proper idle current, because if it is not, then the servo is fighting another servo. Use your prolink wrench to make adjustments to the turnbuckle; if this results in a worse reading, then turn it in the opposite direction to cancel out the servo's preload.


*  Endpoint


With you meter still engaged, move the aileron stick to a full right deflection point. Make sure you are monitoring the meter during this process. The current will be idle in most cases where ailerons are fully deflected against the stop. If you do not stop the servo travel after the ailerons bottom out, then you will see a high current draw. By moving the linkage connection to out on the control service or in on the servo, you can reduce the travel overdrive. Cutting back on end point travel, whether it be via the matchbox adjustment or by using the radio programming, is the next move to make as you fine-tune the total travel.


Complete the procedure again by pushing the aileron stick fully to the left, canceling out any preload using the same procedure. Always keep the throttle from allowing the servo to over drive the idle and high speed stop.  


*  Midrange Travel


Using the stick, we will now show you what the midrange adjustment is.  Move your stick half way and check the current reading; if it jumps in the midtravel, linkages are not adjusted correctly.  If you are utilizing bolt linkages like 8132 or 10132, with linkage fittings connected to the bolt using screws, the binding is the result of the fitting that the pushrod is connected to not being connected the same distance from the hinge line. To demonstrate this issue I will exaggerate the problem.  Let's say you are 20 turns away from the hinge line on one bolt and 10 turns out from the hinge line with the second one, then the mechanical connection may scribe a 1 1/2' radius on one servo and a radius on the other. Imagine what it looks like to see all of the linkages not set up to specifications.


To check the digital servo linkages for perfect parallel symmetry, disconnect the linkage and turn the fitting on the bolt inward a few turns. Reconnect the linkage and read the current meter. If the meter reading is worse, disconnect the linkage again and turn the fitting on the bolt in the opposite direction until the preload is cancelled (zeroed) out. While the servo linkage is disconnected, you can also check for a faulty servo.


If you are utilizing more that an optimal current and there is no linkage hooked up, the servo may be defective; although this is not likely, it is not impossible. It is easy to understand how such a high number of modelers who never complete these checks burn out regulators and servos, and draw so much current they drain their batteries after just a handful of flights. Those who do not know are inclined to believe that the battery, servos, switch or receiver is defective, when the real problem is incorrect setup. The point is not installing several other compensating items,  such as heavier wire, etc., the point is to set up the plane correctly, and nothing else.


Although it is not incorrect to utilize these things, these devices are not required when you set up the systems properly. Mistakes made during the setup process frequently result in plane crashes, where the blame is mistakenly assigned to components that failed due to incorrect setup by modelers. 


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