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SmartWinch and High Power Rudder Servos 

The article on this page was written for the ARYA newsletter "Radio Waves". It goes into detail about the issue of using high power servos for rudder control with the SmartWinch. I will begin this page with a summary of the issue.

Summary 
The SmartWinch voltage regulator can supply a maximum of 1 Amp to the Rx. If the rudder servo requires more than 1 Amp there will be a voltage drop in the Rx. Voltage drop in the receiver can cause loss of control. This is particularly noticable with 2.4 GHz radios where it can take several seconds for the system to recover. 

Detail 
I’ve been answering queries on this topic for a year or a few now so it’s well past time I wrote this article. It’s about glitching (momentary loss of response) when the rudder servo is operated. In the majority of cases it’s due to the use of high power rudder servos. It’s that particular cause I’m writing about here.

In recent years there has been a trend towards the use of high power servos for the rudder. The reason for using high power servos these days is more about the idea that more is better and the availability of cheap high power servos rather than need. It’s easy to think that a servo with a torque spec of 8 kg.cm must be better than one with a spec of 4 kg.cm. But does the extra torque really give any benefit?  For classes up to and including A’s, I don’t believe so. Back in 1980 when I started playing this game I was sailing the old heavy weight A class. There wasn’t the servo choice back then that there is today. But the standard 3.5 kg.cm rudder servos I used did the job.  These A’s were the old heavyweight designs of around 50 to 90 lb and up to about 1500 sq.in sail area. If a 3.5 kg.cm servo is ok for a heavyweight A, surely it’s ok for an IOM. But some skippers are trying rudder servos well over double that torque in their IOM’s. This is quite unnecessary and can cause problems.

As well as high torque a high power servo can be high speed. Even if the torque is moderate, a high speed servo still requires more current than a standard one. The term “standard” has become misleading lately too. Previously, a “standard” servo was standard speed and torque. But now it basically just refers to the case size since there’s several other case sizes available. In the standard size case there’s now a big range of torque and speed specs. So be careful picking a “standard” servo to go with your SmartWinch.

To digress for a moment, I’d like to dispel a myth. I’ve heard some people suggest that the SmartWinch will not work with digital rudder servos. This is not so. Problems are not due to whether the servo is digital or analog. It’s about the amount of current the servo needs. A digital servo and an analog servo with the same speed and torque specs both require roughly the same current.

Anyway, back to glitching. The problem with using high power servos is that the SmartWinch voltage regulator (which supplies not only the SmartWinch controller but also the Rx and rudder servo) has a limited current capacity of 1 Amp and high power servos can require more than this.

When a servo is used that requires more current than the regulator can supply, a voltage drop results and therefore the Rx supply voltage is reduced. When Rx voltage drops below the Rx (or servo) minimum operating level then things will not operate as expected. It might only be a very short glitch that is seen when the radio system is AM, FM, PCM etc. since at the instant of the Rx drop out, the load on the power supply is removed so the voltage recovers and the Rx will instantly restart. But in 2.4 GHz Rx’s drop out can result in several seconds of no response while the Rx is reconnecting to its Tx. During this time, depending on how the 2.4 GHz system is setup the rudder servo and winch may go to failsafe position. And just briefly, here’s a tip; set rudder fail safe as centre and winch failsafe as full out. Setting rudder failsafe as off centre would not be popular with fellow competitors if the system drops out while the boat is in the middle of a pack.

I hunted up a few servos I had lying about in draws and did some tests. The radio system is not really relevant but f.y.i. I used a Futaba 2.4 GHz system. The Rx was supplied with power in the normal manner from a SmartWinch. The SmartWinch was supplied power via a bench top power supply giving 6 volts to the winch power supply leads. It makes very little difference what power supply is used as to rudder servo performance. As long as that supply can give at least 1 Amp at over 5.5 Volts then the winch regulator will behave the same regardless of the type of supply.

The test was simply to rapidly move the Tx stick back and forth to get the servo pulling maximum starting current. This is similar to stall current but only lasts for a very short time. To detect the degree of voltage drop from the no load Rx voltage of 5 Volts an Oscilloscope (CRO) was used. This allowed the very short period drops to be seen and measured. Another method that you can use yourself is our Flash Digital Voltage Display if you have one. The Flash DVD continuously measures its supply voltage over its sampling cycle (about 2 seconds) but displays the lowest value measured during the cycle. This is not as accurate as a CRO but pretty close. I did some comparisons and found the Flash to be within 0.1 Volt of the CRO in these test so it does get the very short period negative voltage spikes quite well. Don’t bother trying to measure voltage drop with a multi meter. They are way too slow.    

Servo voltage drop (V) torque (kg.cm) speed (seconds/600) digital or analog
NES-505 (older std power) 0.2 2.9 0.27 A
DS-559 (std power) 0.3 4.8 0.23 D
ES-539 (std power) 0.3 4.8 0.23 A
DS-8511 (high torque) 1.9 15 0.19 D
HSC-6965 (high speed) 1.9 6.6 0.1 D


All of these servos are in standard case size. All but the Hitec HSC-6965 are JR servos. Torque and speed are shown and it can be seen that as servo power goes up, so does the degree of voltage drop. These specs are the 4.8 V specs. Whether the servo is analog or digital is irrelevant but is included anyway. Interpolating the table suggests that a servo with speed around 0.2 and torque of around 8 kg.cm would result in a drop of under 1 Volt which is about the safe maximum drop.

Considering most Rx’s will drop out at around 3.5 to 3.8 Volts and the regulator is supplying 5 Volts, a drop of over 1 Volt is getting dangerously close to causing trouble. The DS-8511 and HSC-6965 both caused the Futaba Rx to drop out. They were both pulling the Rx supply down to 3.1 Volts.

So using a high power rudder servo with a SmartWinch supplying Rx power is not a good idea. Stick to standard power rudder servos and you won’t have problems. And unless you are sailing something larger than an A, your boat will still turn when you want it to. For what it’s worth, I have a DS-559 in my IOM.

If you do have a requirement for more power in the Rx then there’s a couple of ways to do it. One is to use a separate Rx battery and the other is to use the single battery as per normal practice but run a separate pair of leads from the battery direct to the Rx. In each of these cases, it’s essential to remove the red lead from the winch to Rx lead. Otherwise the winch regulator can be damaged.

If a single battery is used, it should be a voltage that the Rx and servo/s can handle such as a 4 or 5 cell pack. If using a single 6 cell pack or 2 cell LiPo use a voltage regulator such as the MuchMore Racing CTX-Regulator between the battery and Rx. These are available in 3, 5 and 7 Amp capacities. I don’t know if they are in Australian Hobby Shops but they are available online at Tower Hobbies (http://towerhobbies.com). I’m not sure if there are other similar regulators on the market but there probably is.

If using a second battery then you can use a 6 pack NiCad or NimH or a 2 cell LiPo for the winch and a 4 or 5 cell pack for the Rx. But don’t forget to remove the red lead from the winch Rx lead.

But seriously, the only reason this needs to be considered is if the boat is larger than an A or you are using more servos than just a rudder servo out of the Rx. As mentioned earlier, standard power servos are plenty for the rudder servo in all classes up to and including A class. But interestingly the enquiries I get about system glitching due to use of high power servos comes mainly from IOM skippers. Surely if a 3.5 kg.cm rudder servo will do an A it’s going to do for an IOM. So why invite trouble using higher power servos than you need to. Ok so higher power servos are now available. So what, IOM servo power requirements has not changed since the inception of the class. Were skippers saying way back in the early 1990's;  “gee I wish there was a more powerful rudder servo available cos my IOM won’t turn”?                       

Rob Guyatt