Adding A Servo Winch To A Vanquish VS4-10 RC Crawler (And Testing Voltage, Current, And Pulse Width Modulation Signal)

So… I told you I picked up a new hobby. I can’t even remember quite why exactly, but right before Christmas, I got it in my head that I wanted to get a radio controlled truck known as a “Crawler”. The name refers to the fact that the vehicle goes slowly but capably, i.e. crawls, up difficult terrain much like its full-size off-road vehicle counterparts. With the help of a local RC hobby shop, I picked a Vanquish VS4-10 chassis kit and built up a “ten-scale” truck, installing a motor with a combined “ESC” (or “electronic speed controller”), a radio receiver, and a steering servo. I got it built up and even hiked with it up a couple of mountains.

Of course, not obsessing does not seem like an option for me when it comes to my hobbies, so the next plan is to take the little truck to a rc crawler exhibition. A bit of researching and I learned one of the rules for these events is often no “hands of God” on the course. This euphemism describes when you accidentally overturn your vehicle, or otherwise get it stuck, you cannot use your hands to pick it up and put it back on track. You certainly can, however, use a tiny winch to get yourself out of a jam.

This is a LONG blog…here’s a quick table of contents so you can jump through it if you want.

• RC SERVO WINCH OVERVIEW
• RC RECEIVER VOLTAGE TESTING
• RC RECEIVER AUXILLARY SIGNAL TESTING
• A 3D PRINTING INTERMISSION…
• RC SERVO WINCH CURRENT TESTING
• CONCLUSION

RC SERVO WINCH OVERVIEW

After poking around the interwebs, I found this winch based on this youtube video that showed it being installed into a Vanquish VS4-10 Pro crawler. My expectations when it arrived were that I would simply plug the 3-lead (power, ground, signal) servo connector into the crawler’s receiver, screw it into the open slot on the front axle, run the winch “rope” through my new front bumper, and I would be all set. Instead, my servo winch arrived wired to have the signal and ground leads (without a power lead) inserted into the receiver, and a power and ground lead (terminated with a JST connector) which apparently were meant to obtain power from a source separate from the receiver.

Below you can see the components of the servo winch. Notice the heat-shrinked winch controller, apparently a circuit board of some type; I wish I had a schematic. You can see the separate signal and power bundles. Referencing back to the product’s website, the only useful indication of electronic characteristics is “up to 6v through receiver or BEC”. This leaves so many unknowns for me. The servo itself is hidden within the 3D printed housing, and you can see the spool of 500lb test line that that the servo turns. Ultimately, I have zero idea what the specs of the servo are at this point and will need to do some testing to figure it out.

I went to one of the local RC hobby shops on a very busy Saturday to see if I could any insight into how to approach installing the winch on my little truck. The staff did not have the bandwidth to deal with my pesky questions, so the best I could gather was that many people wire a winch directly into their RC battery. I picked up some semi-random connectors hoping they may end up being of some help. I got an extra XT60 female-to-Deans male connector, which matches what I currently have attached to my battery. Stripping away the heat shrink, I could see the solder points on the connector where I could attach a pigtail to power the winch if I decided to go directly into the battery. I also picked up Futaba connectors, which are what connects ground, power, and signal wires to the receiver box. In the picture below, I have wired ground (black) and power (red) leads into the Futaba housing terminating with the Futaba female connectors. The blue signal wire awaits termination. After connecting all three wires, I now had a bundle that I could use to test the properties of the receiver.

RC RECEIVER VOLTAGE TESTING

My first order of business was to test the voltage coming off of the receiver. A bit of research made me think that the ESC of the Hobbywing Quicrun Fusion Pro 2-in-1 brushless motor provided 6V to the receiver and was rated for 3A of current. I inserted my Futaba-type servo cable with its red power wire and black ground wire only into the “AUX1” port of the Spektrum SR515 receiver of my Vanquish truck; the free ends were unterminated wire stripped bare. The picture below shows the receiver with the Futaba termination with hot, ground, and the blue signal wire in place (something I connected later on). Two other Futaba connectors are also plugged into the receiver in positions labeled “STR” for steering (which receives signals from the RC transmitter’s steering wheel, and sends those signals along with power to the steering servo) and “THR” for throttle (which receives power from the motor’s ESC and sends signals from the transmitter back to the motor to control the speed of the truck).

I hooked my Bell Labs KS-14510-LII multimeter to the free ends of the Futaba test jig and turned the power on (from a 11.7V LiPO battery) to my RC crawler. Don’t mind my messy laboratory!

Below you can see that there are 6 volts of electric potential between the power and ground leads coming from the receiver. Read the bottommost part of the black scale at the needle crossing on the multimeter below. Note that at this point there is no blue signal wire attached to the receiver’s AUX1 port as shown above; I put the blue wire on later.

I now have this information to consider. I have an RC servo winch that apparently will run off 6V from the receiver based on the limited information on the product’s website; there is no contact information for the company’s owner that I can see. I have no idea how much current the winch draws, as I do not know what servo is actually used. I have read that RC servo winch’s can average around 7A of current draw which seems ridiculously high (about the amount of an electric drill for instance) and will need to be tested. Particularly since I do not believe the ESC provides more than 3A to the receiver, and that 3A needs to be split between the steering servo, the receiver itself, and any auxillary devices. If the winch draws anything more than an amp or so, it will need to be run directly from the battery. And if I do have to run the winch directly from the battery, I will need to find a way to decrease the voltage from 11.7V (the nominal rating of the 3S LiPO) to 6V (the apparent rating for the winch’s servo).

In order to tackle the issue of running the servo winch directly off the battery, my first thought was that I would need a voltage regulator to drop the nearly 12V output of the battery to 6V. Below is a handy device I built that if I swapped in a LM7806 voltage regulator (I made it to be interchangeable), I could do just that. The problem though was that the LM78XX series of voltage regulators could only output 2.2 amps. So if the winch really required a thirsty 7A, using this device would not work. The other problem is that this particular circuit does not have any current backflow protection, so I would be hesitant to want to actually use this directly off the battery in the use of an RC vehicle.

Figuring this was a common problem, I looked around for solutions. One I kept encountering was called a “BEC” or “Battery Eliminator Circuit”, which promised to eliminate the need for a separate battery with a different voltage to operate your RC accessories. It turns out that a BEC is simply a voltage regulator as far as I can tell. I also wondered about a simple voltage divider circuit, but again, if the winch actually draws 7A, this may be more than my easily accessible components can safely handle. I knew I would need to test the actual amperage of the winch in order to decide how to install it in the crawler.

RC RECEIVER AUXILLARY SIGNAL TESTING

The next order of business, mostly because I was curious, was to take a look at the signal output of the receiver. How exactly did the receiver control the servo winch?

I again tested the hot (red), ground (black), and now signal (blue) wires from the Futaba connector coming off of the receiver. Below I am looking at voltage only again, but this time with the o-scope probe on the red wire and the probe’s ground lead on the black wire.

Channel one is set to ground in the picture below.

Switching channel one to DC shows the actual voltage across the receiver’s auxiliary port.

Changing some display settings on the oscilloscope gives a measurement of approximately 6V, which is the same as what we measured with the multimeter earlier.

But what about when I switch the the oscilloscope probe to the blue signal wire and the probe’s ground lead on the black wire? I get a pulsed signal as shown below.

Again, manipulating the display of the oscilloscope shows a pulse output (the short horizontal bar) around 3.3V, which is a typical value seen in pulse width modulation. The long horizontal bar is at ground, i.e. zero volts.

The Spektrum DX6C radio transmitter that I am using has the ability to program the control signals of the auxiliary ports of the receiver. I went ahead a program a 3-position button on the transmitter to cause the winch to do nothing if in the center position, to unravel if in the forward position, and to rewind if in the rear position.

In the neutral (do nothing) position, the pulsed part of the signal was 1.5 ms in length.

In the forward (unravel) position, the 3.3V pulse portion of the signal cycle decreases to 1.2 ms.

And finally, in the backward (rewind) position, the pulsed portion increases to 2 ms.

The entire signal has a period of 11 ms as shown below, which when inverted gives the frequency of approximately 91 Hz (i.e., 1 divided by 0.0011). In short, the winch controller knows how to behave based on how long of a pulse is sent to it by the receiver unit. This pulse width modulation, as it is called, is a very common way of controlling servos.

A 3D PRINTING INTERMISSION…

I have not talked too much on this blog about my 3d printer, an Elegoo Neptune 4 Pro, which has worked very well for me over the past few years. In order to compete and get points in the RC hobby, “scale” or small-sized model versions of everyday objects earns you points. I spent a bit of time printing a Coleman cooler, a six pack of beer bottles, and two sandwiches.

I also printed an articulated action figure composed of over forty different parts. I yanked his legs off and double-sided taped him to the inside of my truck body. Kinda creepy, right? In fact, this evening as I write this, I have been printing a bunch more odds and ends for my truck. I am hoping to compete in my first RC crawler tournament soon.

RC SERVO WINCH CURRENT TESTING

I needed to make another jig in order to test the current on my the servo winch in question. If you recall, I was not sure what magnitude of amperage this winch would draw. Some sources claimed 7 amps. Which meant that the only power supplies I had that could handle it were my ham radio ones. I do have an Alinco DM-430t, which is a switching power supply, good for 30A peak current, and most importantly can be adjusted to have a voltage ranging from 5 to 15 volts DC. It uses Anderson power pole connectors. And it turns out I already had wired up some power poles, complete with an on-off switch. I just needed to solder on the male JST connector pigtail that would mate with the power and ground leads of the servo. Below is a picture of the parts I was working with.

And here is the finished cable. I would use this to power the servo winch from the Alinco power supply. Simultaneously, I would use the receiver auxiliary port (without the red power wire attached) to get a control signal to the winch.

Below you can see my test set up. And if you look at what the arrow is pointing at, you can see a metal shelf, a leftover from the shelving unit that holds my oscilloscope and other test gear. This was used as the couple-of-pounds weight that the servo would try to lift.

This next picture shows how everything is connected.

With no load attached, the servo only drew 0.22A (according to the Powerwerx power analyzer) at 6.1V.

Lifting the metal shelf, the servo drew 0.38A at 6.1V.

I recruited AA1F to help me with a more scientific study using 2 1/2 and 5 pound weights instead of the spare shelf.

For comparison, my crawler weighs almost 6 1/2 pounds.

Below you can see a table of the various parameters of the tests we conducted. The servo winch did run without melting at 13.9V. I only tested it at this high of voltage because I forgot to switch the Alinco from its pre-set value. And at least for a short period of time it worked extremely well at that voltage.

When I backed off to 6V, the winch struggled to dead lift 5 pounds and completely stalled attempting to lift 7.5 pounds. It was pretty clear that this winch at its rated voltage would be underpowered for my truck.

The current never came close to the internet’s claim that RC servo winches draw 7 amps. The highest we saw in our testing was 0.7A at 13.9V and while lifting 5 pounds.

CONCLUSION

After taking all of this into consideration, I decided to go ahead and run the servo directly from the receiver’s auxiliary port. I could access 6V and draw less than 0.5A. I rewired the cable by splicing the power bundles ground cable into the ground wire of the Futaba connector. The asterisks below show where I cut the ground wire from the JST connector and where I attached it to the signal ground wire. I also needed to cut off the power (red) cable, add a female Futaba connector, and insert that into the middle slot of the Futaba connector.

A little clear heat shrink tubing was added to protect the otherwise exposed soldering.

All that was necessary now was to connect the Futaba connector of the newly rebuilt winch controller cable into the AUX1 of the receiver, and connect the JST connector back together with the one that was hanging off of the winch. Oh yeah, and screw the servo winch into the front of the crawler’s chassis. And inset the winch line through the bumper. And use an anchor knot to tie on the hook.

Whew! There she is! A beautiful, albeit wimpy and underpowered, servo winch sitting proudly next to the steering servo of my little truck!

This blog is a doozy! I need to get some sleep. Hopefully you learned something. Or minimally were amused at the thought of me 3d printing tiny beer bottles…

Hope to see you on the trails!

KM1NDY