ComXim MT200RUWL20 Rotating Electric Turntable review

[Andy Perrin]

My ComXim MT200RUWL20 Rotating Electric Turntable, which can be found many places online, including Amazon, at various prices arrived finally from China. Back in February, Amazon didn't have any units for sale, so I bought it on AliExpress, but it seems to be available there for slightly less money now. When shopping, be VERY careful to check the model number. The model number actually encodes the features of the turntable in question via the following code:

 

MT200RUWL20 = 200mm diameter turntable, with Remote control, USB interface, and WIFI support, and it supports a max load of 20 kg (L20).

The possible diameters are 200mm and 380mm.

The full list of possible letters is: K: Adjusting knob, R: Remote control, U: USB interface, C: Serial port, B: Bluetooth, W: WIFI, S: Synchronized shutter

The maximum load possibilities are 20 kg or 120kg

 

From this breakdown, you can understand how easy it would be to buy the wrong model, particularly since they all look similar and the only visible difference is size and which ports are on the side. 

 

The turntable arrived in good shape from overseas, and it was well-packed. The remote does not come with batteries, but AAA batteries are easily obtained. Setting up the turntable is as easy as plugging it in. By default when you plug it in, it will start to rotate continuously. Pushing the stop button on the remote halts it. Any command to the turntable produces a loud beep by default, but there is a mute button. The turntable is not too light and feels surprisingly well-engineered, especially based on the photos you can see online — it's much more solid than it looks. The 20kg maximum design load would clearly strain the device, but I wouldn't hesitate to put 15kg on there. The remote allows you to set a "0 degree" point, so it is possible to select absolute angles in reference to this point. A button on the remote resets the device back to your chosen zero point. You can set it to rotate in increments as small as 0.1 degree (although I'm not sure how accurate it is). Most of the remote is self-explanatory, although it helps to read through the diagram in the manual the first time.

 

People here who purchase this will almost certainly want to control it by a PC, Raspberry Pi, or Arduino. I was pleasantly surprised to find how easy this was to get started with. My first attempt was via WiFi. If you want to control it by WiFi, you may want to buy a separate WiFi USB stick, because otherwise you have to devote your whole WiFi card to controlling the turntable and you are then cut off from the internet (unless you have a wired ethernet connection). Once you connect to the turntable's hotspot, which is password protected (kinda) with an unchangeable password that is in the publicly available manual (), you can send it commands by TCP/IP to 192.168.181.181 on port 8181. In MATLAB this is done by making a tcpclient object and then using writeline to send commands. I'm sure analogous procedures would work in Python or C with the appropriate libraries. This method of control was easy but because it required my whole WiFi card to be devoted to the turntable, I moved on to control via USB.

 

This version of the device supports USB and comes with a USB cable (again, of higher quality than I was expecting given the source and what I paid for it). The ComXim company has a USB driver that they offer on their website. I did NOT download this driver, which wouldn't work on a Mac in any case. To control the turntable over USB without the company's driver, you need to download and install the FTDI USB serial D3XX series drivers. This driver makes a USB-to-serial connection, and then you can use the serial commands in MATLAB (or your language of choice) to control the turntable. The company posts a guide to secondary development with a list of available commands on its website. Most of what you can do with the remote can also be done with the serial or TCP/IP commands, but there are some baffling omissions. One of these omissions is a way to set the absolute zero degree position in software. You also can't tell it to return to the zero degree position, even though there is a button for that on the remote. What you can do is pre-program a certain number of rotations, by a set amount of degrees, with a pause time of your choice between the rotations, and if your model has a shutter release you can have it snap photos during the pauses. You can also tell it to take just one step by an angle of your choice in a particular direction (clockwise/counterclockwise) and stop. Finally, if you want it to rotate continuously, you can tell it to do that for any number of revolutions.

 

All of this meets my needs, so I am satisfied by the purchase. There are some bad points, however. The biggest of these is the website (www.comxim.com) and provided software. The ComXim website is UNSECURED (http, not https!) and caused my browser to give warnings. It is also mostly Chinese, which I guess makes sense for a Chinese company, but they should really put some effort into making an English language site too, if they are going to sell to English-speaking customers. The documentation is available in PDF form on that site, and the documentation is both good enough that I could figure out how to use the product without problems, yet full of horrible half-translations that range from unintelligible to hilarious:

 

"After electricity: - The start and lose of heartbeat: lights out"? Are we talking about a turntable or a defibrillator here? 

 

Overall, my impression was positive, though. For the fairly low price, you get a mostly-programmable turntable with (nominally) 0.1 degree precision, compared to paying hundreds for a piece of lab-quality equipment. There are many projects of interest to UVP members that could use a turntable, including the use of less-expensive line-scan cameras for SWIR etc.

[colinbm]

I would be interested in how accurate it is in the degrees steps, Andy please ?
200mm dia = 628.38mm circumference, so 1 deg =1.75mm on the outside diameter.
So if it can do multiples of that & be accurate, or 10 tenths of a degree to 1.75mm I would be interested in one.

[Andy Perrin]

I have no way to check the accuracy, colin. I don't have so much as a protractor around here. Measuring distances on a curved edge isn't especially easy either. If I come up with something I'll post it. Since it knows where the zero position is, presumably it does have some kind of encoder (with what resolution, I'm not sure) so it's probably not entirely based on the steps in the stepper motors.

[ulf]

With a laser pointer you can measure angle steps quite well.

If you fasten the pointer to the turntable, (tape, putty...) pointing far away you can measure the spot's movement for a few of the smallest angle steps and also determine if the step lengths are similar

[Andy Perrin]

Ooh good idea. I will try that. 
 

Another thought might be to draw a couple of dots on the turntable and track them under a macro lens.  

[ulf]

59 minutes ago, Andy Perrin said:

Ooh good idea. I will try that. 
 

Another thought might be to draw a couple of dots on the turntable and track them under a macro lens.  

That might work too, but I think a laser pointer is better if you have space enough for a long beam. Their spots are reasonably sized even after tens of meters. If you have limited space you can bounce the beam several times via mirrors.

I remember fine-tuning some optical windows via a mirror fastened in the ceiling.

I have also studied stability of tripods with cameras and long lenses by temporarily mounting a small mirror on the lens. 

Same principle as this:

https://en.wikipedia.org/wiki/Mirror_galvanometer

 

[Andy Perrin]

I don’t really have the space and I don’t own any mirrors really. I live in a very small apartment. I’m not sure how to measure the distance from the turntable to the wall accurately either, which is needed to get an accurate angular measurement.

[colinbm]

This is why I did the calculations for you Andy.

1deg = 1.75mm

10deg = 17.5mm
These are not that far on the circumference to need anything then a simple ruler.

[Andy Perrin]

Ulf, I bought a laser distance measuring tool, so I can combine your idea with the distance from the laser measurement tool and hopefully get a good measurement of everything simultaneously. 
 

There is also the question of how repeatable the movements of the turntable are, which hopefully we can also resolve by doing high quality measurements. 

[ulf]

Looking forward to your findings.
The nice thing with the laser pointer + mirror is that it is so very basic.

Normally you can use just a small pice of glass as a mirror.

The surface reflection is enough in a darkened room.

In measurement situations where the weight of the added mirror must be kept low a microscope cover glass is perfect.

I looked up some siblings of your turntable and they have controls for down to 0.1°.
Repeatability and deviations from ideal behaviour will be present due to drive motor technology and gear design.

[dabateman]

@colinbm,

Your work, experiments and creative process is valued. 

Just remember that everyone thinks a little different and will come at problems in a different way.

 

I love this post for reference: 

https://www.lensrentals.com/blog/2017/09/my-not-nearly-complete-but-rather-entertaining-circular-polarizer-filter-article/

 

Read how Aaron solved the problem at the end.

[dabateman]

For consistency I would probably just tape one of my felt markers to its edge. Then ask it to turn every 1/2 second or 1 second.  With the turn table on a large piece of paper,  the waiting points should have more ink than the turning points. Then I would just look at the dot pattern to see if it's uniform.  

I currently can't think of a project where I would need more accuracy in this measurement. 

[Andy Perrin]

That also seems really imprecise to me, David. Like, just attaching a marker securely enough to the table that I don’t get loosening during the test, but not so rigidly I can’t get it off seems hard. 
 

Anyway, I have acquired a laser distance finder and I will use it in conjunction with Ulf’s suggestion to do the test. My room is roughly 6m long, so each 0.1 degree will be roughly a cm. I have a digital caliper that should allow us to measure individual steps with good accuracy and using the distance from the laser measure will let us get the true average angle and a measurement of the standard deviation.

[Andy Perrin]

Alright, hopefully this will answer all the questions (haha, does that EVER happen in science?). 

 

Procedure

 

I got a Laser Distance Meter (LDM), which bounces a laser off a target and gets distance from time-of-flight. I attached the LDM to the turntable with putty, after first determining where the center of the turntable was by putting a marker on it in different spots while it was spinning continuously until I found a point that didn't draw circles. I then rotated the turntable using the remote control until the distance on the LDF was as small as possible (5.862m from rear of device), and started to increase if I went any further. This was to make sure the laser started off perpendicular to the wall. The marking which is perpendicular is the fourth one from the right. The size of the laser dot is fairly big, so I had to mark the center of it by eye. You can see how big the laser dot was below.

 

I moved the turntable around by the remote control in 0.1 degree (nominal) increments. The steps were pretty consistent as long as I went in the same direction. I measured them afterwards with a digital caliper.

 

Discussion

 

When I did the calculation afterwards, I found that (with 7 measurements...) the mean was 0.10 deg and the standard deviation was 0.01 degree. In other words it was pretty much the nominal value within my ability to measure. HOWEVER, that said, I also found that the top of the turntable was quite slack, and could be rotated by hand by as much as 0.5 degree. This caused me problems because it made the measurements not very repeatable, since every time I got to the end and switched direction, the wiggle room would interfere with the measurement.

 

Conclusions

The 0.1 degree nominal increment value can be trusted (to within 0.01 degree) provided you keep turning the table in the same direction. There is considerable slack (half a degree), so if you want to go back the other way, don't expect to land in the same place. It's probably fair to say that the turntable is accurate to within one degree, taking the wiggle room into account. Obviously there may be variations between devices also, so the above applies only to MY turntable for sure. But hopefully those of you who want to evaluate whether it is accurate enough to meet your own needs can use this as some guidance.

 

Laser Distance Meter (LDM):

 

To show how big the laser dot was in relation to the spaces between markings:

 

Final measurements (note that the L in the photo is measured from the BACK of the LDM, which was 112mm long).

Calculations. Step size is labeled ∆theta, the absolute angle was theta, and x was the displacement from the origin, when the beam was perpendicular to the wall. (note that the L in the calculation is measured from the FRONT of the LDM)

 

[dabateman]

 

Sounds like your back lash is quite high. But not bad and you measured that as well. 

Good results,  so you can trust the box. That can be a rare event for cheap Chinese products. 

[Andrea B.]

Editor's Note: cleaned up topic a bit