Which servos?

[RexAshton]

Apologies if this has been asked before or I've put it in the wrong place. I'm using Peco Smartswitch on a shunting plank. Generally I'm very happy with it apart from the servos. They are rough and noisy unlike the Megapoints set up with Hobby King 15178s I've recently installed on a friends layout. My question is, can you use other servos than the Peco offering on the Peco control board? I've tried the 15178s and they don't respond. I've tried reversing the plugs but this makes no differenece.

Any help would be appreciated.

[Izzy]

I'm afraid I have no experience of the Peco boards, but I believe the servos are the common/standard Towerpro SG90 type (which I agree are coarse and noisy, especially when used at slower speeds) so other servos with the normal wiring plug should work. The Hobbyking ones would appear to be around the same basic spec.

 

You may find more info here if you can trawl through it:

 

http://www.rmweb.co.uk/community/index.php?/topic/88415-experience-with-peco-smartswitch/page-5

 

or perhaps someone with the right knowledge will reply.

 

Izzy

[RexAshton]

I'm afraid I have no experience of the Peco boards, but I believe the servos are the common/standard Towerpro SG90 type (which I agree are coarse and noisy, especially when used at slower speeds) so other servos with the normal wiring plug should work. The Hobbyking ones would appear to be around the same basic spec.

 

You may find more info here if you can trawl through it:

 

http://www.rmweb.co.uk/community/index.php?/topic/88415-experience-with-peco-smartswitch/page-5

 

or perhaps someone with the right knowledge will reply.

 

Izzy

Yep. I thought the Hobby King ones would work. There doesn't seem any obvious reason why they shouldn't but I can't get them to respond at all.

[BG John]

Are they analog or digital? Hobby King seem to have both with a similar model number, 15718 and 15718B. I was trying to make sense of the difference, and I think the digital ones need more power, but I'm not sure if they need to be controlled differently. If there is a different, does the Peco switch handle digital servos, if that's what you've got?

[RexAshton]

Are they analog or digital? Hobby King seem to have both with a similar model number, 15718 and 15718B. I was trying to make sense of the difference, and I think the digital ones need more power, but I'm not sure if they need to be controlled differently. If there is a different, does the Peco switch handle digital servos, if that's what you've got?

Hadn't thought about that. The SG-90 is digital and the 15178 is analogue according to Google. 15178b is digital so I'll have to try one of those. Thanks for the heads up.

[BG John]

Hadn't thought about that. The SG-90 is digital and the 15178 is analogue according to Google. 15178b is digital so I'll have to try one of those. Thanks for the heads up.

There are analog and digital versions of the SG-90 http://www.towerpro.com.tw/product-category/servos-parts/micro-servo/ . As there are also loads of fakes, I bet most of them are analog though.

 

I don't know if it does make a difference, but thought it might. Where are the experts?

[RexAshton]

There are analog and digital versions of the SG-90 http://www.towerpro.com.tw/product-category/servos-parts/micro-servo/ . As there are also loads of fakes, I bet most of them are analog though.

 

I don't know if it does make a difference, but thought it might. Where are the experts?

I know about the potential fakes but as near as I can tell the ones supplied by Peco are genuine. If that's the case I'm now completely confused. The SG-90 appears to be analoge according to Tower Pro as their digital version says so on the label. That would indicate that it has the same structure as the HK15178 so I've no idea why they shouldn't work with Peco board. Very confusing.

[Podhunter]

Where are the experts?

On radio control Forums.  Servos have been in use in robots, model aircraft and boats for donkeys years.  Visit a model shop, or web site, that deals in planes and boats to see a huge variety of servos ranging from tiny devices for indoor fliers to hefty lumps used to control half-size Spitfires.

The sort of servo we need to control a semaphore signal or turnout is fairly modest compared to those that the R/C builders use.

Edited November 21, 2017 by Podhunter

[Izzy]

There are analog and digital versions of the SG-90 http://www.towerpro.com.tw/product-category/servos-parts/micro-servo/ . As there are also loads of fakes, I bet most of them are analog though.

 

I don't know if it does make a difference, but thought it might. Where are the experts?

 

I have used both analogue and digital servos and AFAIK there isn't any real difference as regards connection or power needs, it's more how they respond to instructions. I don't really know because I have never played with R/C equipment and using servos for railway use has been the first contact I have had, with a steep learning curve in understanding where they can fall down for model railway use, i.e. too much EMF interference etc. I gave up in the end and now just hack servos, (cheaper/easier), but that's another matter.

 

The experts with useful knowledge and advice about servos are usually on the DC and DCC forum pages and might miss this thead as a consequence.

 

Izzy

[ScRSG]

That's very strange we use both the TowerPro and Hobbyking servos interchangeably, so don't see any reason why they don't work. Servo experts, please!

[Gordon H]

The HK15178 should behave almost identically to the SG90 or any similar type. Mine do.

All such analogue servos are controlled and powered in much the same way, using typically 50Hz pulses of duration between 1ms and 2ms. The duration of the pulse determines the servo angular position (1.5ms is centre).

Some servo driving circuits disable the drive pulses once the servo has reached its position each time it is moved, I don't know whether the Peco Smartswitch does this or not.

[Suzie]

It looks like the "SG90 digital" is a hybrid SG90 with SG92 electronics in it.

 

As long as the servo has the positive in the centre of the 3-wire cable it should work. The interface is the same whether analogue or digital, they just behave a bit differently. Some people prefer the analogue versions with their slow starting torque and natural ability to smooth out jitter on the control signal, but I go for the digital every time for its high instantaneous torque when you want it to move and natural immunity from interference.

 

Strange as it may seem, most of the fake SG90 you will find actually have digital electronics in them. The analogue electronics is more complex/expensive to manufacture than the digital! Typically I have found the real (analogue) SG90 to be a similar price to the digital carbon fibre geared higher torque SG92R for the most part, I suspect the new digital SG90 is an attempt to make a cheaper servo to better compete with the fakes - just be careful you don't strip the gears with all that extra torque!

[BG John]

Is there any difference in the way you control analogue and digital? They may perform differently, but if you replace one with the other without changing the software that controls it, will both produce the same end result? I think that's the important question.

[RexAshton]

That's very strange we use both the TowerPro and Hobbyking servos interchangeably, so don't see any reason why they don't work. Servo experts, please!

The two both work happily together on the Megapoints board but not on the the Peco one. The layout only has two points on it so it's difficult to justify the Megapoints one especially as I have the Peco one already.

Edited November 21, 2017 by RexAshton

[BG John]

An Arduino should work. They can handle up to six servos. You can get them for a few quid.

http://www.ebay.co.uk/itm/Arduino-UNO-R3-ATMEGA328-CH340G-Development-board-Microcontroller-w-Data-Cable/253240204926

[Butler Henderson]

Always used SG90s although bit miffed that as none have had particularly been extensively used one died the other week.

Edited November 21, 2017 by Butler Henderson

[Suzie]

Is there any difference in the way you control analogue and digital? They may perform differently, but if you replace one with the other without changing the software that controls it, will both produce the same end result? I think that's the important question.

 

Yes, you control them exactly the same.

 

You can use subtle differences in the way you control them to eliminate some little quirks, but fundamentally all control systems send a pulse between 1us 1ms and 2us 2ms fifty times a second and don't care which type of servo is connected.

 

 

Edited to correct units! Thank you Gordon.

Edited November 21, 2017 by Suzie

[Gordon H]

Yes, you control them exactly the same.

 

You can use subtle differences in the way you control them to eliminate some little quirks, but fundamentally all control systems send a pulse between 1us and 2us fifty times a second and don't care which type of servo is connected.

Between 1ms and 2ms (not us) I think you mean.

The only difference I have heard of between analogue and digital is that some, if not all digital ones will try to maintain their position even if the control pulses stop.

[Suzie]

...The only difference I have heard of between analogue and digital is that some, if not all digital ones will try to maintain their position even if the control pulses stop.

 

1. Digital servos will 'tick' if there is any jitter in the control pulses as they operate the motor at full power to gain the new position, whereas an analogue servo will just apply a very small correction voltage to the motor and move it slowly and quietly. this is significant in battery powered environments when power consumption can be high with a digital servo on a jittery signal perhaps from an analogue radio source, and can be irritating if audible. A good controller should not cause ticking.

 

2. An analogue servo will respond to every pulse that it sees, and if it is a glitch it will move wildly. A digital servo will not move until it sees a consistent pulse, filtering out wild glitches and appearing to be a lot more stable. Glitches are usually induced in to the servo wiring from outside sources, and can often occur at switch on if the controller does not handle startup correctly.

 

3. A digital servo will always apply full power to the motor and move with full power so is hard to stall, but if stalled will put a lot of stress on the linkage and gears. An analogue servo will not apply full power for small movements so can be stalled and appear 'sticky' a lot more easily and will be gentle on the linkage. A stalled servo is very bad because the servo draws a lot of power and can burn out or damage the controller PSU or flatten batteries.

 

4. As Gordon said a digital servo will tenuously retain its current position until it sees a good sequence of pulses, whereas an analogue servo will not maintain its position in the absence of pulses, or rigidly resist attempts to move the arm small amounts between pulses.

 

5. Digital servos will often have stronger gearing made from metal or carbon fibre with stronger linkages and better bearings reflecting that they can exert more force to move things, and are therefore often quieter and smoother operating as well as being more robust.

 

If I had a simple RC plane with an analogue radio control running from batteries I would use an analogue servo, for everything else I would use a digital one, from radio control steering of a car to operating points and signals on a layout.

[idd15]

Hi

 

" 5. Digital servos will often have stronger gearing made from metal or carbon fibre with stronger linkages and better bearings reflecting that they can exert more force to move things, and are therefore often quieter and smoother operating as well as being more robust."

 

I have flown RC models for about 40 years and as a club/sport flyer never felt the need for digital servos. But there again I don't fly 3D helicopters, turbine powered jets or 100cc petrol powered aerobatic jobs.  It's "horses for courses" really IMHO.  However, I would say that not all digital servos are quiet in operation, some do "squeal" even when stationary.  

 

Analogue servos can also buzz when in position and I have found with my Megapoints controller that certain brands are more likely to buzz than others.  For example my favourite brand  of servos Hitec do, whereas my Spektrum S75's are silent.  I've just bought some of the HK servos Dave recommends to try so it will be interesting to hear how quiet they are,

 

It is very puzzling that the Peco unit will not drive the HK units.  There is no electrical difference as to how the two types of servo are driven from the controller. 

 

Check the power supply to the controller is "man" enough for the job in hand.  The starting current of a servo can be surprisingly high and may be affecting operation of the controller.  If this proves to be the case you can feed the servos with their own independent power supply and just use the ground and signal wires from the controller to the servo.  WARNING! If you do this 

DO NOT connect the controller and and servo power leads together because that will let the magic smoke out!!!!

 

A servo tester is a very useful tool for helping to set up and diagnose servo problems.  You can also get in line current meters for checking servos and linkages.  Hobbyking do both of these devices for quite reasonable prices.

 

HTH

idd

[Butler Henderson]

Well the replacement SG90 for the one I referred to as having died has gone crazy with the driving ESU Switchpilot flashing its light madly once power is applied. Having disconnected it and plugged into its connection the other SG90 that the same Switchpilot works that other servo works okay. Where can you get a guaranteed original from?

 

EDIT - Ordered an Hobby King analogue servo.

Edited December 17, 2017 by Butler Henderson

[Suzie]

Does your Switchpilot have adequate power? An SG90 can take up to 0.9A at start up, and clones which are usually clones of digital versions will take the full 0.9A.

[Butler Henderson]

Does your Switchpilot have adequate power? An SG90 can take up to 0.9A at start up, and clones which are usually clones of digital versions will take the full 0.9A.

Could be the issue as the spec is 250mA continuous, 500mA peak (20 seconds).

[RexAshton]

All my trials up to now have used the Smartswitch board powered from the dcc bus rail which is powered by a 5 amp transformer through a Lenz100. I would have thought that should be enough power but I'll try the same set up powered on the dc supply to the Megapoints board - powered with 2½ amps and see what happens as this setup is quite happy with either make of servo.

[Butler Henderson]

Finally got round to fitting the Hobby King servo yesterday - exactly the same with the Servo working erratically and the Switchpilots LED flashing wildly. Decided that their must have been something wrong with the Switchpilot. Fitted a further Servo only Switchpilot that I had spare and both the Hobby King servo and the "faulty" SG90 worked fine - the original Switchpilot is now relegated to merely switching some lights on/off.