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OpenServo V4 (was OpenServo V3.x hardware update)
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jharvey
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Joined: 15 Mar 2009
Posts: 359
Location: Maine USA

PostPosted: Mon Feb 07, 2011 5:26 pm    Post subject: Reply with quote

I guess a first question would be, what are your skills? Do you know KICAD or are you willing to learn KICAD?
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RasmusB



Joined: 15 Dec 2007
Posts: 4
Location: Södertälje, Sweden

PostPosted: Mon Feb 07, 2011 6:31 pm    Post subject: Reply with quote

Yes, I know kicad. I use it for hobby projects as well as some work related stuff. Everything from simple amplifiers to fpga based waveform generators. Smile
I have some experience of microchip and atmel microcontrollers, and at work I have access to equipment for smd soldering and rework, and a pcb mill.
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Robotaxonomist



Joined: 11 Feb 2011
Posts: 4
Location: Boston, Massachusetts

PostPosted: Fri Feb 11, 2011 8:02 pm    Post subject: Reply with quote

I was just wondering, would it at all make sense to make one pin either the pot input or a second I2C input for the encoder? Very few people are going to be using both an encoder and a pot, I suspect. Does this work from an IC & schematic point of view?
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Robotaxonomist



Joined: 11 Feb 2011
Posts: 4
Location: Boston, Massachusetts

PostPosted: Fri Feb 11, 2011 8:10 pm    Post subject: Reply with quote

Oh; since you were talking about KICAD... How is the Mac version of that?
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ginge
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Joined: 14 Jan 2006
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Location: Manchester, UK

PostPosted: Fri Feb 11, 2011 10:40 pm    Post subject: Reply with quote

There are so many pins on these things it is often easier to add both I2C and pot. You can possibly use both at the same time. Start up and sample absolute, and then switch to encoder incremental.
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Robotaxonomist



Joined: 11 Feb 2011
Posts: 4
Location: Boston, Massachusetts

PostPosted: Sat Feb 12, 2011 4:28 am    Post subject: Reply with quote

I just mention it because of the concern about lack of board space. The through-hole or pad where you solder the pot down might as well be the same one you solder the encoder to, so long as no one has an interest in using both at once. If it’s not a matter of the pinout, maybe you can use adjacent pins, both on the same trace, and just activate one and deactivate the other.
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BasicFox



Joined: 15 Mar 2009
Posts: 59
Location: Belgium

PostPosted: Fri Jan 27, 2012 11:06 pm    Post subject: Reply with quote

Robotaxonomist has a point, you can save even more space if you don't use trough hole pads, you can solder a flat cable directly onto the bottom side of the PCB, something like this:
http://search.digikey.com/us/en/products/HF365%2F04SF/MD04R-100-ND/2416317
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BasicFox



Joined: 15 Mar 2009
Posts: 59
Location: Belgium

PostPosted: Sat Jan 28, 2012 4:27 pm    Post subject: Re: OpenServo V4 (was OpenServo V3.x hardware update) Reply with quote

ginge wrote:

* more gpio on header
* analogue input on header
* gpio on main OpenServo board
* external 5v power input (debatable)

This would be problematic when we use the same connector as in v3. Do all that wiring by hand. Maybe it's good to split up the connectors:
power connector (6-7.4V) soldered on the powerPCB
These are very common and can handle 8A
http://be.farnell.com/camden/ctb9308-2a/header-top-entry-2way-5-08mm/dp/1717022

Another connector for the 5V and IO, something like this:
http://be.farnell.com/multicomp/4409as-10/socket-top-entry-10way/dp/1565548

If we position the important pins (GND MISO SCL MOSI) in the beginning then its possible control the servo with 4 wires:
http://be.farnell.com/te-connectivity-amp/7-215083-4/connector-male-4way/dp/149032
all we have to do is cut off the left or right side of it.

It will also look neater than all separate wires.
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douglas



Joined: 01 Nov 2012
Posts: 66

PostPosted: Fri Nov 02, 2012 2:33 am    Post subject: my 2c worth Reply with quote

Hi,
I have ordered some v2 open servo boards to play with (I am using Vigor VSD-11YMB HV winch servos that deliver up to 40kg/cm for a hexapod) to play with and then found this forum.
I am looking at modifing V2 to add multi-drop RS485, hot plug, auto-discovery of servos (I wrote a bunch of AVR code a while ago to do this for another project, including funtions to uniquely assign an identifier to a AVR CPU, etc).
I like the idea of the H-bridge in a single component, especially as some have a built in temp sensor and maybe to change the potentiometer to a hall effect sensor.
I am looking to try and fit this onto a double sided board as 4 layers seems to cost a lot to make; If I have to stack two board then that is fine for my purposes; but if 4 layers proves cheaper than it doesn't worry me.
Cheers
Douglas.
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jharvey
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Joined: 15 Mar 2009
Posts: 359
Location: Maine USA

PostPosted: Fri Nov 02, 2012 11:07 am    Post subject: Reply with quote

Hello and welcome along.

There has been some talk about a V4, that might end up ARM based. Have you see the Discovery and combined it with the CooCox? It's a $12 step through your live code development platform. Very powerful for nearly no $. Any how, this allows for some really nice features, and ARMs are the most popular embedded processor out there. As for the 485, I may have part of another project I can share to help out. I'll look into it. Do you have a schematic for how you plan to implement 485?

Also look for OpenEncoder (OE) it is and was a magnetic encoder option for OpenServo. It allowed full rotation with absolute position control.
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douglas



Joined: 01 Nov 2012
Posts: 66

PostPosted: Sun Nov 04, 2012 8:17 am    Post subject: Reply with quote

I have been thinking about this CPU: STM32F103T6U6A
as:
(1) it is not very expensive
(2) Already set up as a motor driver (emergency stop, etc)
(3) has a CRC generator (good to confirm the received command is not corrupted), and to add a CRC to sent packets
(4) has a brownout and WDT timers
(5) in the LQFP48 package seems to be the same size as the CPU used in OpenServo V2.1
(6) has a built in temperature sensor
(7) Can run at 8MHz without an external crystal
Note: I already have an Olimex ARM-USB-OCB JTAG device.

For RS-485 I am considering the SN75HVD12D as:
(1) It is fault tolerant (open and short circuit)
(2) Supports Hot Plug
(3) Is nice and small (SOIC-Cool

I will put together up a circuit in Eagle PCB over the next few days for comments...

Cheers
Douglas.
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jharvey
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Joined: 15 Mar 2009
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Location: Maine USA

PostPosted: Sun Nov 04, 2012 9:28 am    Post subject: Reply with quote

Any chance I can convince you to try KICAD?

Also if you can implement the STM Discovery connector, it's much lower cost and simpler to use than the Olimex JTAG.

Also I've used ST1480ABDR for 485 before. Might be worth considering, might be the same chip, different MFG. Key difference is price.
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douglas



Joined: 01 Nov 2012
Posts: 66

PostPosted: Sun Nov 04, 2012 9:09 pm    Post subject: Reply with quote

I will have a look at KICAD over the next day or so, as well as the STM discovery connector.

One thought; everyone seems focused on getting everything into the servo itself, what would happen if we left the pot (or hall effect sensor) and motor in the case and centralised *all* the electronics.

The ARM processor is more than capable of handling all the PWM in software, perhaps go for a digital Hall Effect sensor (AS5132, using RS-485 with the chip select and clock common to all servos and the data uniquely going back to the CPU, therefore each servo would contain 2xRX and 1(or 2) TX interfaces, e.g. ISL81387IBZ)?

That leaves two wires for the motor, and 5 for the remote hall effect sensor; the net result is that we would use in terms of I/O ports on the ARM:
1 x Chip select for *all* servos (1xRS-485 driver)
1 x Clock for *all* sensors (1x RS-485 driver)
1 x Data in *per* sensor (1x RS-485 receiver)
2 x motor wires with the H-Bridge held centrally.

So for a 3DOF hexapod = 18 servos, which gives:
1 x Chip for the servos (1xRS-485 driver)
1 x Clock for the servos (1x RS-485 driver)
18 x Data for the servos (18x RS-485 receivers)
36 x motor wires with the H-Bridge held centrally.
Total 56 I/O ports, plus a UART to allow the unit to be talked to.

The something like a LM3S1601 or ATSAM3X8CA?

We may end up with a large PCB, but it would work with any servo as size is no longer an issue.

Interested in comments....

Cheers
Douglas
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jharvey
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Joined: 15 Mar 2009
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Location: Maine USA

PostPosted: Mon Nov 05, 2012 10:01 am    Post subject: Reply with quote

Long leads in the POT will lead to an increase noise floor of the sensor signal, inducing an error. Also long PWM motor leads with motor currents and voltage spikes will emit more RF which will get into the pot signal. You generally want to keep these lead lengths minimal to have a more accurate device.
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douglas



Joined: 01 Nov 2012
Posts: 66

PostPosted: Mon Nov 05, 2012 10:11 am    Post subject: Reply with quote

Good points,
I am devising a prototype using an Atmel ATSAM3N00AA-AU CPU and MC33887APVW to drive the motor for in servo use and hope to post a schematic by the weekend.
Cheers
Douglas
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