DIY Servo controller

[kreutz]

Hello Guys:

I am planing to finish the PCB layout tomorrow and send it to the fab next Wednesday. There are a few differences to the original UHU schematics, one experimental difference is my approach to torque control (it can always be bypassed by eliminating a few components if you want to keep the original peak current control).

Any Permanent magnet brushed DC motor is specified for maximum continuous torque and peak stall torque, which translates to maximum continuous current and Peak stall current. Usually the later is more than twice the former. If you adjust the peak current to the former, you are losing a short time reserve torque that could be more than double the maximum continuous torque. That can make the difference between a completed and a ruined job.

My approach consists in two different current limits; A short time limit is 2.1 times the continuous current, and a long time limit that is the maximum average continuous current. Since I like things simple, there is only one current adjustment: Average maximum continuous Current limit.

I know that many of you know about other commercial servo amplifiers, and would like to see if something like this has been implemented before. I haven't been able to find much information around.

[contactirfu]

Hello Guys:

I am planing to finish the PCB layout tomorrow and send it to the fab next Wednesday. There are a few differences to the original UHU schematics, one e................................................. .................................................. .........................................other commercial servo amplifiers, and would like to see if something like this has been implemented before. I haven't been able to find much information around.

Thats great news Kreutz, I have 3 more of the UHU chips and would utilize for them for the larger PS which I already have, with your PCB, Good to know that u are testing your circuit ...... your circuit might be the latest for the UHU, my suggestion would be that you offer the gerbers for a very nominal fee, after all the hard work u put into it. Also it might help your email inbox to have lesser mails than intended and only serious people will contact you. I would personally buy the PCB layout files from you and use them for myself.

I might be wrong here, if you have a different approach for the diy and dont want to charge any one for anything it probably is your happiness factor, but for the amount of great work you do for the CNC DIYer's ON CNC zone .............

I SALUTE YOU

Keep the great work goin on and on Kreutz.

Regards
IRfan

[kreutz]

Thats great news Kreutz, I have 3 more of the UHU chips and would utilize for them for the larger PS which I already have, with your PCB, Good to know that u are testing your circuit ...... your circuit might be the latest for the UHU, my suggestion would be that you offer the gerbers for a very nominal fee, after all the hard work u put into it. Also it might help your email inbox to have lesser mails than intended and only serious people will contact you. I would personally buy the PCB layout files from you and use them for myself.

I might be wrong here, if you have a different approach for the diy and dont want to charge any one for anything it probably is your happiness factor, but for the amount of great work you do for the CNC DIYer's ON CNC zone .............

I SALUTE YOU

Keep the great work goin on and on Kreutz.

Regards
IRfan

I won't charge for the schematics or Gerbers. I won't post them unless they are well tested.


Thanks,

Kreutz.

[kreutz]

Does any of you have the female connectors for the SEM MT30U4-31 servo?, only need them for a few weeks, I will send them back as soon as finished testing the servo. At least having a part number (for each connector) will help me a lot, I don't have much time available.

[lingdao]

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That brothers have of, please leave a telephone
help me

[kreutz]

Hello Guys:

I am planing to finish the PCB layout tomorrow and send it to the fab next Wednesday. There are a few differences to the original UHU schematics, one experimental difference is my approach to torque control (it can always be bypassed by eliminating a few components if you want to keep the original peak current control).

Any Permanent magnet brushed DC motor is specified for maximum continuous torque and peak stall torque, which translates to maximum continuous current and Peak stall current. Usually the later is more than twice the former. If you adjust the peak current to the former, you are losing a short time reserve torque that could be more than double the maximum continuous torque. That can make the difference between a completed and a ruined job.

My approach consists in two different current limits; A short time limit is 2.1 times the continuous current, and a long time limit that is the maximum average continuous current. Since I like things simple, there is only one current adjustment: Average maximum continuous Current limit.

I know that many of you know about other commercial servo amplifiers, and would like to see if something like this has been implemented before. I haven't been able to find much information around.

SAFETY Note: This modified UHU board is specifically designed for High Power Servo motors > 80 Volt > 10 Amps. I will limit my tests to 150 volts, 13.5 Amps (continuous) 25 Amp maximum due to the used test motor's specs. Changing the Output mosfets will require extra modifications This board is not a plug and play servo test station, working at high power levels requires a safe and controlled environment (both, electrical and mechanically safe). I use an isolated 3KVA, 0 to 170 Vdc power supply (made specifically for this tests) and a thick lexan shield when powering the boards, exploding semiconductors could do a lot of damage to your face/body if you are still alive after the accident!! Please, be responsible.

[contactirfu]

, working at high power levels requires a safe and controlled environment (both, electrical and mechanically safe). I use an isolated 3KVA, 0 to 170 Vdc power supply (made specifically for this tests) and a thick lexan shield when powering the boards, exploding semiconductors could do a lot of damage to your face/body if you are still alive after the accident!! Please, be responsible.

I myself use heavy High Voltage graded gloves and wear protective glasses now that you mentioned Lexan shields, I will add them on to my list of protective items to be bought on priority. DO u think the Lexan should be shaped like a Box with five sides or just a plain sheet protecting the face.

Thanks Kreutz

Regards
IRfan

[kreutz]

I myself use heavy High Voltage graded gloves and wear protective glasses now that you mentioned Lexan shields, I will add them on to my list of protective items to be bought on priority. DO u think the Lexan should be shaped like a Box with five sides or just a plain sheet protecting the face.

Thanks Kreutz

Regards
IRfan

Mine is like a full body 1/2 inch (4 parallel sheets) vertical wall supported by heavy feet. The Lexan I use is the same sold here for protecting windows against hurricane force winds and high speed debris.

[kreutz]

A couple of more changes:

1- RS232 communication between the original UHU board and the PC is not isolated. When working with High voltage power supplies is is highly desirable to have electrical isolation between both of them, so I replaced the Max232 chip and associated components by a discrete opto-isolated Rs-232 interface that takes the PC side supply from the data lines. On the new design the -12 Vdc supply voltage, for the current limit comparator chip, is not necessary anymore (I also replaced the TL082 op-amp by the LM339 quad comparator).

2- There is no protection on the original UHU board against unconnected Encoder cable. When the encoder is left unconnected the motor will turn at high speed without stopping (once a step command is received from the PC). This behavior is a safety hazard. On the modified board if you left disconnected the encoder cable the board will be under Emergency Stop mode. There is a jumper on the encoder cable that feeds back information about encoder connection integrity. It is necessary to wire that jumper on the motor encoder side of the interface, as shown on the schematics.

[kreutz]

This is the time to ask

I am sure some of you have long encoder cables. Do you think that adding an independently adjustable +5 Volts supply for the encoder is justifiable? It will add a few components but will compensate for the encoder cable drop and will be short circuit proof.

Let me know your opinion, this is the time!!

[contactirfu]

This is the time to ask

I am sure some of you have long encoder cables. Do you think that adding an independently adjustable +5 Volts supply for the encoder is justifiable? It will add a few components but will compensate for the encoder cable drop and will be short circuit proof.

Let me know your opinion, this is the time!!

I am in for it Kreutz, if it helps then why not, I particularly have long encoder cables as i am on a 4x8 router build.

REgards
IRfan

[LZ1TWB]

Hello kreutz,
I myself will have 6-7meters of cable till I reach the Z axis, some will say this is a lot I don't know. I was thinking about the voltage drop across the encoder line and that an encoder never takes much current, say will 0.1A do something significant? Most of the cases like me power is distributed across 2 or even 3 wires together for the GND and the +5V. Maybe you are right, because I haven't tried this yet.
I want to share another problem I faced recently. Is it normal that the motor heats up like a cooker even without moving?
I have one small and one big motor both 55Volt maximum. When I connect the small motor and turn it on /Using 65V/ it heats for about 10-15minutes like I cannot touch it. The big motor behaves much better maybe it cannot heat up that quick because it is much bigger a piece but sooner or later faces the same problem. This was quite visible when the weather was hot, say 30degrees in the room.
I blame this square wave which is double the voltage at the output. I think it should not be there when the motor is not moving. At first I thought 20Khz wont pass through the windings due to the inductance but maybe it is not the case. I was wondering why not put a D-class audio amplifier filter at the end? It will leave just the DC part of the signal. Just a suggestion. You will correct me if this is not possible with the project.

Thanks, Todor

[kreutz]

Hello kreutz,
I myself will have 6-7meters of cable till I reach the Z axis, some will say this is a lot I don't know. I was thinking about the voltage drop across the encoder line and that an encoder never takes much current, say will 0.1A do something significant? Most of the cases like me power is distributed across 2 or even 3 wires together for the GND and the +5V. Maybe you are right, because I haven't tried this yet.
I want to share another problem I faced recently. Is it normal that the motor heats up like a cooker even without moving?
I have one small and one big motor both 55Volt maximum. When I connect the small motor and turn it on /Using 65V/ it heats for about 10-15minutes like I cannot touch it. The big motor behaves much better maybe it cannot heat up that quick because it is much bigger a piece but sooner or later faces the same problem. This was quite visible when the weather was hot, say 30degrees in the room.
I blame this square wave which is double the voltage at the output. I think it should not be there when the motor is not moving. At first I thought 20Khz wont pass through the windings due to the inductance but maybe it is not the case. I was wondering why not put a D-class audio amplifier filter at the end? It will leave just the DC part of the signal. Just a suggestion. You will correct me if this is not possible with the project.

Thanks, Todor

Your dv/dt could be too high and your motors are suffering because of that, your power supply should be only (5% +1.5 Volts) higher than the motor rated voltage, no more than that. Were your motors originally made as servo motors, or just DC motors with home attached encoders?

The square wave is normal, this drive works that way, no movement corresponds to 50% duty cycle, and it is also normal for the motor to heat up more when idling, but not that much.

I added an RLC filter to the output (actually two filters, one per half bridge) in order to control dv/dt to 3 to 5 V/ns at the motor terminals. The filter parameters depend on the Power supply Voltage and motor current, so they need to be individually calculated.


Regards,

Kreutz.

[tenmetalman]

Amphenol Connectors - Kreutz,
Give me a call (I'll e-mail my phone) I think i fopund what you need & can get them in the mail today if needed... I've already e-mailed you with the specifics
tenmetalman

[LZ1TWB]

Hello Kreutz,
UHU Servo controller manual says: "...voltage of the output stage has to be 15% higher than the voltage specified on the motor to provide maximum performance" I have done this because this was written so.
My motors are made servo especially the big one has a back cap with the encoder mounted inside, wires coming out from a connector. I doubt that lowering the PS voltage from 65 to 55 will make any significant difference.

Originally Posted by kreutz
and it is also normal for the motor to heat up more when idling

I thought motors should heat up more when loaded rather than at idle.

What is the purpose of the RLC network that you have used?

Thanks, Todor

[kreutz]

Hello Kreutz,
UHU Servo controller manual says: "...voltage of the output stage has to be 15% higher than the voltage specified on the motor to provide maximum performance" I have done this because this was written so.
My motors are made servo especially the big one has a back cap with the encoder mounted inside, wires coming out from a connector. I doubt that lowering the PS voltage from 65 to 55 will make any significant difference.



I thought motors should heat up more when loaded rather than at idle.

What is the purpose of the RLC network that you have used?

Thanks, Todor

When loaded, the motor will heat up due to the I2*R effect on the rotor coil (plus magnetic circuit losses due to the high frequency switching), but the rotation makes the air around it to move, so some of that heat is going to be dissipated, when idle it will heat up due to losses in the magnetic circuit, I2*R losses are also present, due to the 50% duty cycle the same current will flow in both directions so the resulting movement is zero, and there is no airflow to help dissipating it.

The designer sure knows better than I what % margin he left at both ends of the PWM cycle in order to recharge the bootstrap capacitors, I assumed I was 5% (standard design margin), so then your voltage is OK.

The PWM frequency for the UHU board is 23.5 KHz, some old servo drives used from 5 to 15 KHz, the older technology, or higher power, the lower the switching frequency they use. So I expect a little more magnetic losses than usual on those motors.

The filters I use are designed to control DV/dt on the motor. Depending on the turn-on time of the power Mosfets used in the H bridge, and the size of the motor; cable lengths as short as eight feet can result in motor peak voltages that exceed the rating of the motor's insulation system. However, the longer the cable the greater the problem. Dampening the rate of voltage increase will minimize the peak voltage that occurs at the motor terminals.

When using low power/voltage motors the dv/dt could be effectively controlled using lower switching times, like in the original UHU design. When handling high voltages and currents, it is better controlled with external Snubbers, since slowing down the switching times could destroy the switching semiconductors.

[contactirfu]

" However, the longer the cable the greater the problem. Dampening the rate of voltage increase will minimize the peak voltage that occurs at the motor terminals. ..............

When handling high voltages and currents, it is better controlled with external Snubbers, since slowing down the switching times could destroy the switching semiconductors."

Kreutz,

What happens when we use longer power cables? In my case I think i will be using long power cables to the motors, what caution should we take for longer cables and also does external snubbers mean some circuit outside the controller boards nearer to the motors. I have seen ferrite rings with power cable wound around .... is this you are talking about?

Regards
IRfan

[kreutz]

Kreutz,

What happens when we use longer power cables? In my case I think i will be using long power cables to the motors, what caution should we take for longer cables and also does external snubbers mean some circuit outside the controller boards nearer to the motors. I have seen ferrite rings with power cable wound around .... is this you are talking about?

Regards
IRfan

Hopefully, it will be all taken care of on the new modified board. The RLC filter is supposed to take care of that. Component values will be calculated for every individual motor, there is no one size fits all. I will post an spreadsheet I made for this calculations, but it is only a guide, I don't have commercial values database incorporated, and the right values should be re-calculated using the nearest commercial values (or available values). That is why I will do it myself upon request.

[danylipsker]

aaa

[kreutz]

This is the time to ask

I am sure some of you have long encoder cables. Do you think that adding an independently adjustable +5 Volts supply for the encoder is justifiable? It will add a few components but will compensate for the encoder cable drop and will be short circuit proof.

Let me know your opinion, this is the time!!

I closed the prototype Revision (000a) already, this feature will be implemented in the next revision....

Regards,

Kreutz.