Support (Admin)
03-22-1999, 11:43 PM
THREAD FROM ARCHIVE
========================================
The following are portions of a thread from the BotHead Forum and are provided here in the event that some people may have an interest in this somewhat technical issue.
The first posting is a comment from Ted with respect to the question of whether stepper motors generate voltages/currents when spun by hand ... and more generally in relation to the point that we believe it is bad practice to move your ShopBot by hand and recommend against it.
Re voltages from stepper motors:
Steppers are not 'little generators' in the sense that they will generate usable output, however, they will and do generate large voltage pulses when they are spun quickly.
Let me lead you through how you can explore this for yourself. The most instructive approach is using an oscilloscope. Just hook it up to two motors wires on the same coil, for example black and green or blue and red (hopefully you will be using an autoranging scope). Just spin the rotor and you will see the pulsing current (as an AC sine) as the rotor turns. The faster you spin, the higher the voltage.
Since you may not have a scope handy, lets move to your voltmeter. The slow integration and averaging of modern digital multimeters make the alternating current pulses a little more difficult to see ... but they are there. Don't use DC setting, because the swings will tend to average out. Set your meter to something like 40V AC and again connect the leads from a single coil, say the black and the green. Spin the motor and keep it going for a few seconds so the meter will integrate. You will be able to easily generate average voltages in the 20v range.
These are real voltages and currents that can do damage to solid state devices (like our current drivers) that are attached to them. Let me illustrate how powerful the current is. Get a little jumper wire with an aligator clip on both ends. Using a disconnected stepper motor, first spin it a bit to get a feel for the motion. Then, using your jumper, short the black and the green wires. You will instantly find the motor much more difficult to spin. You can turn it slowly without a problem, but when you try to spin it quickly, the motor feeds current back into the coil and 'brakes' itself. There is enough power, that the motor becomes very difficult to turn.
For an even more impressive demonstration of the currents produced by a turning motor, take two disconnected steppers and wire them together, matching color to color in the wires. Turn one of the motors very slowly ... nothing happens. But turn it fast and you'll see that it generates enough power to move the other motor in synchrony ... with force.
We may be being a little overprotective by telling ShopBotters not to move their tools by hand. In fact, 99 out of 100 times, even a big fast movement is unlikely to damage the driver boards. The driver chips themselves are very sophisticated and have protection against back surges. Nonetheless, on a few occasions we are aware of having damaged a driver here ourselves by rapidly moving the tool around by hand ... and I see no reason to take chances. A driver chip costs $20 but because it is soldered into the board (for motor-driving efficiency), we effectively loose the whole $200 board.
Now, its probably fine to turn the motors off to move the Z axis a bit by hand in order to zero it to a surface, but because the X and Y are geared at about 4:1 it is just too easy to produce rapid motor turning and risk driver damage.
The new software has an arrow key control of tool movement that makes moving the tool around very convenient. There should be little need to move an axis by hand. Additionally, we have installed convenient plugs at each motor so that if for some reason it is necessary to move an axis by hand, the motor can be unplugged and the axis safely moved.
John Tesdal March 19, 1999
Thats fine and dandy, but fortunately for us the Chain Drive dosen't spin the motor as fast as the small spindle on the cable drive. Instead of saying "NO", you should really give us the tolerance of the driver
transistor. I could figure this out, but the manufacturer surely has this information....right?
Ted Hall March 20, 1999
Hi John,
Please send me your FAX number and I will get a copy of the spec sheets for the driver chips off to you (I don't see one in our database).
On gear ratios. I don't know what ratio the chain drive is ... but I don't imagine it's that much different than our current R&P. What, for example, is the typical Unit/Calibration value that you're using? Multiply this by 4 and you'll have the approximate steps/inch (or divide by 400 for rotations/inch) for the X and Y axis. The Z axis is geared at 2000steps/inch. A R&P tool as we are shipping it is geared (after the reduction box) at about 500/1 as is a cable drive tool. If you're operating too much different than this I'd be concerned that you may be going out of the optimal range for the control system. If the ratio gets too much smaller, the tool will have speed, but low power and resolution ... it gets to much higher, you'll up the power but not be able to reach very high end speeds. (This latter problem is why we are no longer selling the ball-screw PR32 unless a customer wants very high res and is willing to sacrifice a little speed).
FYI, we ship the R&P tools with an stand-alone power supply that supplies a little higher voltage (13.8) than the PC box and this increases the top end speed. If we can find a power supply at a fair price, we'd like to switch to 28v (and abt 25A) to really drive the tool right. These higher voltage supplies should also improve the speed performance of a chain drive. [Given an adequate current is available from the supply, the voltage will have no real effect on the amount of power delivered at slower cutting speeds (i.e. less than 1000 steps/sec)].
Posted by John Tesdale from his Discussion Board March 22,1999
Mr. Hall: I find your response to John about moving the carrage of the machine interesting but without real substance. It matters very little that the motors resist hand turning under certain configurations. It's been my professional experience (40 years in the electronics industry with the last 25 in an engineering position with the same employer) the real issue is the breakdown voltages of the driver. If you have those specs available, please publish them. If impulses produced by the motors manual turning exceed the breakdown of the driver, then you are particially correct,damage could occur. Notice I said partically correct. The solution is not to cease moving the carrage by hand, the solution would be for Shopbot to redesign the circuit as it should have been in the first place. I have a feeling there is no real problem here. Not having measured the pulses myself, I will take your word they are in the 20 volt range. I believe there is a greater danger from static discharge from ones body than low voltage pulses. I always find a difference of understanding interesting. I would be more then willing to discuss the technical aspects further with you, but I doubt it is of interest to most of the fourm readers
Ted Hall March 22, 1999
The message above makes several good points, though like the writer, I worry we may be wearing the subject thin. My initial posting about steppers generating current was a reply to an earlier posting in which a failure to measure a voltage from a stepper was reported. Because I imagined that it was just a matter of correcting how the measurement was made, I thought I should explain how one can readily convince themselves that steppers are able to generate current.
In response to the question about specs, we are always happy to share whatever technical information is available on our tools. I will post a summary of the motor specs at the end of this message. But more generally, the data on the Allegro chips (SLA7026M) which are the core of our drivers is available and downloadable as a pdf file from the manufacturer at: http:/www.allegromicro.com
These ICs are wonderful little drivers for our purposes, and we spent considerable effort in selecting them. They are robust and have avalanche-rated FETs (~100v). They have performance characteristics that are well optimized for our needs. Probably owing to the complexity of the integrated logic, diodes, and FETs the manufacturer does not provide a specific "breakdown" voltage. However, as my original posting indicated and as the above writer notes, the chances of damaging the drivers by moving your tool by hand is slight ... and for small, slow moves not likely to be a problem. Nonetheless, I see no reason to take a chance ... and having recognized the potential to do damage by rapidly spinning the geared motors when doing an abrupt move by hand, I believe this is poor practice.
In addition, the writer notes the problem of static. We concur and note that this is a potentially significant problem in woodworking shops running dust collectors. Thus, our recommendation regarding electrical protection of the driver electronics is two-fold.
1. Don't move your ShopBot rapidly manually.
2. Properly ground your ShopBot and dust collection system to reduce static.
SLA7026M (Allegro Microsystems)
Abs.Maximum Ratings
Load Supply Voltage 46v
FET Output Voltage 100v
COntrol Supply Voltage 46v
Peak Output Cur(<100us) 5.0a
Continuous Ouptut Cur 3.0a
Input Voltage Range -.3 to 7.0v
Reference Voltage 2.0v
Juction Temp 150C
Operating Temp Range -20 to 85C
Storage Temp Range -40 to 150C
========================================
The following are portions of a thread from the BotHead Forum and are provided here in the event that some people may have an interest in this somewhat technical issue.
The first posting is a comment from Ted with respect to the question of whether stepper motors generate voltages/currents when spun by hand ... and more generally in relation to the point that we believe it is bad practice to move your ShopBot by hand and recommend against it.
Re voltages from stepper motors:
Steppers are not 'little generators' in the sense that they will generate usable output, however, they will and do generate large voltage pulses when they are spun quickly.
Let me lead you through how you can explore this for yourself. The most instructive approach is using an oscilloscope. Just hook it up to two motors wires on the same coil, for example black and green or blue and red (hopefully you will be using an autoranging scope). Just spin the rotor and you will see the pulsing current (as an AC sine) as the rotor turns. The faster you spin, the higher the voltage.
Since you may not have a scope handy, lets move to your voltmeter. The slow integration and averaging of modern digital multimeters make the alternating current pulses a little more difficult to see ... but they are there. Don't use DC setting, because the swings will tend to average out. Set your meter to something like 40V AC and again connect the leads from a single coil, say the black and the green. Spin the motor and keep it going for a few seconds so the meter will integrate. You will be able to easily generate average voltages in the 20v range.
These are real voltages and currents that can do damage to solid state devices (like our current drivers) that are attached to them. Let me illustrate how powerful the current is. Get a little jumper wire with an aligator clip on both ends. Using a disconnected stepper motor, first spin it a bit to get a feel for the motion. Then, using your jumper, short the black and the green wires. You will instantly find the motor much more difficult to spin. You can turn it slowly without a problem, but when you try to spin it quickly, the motor feeds current back into the coil and 'brakes' itself. There is enough power, that the motor becomes very difficult to turn.
For an even more impressive demonstration of the currents produced by a turning motor, take two disconnected steppers and wire them together, matching color to color in the wires. Turn one of the motors very slowly ... nothing happens. But turn it fast and you'll see that it generates enough power to move the other motor in synchrony ... with force.
We may be being a little overprotective by telling ShopBotters not to move their tools by hand. In fact, 99 out of 100 times, even a big fast movement is unlikely to damage the driver boards. The driver chips themselves are very sophisticated and have protection against back surges. Nonetheless, on a few occasions we are aware of having damaged a driver here ourselves by rapidly moving the tool around by hand ... and I see no reason to take chances. A driver chip costs $20 but because it is soldered into the board (for motor-driving efficiency), we effectively loose the whole $200 board.
Now, its probably fine to turn the motors off to move the Z axis a bit by hand in order to zero it to a surface, but because the X and Y are geared at about 4:1 it is just too easy to produce rapid motor turning and risk driver damage.
The new software has an arrow key control of tool movement that makes moving the tool around very convenient. There should be little need to move an axis by hand. Additionally, we have installed convenient plugs at each motor so that if for some reason it is necessary to move an axis by hand, the motor can be unplugged and the axis safely moved.
John Tesdal March 19, 1999
Thats fine and dandy, but fortunately for us the Chain Drive dosen't spin the motor as fast as the small spindle on the cable drive. Instead of saying "NO", you should really give us the tolerance of the driver
transistor. I could figure this out, but the manufacturer surely has this information....right?
Ted Hall March 20, 1999
Hi John,
Please send me your FAX number and I will get a copy of the spec sheets for the driver chips off to you (I don't see one in our database).
On gear ratios. I don't know what ratio the chain drive is ... but I don't imagine it's that much different than our current R&P. What, for example, is the typical Unit/Calibration value that you're using? Multiply this by 4 and you'll have the approximate steps/inch (or divide by 400 for rotations/inch) for the X and Y axis. The Z axis is geared at 2000steps/inch. A R&P tool as we are shipping it is geared (after the reduction box) at about 500/1 as is a cable drive tool. If you're operating too much different than this I'd be concerned that you may be going out of the optimal range for the control system. If the ratio gets too much smaller, the tool will have speed, but low power and resolution ... it gets to much higher, you'll up the power but not be able to reach very high end speeds. (This latter problem is why we are no longer selling the ball-screw PR32 unless a customer wants very high res and is willing to sacrifice a little speed).
FYI, we ship the R&P tools with an stand-alone power supply that supplies a little higher voltage (13.8) than the PC box and this increases the top end speed. If we can find a power supply at a fair price, we'd like to switch to 28v (and abt 25A) to really drive the tool right. These higher voltage supplies should also improve the speed performance of a chain drive. [Given an adequate current is available from the supply, the voltage will have no real effect on the amount of power delivered at slower cutting speeds (i.e. less than 1000 steps/sec)].
Posted by John Tesdale from his Discussion Board March 22,1999
Mr. Hall: I find your response to John about moving the carrage of the machine interesting but without real substance. It matters very little that the motors resist hand turning under certain configurations. It's been my professional experience (40 years in the electronics industry with the last 25 in an engineering position with the same employer) the real issue is the breakdown voltages of the driver. If you have those specs available, please publish them. If impulses produced by the motors manual turning exceed the breakdown of the driver, then you are particially correct,damage could occur. Notice I said partically correct. The solution is not to cease moving the carrage by hand, the solution would be for Shopbot to redesign the circuit as it should have been in the first place. I have a feeling there is no real problem here. Not having measured the pulses myself, I will take your word they are in the 20 volt range. I believe there is a greater danger from static discharge from ones body than low voltage pulses. I always find a difference of understanding interesting. I would be more then willing to discuss the technical aspects further with you, but I doubt it is of interest to most of the fourm readers
Ted Hall March 22, 1999
The message above makes several good points, though like the writer, I worry we may be wearing the subject thin. My initial posting about steppers generating current was a reply to an earlier posting in which a failure to measure a voltage from a stepper was reported. Because I imagined that it was just a matter of correcting how the measurement was made, I thought I should explain how one can readily convince themselves that steppers are able to generate current.
In response to the question about specs, we are always happy to share whatever technical information is available on our tools. I will post a summary of the motor specs at the end of this message. But more generally, the data on the Allegro chips (SLA7026M) which are the core of our drivers is available and downloadable as a pdf file from the manufacturer at: http:/www.allegromicro.com
These ICs are wonderful little drivers for our purposes, and we spent considerable effort in selecting them. They are robust and have avalanche-rated FETs (~100v). They have performance characteristics that are well optimized for our needs. Probably owing to the complexity of the integrated logic, diodes, and FETs the manufacturer does not provide a specific "breakdown" voltage. However, as my original posting indicated and as the above writer notes, the chances of damaging the drivers by moving your tool by hand is slight ... and for small, slow moves not likely to be a problem. Nonetheless, I see no reason to take a chance ... and having recognized the potential to do damage by rapidly spinning the geared motors when doing an abrupt move by hand, I believe this is poor practice.
In addition, the writer notes the problem of static. We concur and note that this is a potentially significant problem in woodworking shops running dust collectors. Thus, our recommendation regarding electrical protection of the driver electronics is two-fold.
1. Don't move your ShopBot rapidly manually.
2. Properly ground your ShopBot and dust collection system to reduce static.
SLA7026M (Allegro Microsystems)
Abs.Maximum Ratings
Load Supply Voltage 46v
FET Output Voltage 100v
COntrol Supply Voltage 46v
Peak Output Cur(<100us) 5.0a
Continuous Ouptut Cur 3.0a
Input Voltage Range -.3 to 7.0v
Reference Voltage 2.0v
Juction Temp 150C
Operating Temp Range -20 to 85C
Storage Temp Range -40 to 150C