richards
04-08-2007, 04:56 PM
There have been a lot of posts made on this forum promoting various ways to use Gecko stepper drivers to get better performance out of a stock PRT or PR Shopbot. Indeed, the 4G upgrade is somewhat related to users wanting to get more resolution and higher speeds out of their PRT and PR machines. If you own an Alpha model, or if you own a PR or PRT with stock stepper drivers, what I'm posting here will have no practical application. The Alpha motors are married to the Alpha stepper drivers. Using those motors with Gecko drivers, even if it were possible, would be a step in the wrong direction. (No pun intended.) On the other hand, modifying a non-Gecko PR or PRT by changing the power supply or somehow increasing current through the stock stepper drivers would probably destroy your stepper drivers. So, don't do it. You have been warned!
First, I want to thank Bruce Clark for posing some questions that caused me to re-think everything that I thought that I knew about stepper motors and stepper drivers. The end result of that self-examination was to ask Mariss, the designer or the Gecko G20x series of stepper drivers, to recommend the 'best' way to wire a stepper motor to get the 'most' performance from that motor/driver combination. I thought that he would give an answer with two-parts. Part one would be the best connection diagram for motors that are used at low speeds - such as a motor that directly drives a spur gear without using a gear box. With that kind of application, a motor running at 200 RPM would be moving an axis 10 to 15 inches per second, depending on the spur gear being used. Part two would be the best connection diagram for motors that drive the spur gear through a gear box. Those motors would need speeds about 3.6X higher (depending on the gear box ratio) to move the axis at the same speed. Instead of making a recommendation, Mariss simply posted a forumla that can be used to determine how to get the most out of your motor.
The formula is: Voltage / SQRT(Inductance) = Power. Isn't that both simple and elegant? By using that formula, it becomes a very simple matter to compare the two or three possible wiring connections that a motor may allow.
The six-wire Oriental Motor stepper motors that came with the PRT machines allow two types of wiring connections: Unipolor (half-coil) or Bipolor (serial). Eight-wire motors, such as the PK299-F4.5 would also allow a third option, Parallel.
The mechanics of HOW to connect a motor can be found on the Oriental Motor web site. If you can't find the information, please email me directly at miker@xmission.com (mailto:miker@xmission.com) and I'll send you the Oriental Motor PDF file that contains the information. Don't pester Shopbot for this information. Remember, we're talking about modifications and variations to a stock machine.
With that formula in hand, we need to determine the optimum power supply voltage to use. Mariss has stated that we can use a voltage that is at least 4X the motor's rated voltage but not more than 25X the motor's rated voltage. He also specifies that the voltage range must be clipped at 24V on the low end and 80V on the high end, meaning that his G20x stepper drivers must be used in the range of 24V to 80V.
If we look at the two most common Oriental Motor stepper motors likely to be found on a PRT, we'll see the non-geared PK299-01AA and the geared PK296A1A-SG3.6 motors. If we first look at the PK299-01AA motor, and run the numbers for the Series connection, we would use the forumula: 80V / SQRT(56) = 10.69. (Remember, even though 80V is much less than 25 X 9V, it is the maximum voltage allowed by the G20x stepper driver.) Then, if we run the numbers for the Unipolor (half-coil) connection we get: 80V / SQRT(14) = 21.38. That shows that the half-coil connection would give about 2X the power of the Series connection with the PK299-01AA motor when the G20x stepper driver is run at its maximum allowable voltage level.
If we do the same thing for the PK296A1A-SG3.6 geared motor, the Series numbers are: 80V / SQRT(30.8) = 14.41. The half-coil connection numbers are: 80V / SQRT(7.7) = 28.82. Again, the half-coil connection has 2X the power of the Series connection - because the Series connection requires more voltage than the G20x stepper driver can furnish to maximize the Series connected motor.
I am NOT recommending that anyone increase the voltage of their power supply to 80V or that they change the wiring from Series to half-coil. As far as I'm concerned, it's much better to be conservative and run a machine at a moderate voltage that will produce moderate speeds than it is to squeeze out the last fraction of torque from a motor/driver pair. Electro/mechanical things seem to last much longer when used conservatively than when they are pushed to their limits. Sometimes just knowing that you could get more out of your machine, and knowing that making those modifications will make the motors run hot enough to fry eggs on, is enough to make you content to leave the machine as it is.
First, I want to thank Bruce Clark for posing some questions that caused me to re-think everything that I thought that I knew about stepper motors and stepper drivers. The end result of that self-examination was to ask Mariss, the designer or the Gecko G20x series of stepper drivers, to recommend the 'best' way to wire a stepper motor to get the 'most' performance from that motor/driver combination. I thought that he would give an answer with two-parts. Part one would be the best connection diagram for motors that are used at low speeds - such as a motor that directly drives a spur gear without using a gear box. With that kind of application, a motor running at 200 RPM would be moving an axis 10 to 15 inches per second, depending on the spur gear being used. Part two would be the best connection diagram for motors that drive the spur gear through a gear box. Those motors would need speeds about 3.6X higher (depending on the gear box ratio) to move the axis at the same speed. Instead of making a recommendation, Mariss simply posted a forumla that can be used to determine how to get the most out of your motor.
The formula is: Voltage / SQRT(Inductance) = Power. Isn't that both simple and elegant? By using that formula, it becomes a very simple matter to compare the two or three possible wiring connections that a motor may allow.
The six-wire Oriental Motor stepper motors that came with the PRT machines allow two types of wiring connections: Unipolor (half-coil) or Bipolor (serial). Eight-wire motors, such as the PK299-F4.5 would also allow a third option, Parallel.
The mechanics of HOW to connect a motor can be found on the Oriental Motor web site. If you can't find the information, please email me directly at miker@xmission.com (mailto:miker@xmission.com) and I'll send you the Oriental Motor PDF file that contains the information. Don't pester Shopbot for this information. Remember, we're talking about modifications and variations to a stock machine.
With that formula in hand, we need to determine the optimum power supply voltage to use. Mariss has stated that we can use a voltage that is at least 4X the motor's rated voltage but not more than 25X the motor's rated voltage. He also specifies that the voltage range must be clipped at 24V on the low end and 80V on the high end, meaning that his G20x stepper drivers must be used in the range of 24V to 80V.
If we look at the two most common Oriental Motor stepper motors likely to be found on a PRT, we'll see the non-geared PK299-01AA and the geared PK296A1A-SG3.6 motors. If we first look at the PK299-01AA motor, and run the numbers for the Series connection, we would use the forumula: 80V / SQRT(56) = 10.69. (Remember, even though 80V is much less than 25 X 9V, it is the maximum voltage allowed by the G20x stepper driver.) Then, if we run the numbers for the Unipolor (half-coil) connection we get: 80V / SQRT(14) = 21.38. That shows that the half-coil connection would give about 2X the power of the Series connection with the PK299-01AA motor when the G20x stepper driver is run at its maximum allowable voltage level.
If we do the same thing for the PK296A1A-SG3.6 geared motor, the Series numbers are: 80V / SQRT(30.8) = 14.41. The half-coil connection numbers are: 80V / SQRT(7.7) = 28.82. Again, the half-coil connection has 2X the power of the Series connection - because the Series connection requires more voltage than the G20x stepper driver can furnish to maximize the Series connected motor.
I am NOT recommending that anyone increase the voltage of their power supply to 80V or that they change the wiring from Series to half-coil. As far as I'm concerned, it's much better to be conservative and run a machine at a moderate voltage that will produce moderate speeds than it is to squeeze out the last fraction of torque from a motor/driver pair. Electro/mechanical things seem to last much longer when used conservatively than when they are pushed to their limits. Sometimes just knowing that you could get more out of your machine, and knowing that making those modifications will make the motors run hot enough to fry eggs on, is enough to make you content to leave the machine as it is.