richards
12-05-2007, 05:20 PM
Note: I've used stepper motors for about 25 years in various process control applications. For the last three years, I've been heavily involved in testing various Gecko based stepper motor systems. The following post is mostly based on questions that Shopbotters have emailed me. Please note that this post is not an attempt to try to sway anyone to use Gecko based steppers. You'll note at the very end of the post that I clearly state that I use Alpha motors on my machine and that I've ordered the 7.2 Alpha motors to replace the motors that came with my machine.
Stepper motors are the devices that cause an axis to move. In effect, they are the heart and soul of our machines. Most of us have a pretty good idea of how they work. The stepper motor is connected to a stepper driver and the stepper driver is connected to the controller board. When we want an axis to move, we send a pulse to the controller board and the controller board sends that pulse to the stepper driver. The stepper driver does its magic and energizes the proper coils on the stepper motor - and the axis moves one step. How far the axis moves with that single step and how fast the axis moves with repeated steps is largely determined by the type of stepper driver that we use and how we configure the stepper driver and the stepper motor.
How many steps does it take to move the shaft of the stepper motor one revolution? That depends on which stepper motor we use. Most stepper motors are designed to move the shaft 1.8 degrees per step. Most stepper motors have fifty rotor teeth and four coil combinations which gives two-hundred steps per revolution. Three-hundred-sixty degrees divided by two-hundred gives 1.8 degrees per step. Not all stepper motors have that exact configuration, but Oriental Motor stepper motors that are used with Gecko stepper drivers use that particular configuration.
What is half-stepping or micro-stepping? Although the stepper motor is designed to step two hundred times per revolution, a stepper driver determines whether the motor's shaft will actually rotate 1.8 degrees, or whether the shaft will rotate some fraction of 1.8 degrees. Some stepper drivers can be configured by the end user to move 0.9 degrees (400 steps per rotation) or 0.18 degrees (2,000) steps per rotation. Depending on the stepper driver used, many other possibilities exist; however, the Gecko stepper drivers divide each full step into ten micro-steps which requires 2,000 step pulses from the controller to rotate the motor's shaft one revolution.
How much current and voltage does a stepper motor require? The stepper motor manufacturer labels a motor's power requirements on the motor's label. I have an Oriental Motor PK268-02A motor sitting on the bench that has the following information on the motor's label: 2-Phase 1.8 degree/Step, DC 2A, 2.25 ohm. Unfortunately, the label doesn't specify a voltage. Also, the 2A current rating may not be correct. Many motor makers list the current rating of the motor when the motor is connected to the stepper controller using the UNIPOLAR wiring method. Shopbot uses the Bipolar Series wiring method to connect stepper motors to the stepper driver. The data sheet for the PK268-02A motor shows that the current rating of that motor when wired Bipolar Series is 1.4A. (A motor's Bipolar Series current rating is always 70% of the motor's Unipolar current rating.) So, what about the voltage? Using ohms' law, we know that Voltage = Resistance X Current, so the voltage rating for that motor, when it is wired using Unipolar wiring is 2.25 ohms X 2A = 4.5V. On the other hand, when that motor is used with Bipolar Series wiring, the data sheet shows that the resistance is 4.5 ohms and that the current is 1.4A, so 1.4A X 4.5 ohms = 6.3 volts, which is exactly what the data sheet shows.
Why does my controller have a power supply that is much larger than the motor requires? The Gecko stepper driver (as well as most other stepper drivers) is a chopper driver. That means that it uses a high voltage to force current into a motor's coils and then reduces that high voltage to a much lower level or turns the high voltage on/off rapidly to simulate a low voltage. The actual method used by Gecko or other manufacturers is beyond our control, so it really doesn't matter what method they use. However, it is important to know that Gecko specifies that we should use a power supply that supplies at least four times the voltage required by the motor, but not more than twenty-five times the voltage required by the motor. Gecko also requires that the voltage range fall between twenty-four volts and eighty-volts (in order for the stepper driver to work properly). Knowing exactly what voltage works best is part art and part science. Your requirements may dictate a different voltage than my requirements; however, most of us use our machines to cut a variety of materials at a variety of speeds, so most of us can use the same power supply. Gecko published a formula that helps to find the maximum voltage that we can use with a particular motor. That formula is: MAXIMUM voltage = 1000 X SQRT(Inductance). The motor's data sheet will list its inductance. In the case of the PK296-02A motor, the data sheet lists the motor's inductance as 14.4mH when the motor is wired Bipolar Series and 3.6mH when the motor is wired Unipolar. Using the formula, we could use an 80V power supply if we wire the motor Bipolar Series. (The formula gives the maximum voltage as 120V, but the Gecko stepper driver has a maximum voltage of 80V, so we must use 80V or less.) If we use that same motor with the Unipolar wiring method, we can use a power supply up to 60V. It is really important to know that just because we can use a power supply that gives 60V doesn't mean that we must use 60V. If we use the maximum voltage for a motor, that motor is going to run hot - really, really hot; hot enough to burn the skin off a finger tip. If we reduce the voltage about 25% from the maximum, the motor will still work very well and we'll save our finger tips.
How do we control how much current a motor uses? Because a Gecko stepper driver can deliver as much as seven amps of current to a motor, we must limit the current in some way to keep the motor from destroying itself. The Gecko stepper driver has two terminals on the driver to which we attach a 1/4 watt resistor. The resistor will limit the amount of current that the stepper driver is allowed to pass to the motor. Gecko has a very simple formula that computes the size of the resistor in K ohms. The formula is: 47 X Current / ( 7 - Current ) = Resistor size (in K ohms). So, using a motor that can handle 1A of current, we would compute the resistor like this: 47 X 1 / ( 7 - 1 ) = 7.8K. Gecko's data sheet permits us to use the closest standard size resistor, so the actual resistor that we would use with a 1A motor would probably be 8.2K.
What is auto-current-reduction? Stepper motors pull the most current when they are stopped; therefore, they also produce the most heat when they are stopped. Most stepper drivers have an auto-current-reduction feature. Sometimes that feature can be turned on or off by the user. When auto-current-reduction is turned on, the current to the stepper motor is automatically reduced a short time after the stepper driver stops receiving pulses. On the Gecko G202 stepper driver, that feature activates about one-second after the last pulse has been received. The G202 automatically reduces the current to about 30% of normal. The good thing is that by reducing the current, the electronics run much cooler. The bad thing is that holding torque is also greatly reduced. It's up to the user to determine whether the stepper motor has enough holding torque with auto-current-reduction activated or whether the stepper motor needs to be held at full torque when stopped. On the G202, the amount of current reduction is not adjustable. It is either turned on or it's turned off.
What happens if we let the motor pull too much current? Bad things will happen - eventually. Electrical components are built to handle a certain amount of heat. Heat is rated in Watts. We're all familiar with how much hotter a 100W light bulb is hotter than a 25W light bulb. Stepper motors are wound with wire and that wire is insulated to withstand a certain temperature. As long as the motor runs at or below that rated temperature, the motor will have a long life; but, if we allow the motor to get too hot by allowing the motor to pull more current through its windings than the windings were designed to handle, the motor will have a shortened life.
Should I choose a motor with high inductance or a motor with low inductance? Generally speaking inductance is to AC as resistance is to DC. When you look at the torque charts and compare various motors, usually you'll notice that motors with high inductance have a smoother low-end torque than do motors with low inductance. You'll usually also note that motors with high inductance have very limited high-end speed when compared to motors with low inductance. Personally, I prefer using motors with low to moderate inductance for general all-around use. Stepper drivers are designed to handle a limited range of inductance. The Gecko G202 stepper driver handles high inductance better than the G203v stepper driver. So, because I really, really, really like the G203v, I make it a point to always select a motor that works well with that driver.
Why does a stepper motor sometimes shake and make loud noises? Stepper motors resonate when running at certain speeds - typically at speeds less than 100 RPM. A user has almost no control over that resonating; however, wiring a motor Bipolar Series usually lets the motor run at slower speeds without resonating - but Bipolar Series wiring also severely limits the high speed performance of the motor. If a motor must run at very low speeds, then the usual practice is to add gearing to the motor to enable the motor to run at a faster speed.
Why do some stepper motors have gearboxes and others have direct drive? How a motor is to be used determines whether it needs a gearbox or whether it can be connected directly to its load. If a motor is attached to a ball-screw, each revolution of the motor's shaft might move the axis 1/5th of an inch. If a motor is attached to a spur gear that rides on a rack, each revolution of the motor's shaft might move the axis 3.14 inches or even 4.71 inches. It's easy to see that if we divide 3.14 inches by 2000 steps, that the axis will move 0.0015 inches. That's not bad for a CNC router, but if we used a 3.6:1 gearbox, each step would only move the axis 0.000436 inches AND the gearbox would also increase the motor's torque 3.6 times (within the limit of the gearbox's gears). So gearing is highly desirable.
Is there really any difference between Gecko driven motors and high priced motors that have their own stepper drivers? That is a very hard question to answer. The easy answer is, YES, there is a difference between a motor/driver that costs about $350 compared to a motor/driver that costs about $1,500 - as would be expected. However, knowing that there is a difference and justifying the cost of that difference is another matter entirely. First, lets look at resolution. Shopbot offers two competing motor systems, a Gecko driven 3.6:1 motor and an Oriental Motor Alpha 7.2:1 motor. On the surface it looks like the Alpha motor would have 2X the resolution when compared to the Gecko driven motor, but that is not the case. The Gecko driven motor requires 2,000 steps per revolution, while the Alpha motor requires 1,000 steps per revolution. The net effect is that both motors drive the axis the same distance per step. However, the 3.6:1 motor is basically a 300 oz*in motor connected to a gearbox that is limited by the gearbox's gears to about 350 oz*in even though the gearing would normally change that rating to about 1,100 oz*in. The 7.2:1 motor is basically a 600 oz*in motor connected to a gearbox that allows about 1,300 oz*in to be transfered to the axis. So, that comparison shows that if you require fast, deep cuts, the Alpha motor is clearly superior. The Alpha 7.2:1 motor has less slop or backlash than does the 3.6:1 motor. The Alpha 7.2:1 motor handles slow speeds more smoothly than does the 3.6:1 motor. So the only way that you can decide which motor is best for you is to look very hard at your requirements and then pick the motor that best meets those requirements. One thing to keep in mind when making that decision is that if high speed and deep cuts are on your required list, that you might also require a large spindle and a very large vacuum hold-down system to make deep, fast cuts a practical reality. One further point that must also be made is that making deep, fast cuts can cause a machine to flex substantially. The quality of cuts made at low to moderate speeds may be vastly superior to cuts made at top speed. My own machine is the PRT-Alpha that will soon have the 7.2:1 motors. Even though I know that I can build a custom belt-drive gearbox that would allow me to use a large stepper motor at 2X the torque of the 7.2:1 Alpha, I have been very pleased with the PRT-Alpha overall and hope to have substantially smoother cuts from the 7.1:1 motors than I had with the unmodified 1:1 motors that were originally shipped with my machine.
Stepper motors are the devices that cause an axis to move. In effect, they are the heart and soul of our machines. Most of us have a pretty good idea of how they work. The stepper motor is connected to a stepper driver and the stepper driver is connected to the controller board. When we want an axis to move, we send a pulse to the controller board and the controller board sends that pulse to the stepper driver. The stepper driver does its magic and energizes the proper coils on the stepper motor - and the axis moves one step. How far the axis moves with that single step and how fast the axis moves with repeated steps is largely determined by the type of stepper driver that we use and how we configure the stepper driver and the stepper motor.
How many steps does it take to move the shaft of the stepper motor one revolution? That depends on which stepper motor we use. Most stepper motors are designed to move the shaft 1.8 degrees per step. Most stepper motors have fifty rotor teeth and four coil combinations which gives two-hundred steps per revolution. Three-hundred-sixty degrees divided by two-hundred gives 1.8 degrees per step. Not all stepper motors have that exact configuration, but Oriental Motor stepper motors that are used with Gecko stepper drivers use that particular configuration.
What is half-stepping or micro-stepping? Although the stepper motor is designed to step two hundred times per revolution, a stepper driver determines whether the motor's shaft will actually rotate 1.8 degrees, or whether the shaft will rotate some fraction of 1.8 degrees. Some stepper drivers can be configured by the end user to move 0.9 degrees (400 steps per rotation) or 0.18 degrees (2,000) steps per rotation. Depending on the stepper driver used, many other possibilities exist; however, the Gecko stepper drivers divide each full step into ten micro-steps which requires 2,000 step pulses from the controller to rotate the motor's shaft one revolution.
How much current and voltage does a stepper motor require? The stepper motor manufacturer labels a motor's power requirements on the motor's label. I have an Oriental Motor PK268-02A motor sitting on the bench that has the following information on the motor's label: 2-Phase 1.8 degree/Step, DC 2A, 2.25 ohm. Unfortunately, the label doesn't specify a voltage. Also, the 2A current rating may not be correct. Many motor makers list the current rating of the motor when the motor is connected to the stepper controller using the UNIPOLAR wiring method. Shopbot uses the Bipolar Series wiring method to connect stepper motors to the stepper driver. The data sheet for the PK268-02A motor shows that the current rating of that motor when wired Bipolar Series is 1.4A. (A motor's Bipolar Series current rating is always 70% of the motor's Unipolar current rating.) So, what about the voltage? Using ohms' law, we know that Voltage = Resistance X Current, so the voltage rating for that motor, when it is wired using Unipolar wiring is 2.25 ohms X 2A = 4.5V. On the other hand, when that motor is used with Bipolar Series wiring, the data sheet shows that the resistance is 4.5 ohms and that the current is 1.4A, so 1.4A X 4.5 ohms = 6.3 volts, which is exactly what the data sheet shows.
Why does my controller have a power supply that is much larger than the motor requires? The Gecko stepper driver (as well as most other stepper drivers) is a chopper driver. That means that it uses a high voltage to force current into a motor's coils and then reduces that high voltage to a much lower level or turns the high voltage on/off rapidly to simulate a low voltage. The actual method used by Gecko or other manufacturers is beyond our control, so it really doesn't matter what method they use. However, it is important to know that Gecko specifies that we should use a power supply that supplies at least four times the voltage required by the motor, but not more than twenty-five times the voltage required by the motor. Gecko also requires that the voltage range fall between twenty-four volts and eighty-volts (in order for the stepper driver to work properly). Knowing exactly what voltage works best is part art and part science. Your requirements may dictate a different voltage than my requirements; however, most of us use our machines to cut a variety of materials at a variety of speeds, so most of us can use the same power supply. Gecko published a formula that helps to find the maximum voltage that we can use with a particular motor. That formula is: MAXIMUM voltage = 1000 X SQRT(Inductance). The motor's data sheet will list its inductance. In the case of the PK296-02A motor, the data sheet lists the motor's inductance as 14.4mH when the motor is wired Bipolar Series and 3.6mH when the motor is wired Unipolar. Using the formula, we could use an 80V power supply if we wire the motor Bipolar Series. (The formula gives the maximum voltage as 120V, but the Gecko stepper driver has a maximum voltage of 80V, so we must use 80V or less.) If we use that same motor with the Unipolar wiring method, we can use a power supply up to 60V. It is really important to know that just because we can use a power supply that gives 60V doesn't mean that we must use 60V. If we use the maximum voltage for a motor, that motor is going to run hot - really, really hot; hot enough to burn the skin off a finger tip. If we reduce the voltage about 25% from the maximum, the motor will still work very well and we'll save our finger tips.
How do we control how much current a motor uses? Because a Gecko stepper driver can deliver as much as seven amps of current to a motor, we must limit the current in some way to keep the motor from destroying itself. The Gecko stepper driver has two terminals on the driver to which we attach a 1/4 watt resistor. The resistor will limit the amount of current that the stepper driver is allowed to pass to the motor. Gecko has a very simple formula that computes the size of the resistor in K ohms. The formula is: 47 X Current / ( 7 - Current ) = Resistor size (in K ohms). So, using a motor that can handle 1A of current, we would compute the resistor like this: 47 X 1 / ( 7 - 1 ) = 7.8K. Gecko's data sheet permits us to use the closest standard size resistor, so the actual resistor that we would use with a 1A motor would probably be 8.2K.
What is auto-current-reduction? Stepper motors pull the most current when they are stopped; therefore, they also produce the most heat when they are stopped. Most stepper drivers have an auto-current-reduction feature. Sometimes that feature can be turned on or off by the user. When auto-current-reduction is turned on, the current to the stepper motor is automatically reduced a short time after the stepper driver stops receiving pulses. On the Gecko G202 stepper driver, that feature activates about one-second after the last pulse has been received. The G202 automatically reduces the current to about 30% of normal. The good thing is that by reducing the current, the electronics run much cooler. The bad thing is that holding torque is also greatly reduced. It's up to the user to determine whether the stepper motor has enough holding torque with auto-current-reduction activated or whether the stepper motor needs to be held at full torque when stopped. On the G202, the amount of current reduction is not adjustable. It is either turned on or it's turned off.
What happens if we let the motor pull too much current? Bad things will happen - eventually. Electrical components are built to handle a certain amount of heat. Heat is rated in Watts. We're all familiar with how much hotter a 100W light bulb is hotter than a 25W light bulb. Stepper motors are wound with wire and that wire is insulated to withstand a certain temperature. As long as the motor runs at or below that rated temperature, the motor will have a long life; but, if we allow the motor to get too hot by allowing the motor to pull more current through its windings than the windings were designed to handle, the motor will have a shortened life.
Should I choose a motor with high inductance or a motor with low inductance? Generally speaking inductance is to AC as resistance is to DC. When you look at the torque charts and compare various motors, usually you'll notice that motors with high inductance have a smoother low-end torque than do motors with low inductance. You'll usually also note that motors with high inductance have very limited high-end speed when compared to motors with low inductance. Personally, I prefer using motors with low to moderate inductance for general all-around use. Stepper drivers are designed to handle a limited range of inductance. The Gecko G202 stepper driver handles high inductance better than the G203v stepper driver. So, because I really, really, really like the G203v, I make it a point to always select a motor that works well with that driver.
Why does a stepper motor sometimes shake and make loud noises? Stepper motors resonate when running at certain speeds - typically at speeds less than 100 RPM. A user has almost no control over that resonating; however, wiring a motor Bipolar Series usually lets the motor run at slower speeds without resonating - but Bipolar Series wiring also severely limits the high speed performance of the motor. If a motor must run at very low speeds, then the usual practice is to add gearing to the motor to enable the motor to run at a faster speed.
Why do some stepper motors have gearboxes and others have direct drive? How a motor is to be used determines whether it needs a gearbox or whether it can be connected directly to its load. If a motor is attached to a ball-screw, each revolution of the motor's shaft might move the axis 1/5th of an inch. If a motor is attached to a spur gear that rides on a rack, each revolution of the motor's shaft might move the axis 3.14 inches or even 4.71 inches. It's easy to see that if we divide 3.14 inches by 2000 steps, that the axis will move 0.0015 inches. That's not bad for a CNC router, but if we used a 3.6:1 gearbox, each step would only move the axis 0.000436 inches AND the gearbox would also increase the motor's torque 3.6 times (within the limit of the gearbox's gears). So gearing is highly desirable.
Is there really any difference between Gecko driven motors and high priced motors that have their own stepper drivers? That is a very hard question to answer. The easy answer is, YES, there is a difference between a motor/driver that costs about $350 compared to a motor/driver that costs about $1,500 - as would be expected. However, knowing that there is a difference and justifying the cost of that difference is another matter entirely. First, lets look at resolution. Shopbot offers two competing motor systems, a Gecko driven 3.6:1 motor and an Oriental Motor Alpha 7.2:1 motor. On the surface it looks like the Alpha motor would have 2X the resolution when compared to the Gecko driven motor, but that is not the case. The Gecko driven motor requires 2,000 steps per revolution, while the Alpha motor requires 1,000 steps per revolution. The net effect is that both motors drive the axis the same distance per step. However, the 3.6:1 motor is basically a 300 oz*in motor connected to a gearbox that is limited by the gearbox's gears to about 350 oz*in even though the gearing would normally change that rating to about 1,100 oz*in. The 7.2:1 motor is basically a 600 oz*in motor connected to a gearbox that allows about 1,300 oz*in to be transfered to the axis. So, that comparison shows that if you require fast, deep cuts, the Alpha motor is clearly superior. The Alpha 7.2:1 motor has less slop or backlash than does the 3.6:1 motor. The Alpha 7.2:1 motor handles slow speeds more smoothly than does the 3.6:1 motor. So the only way that you can decide which motor is best for you is to look very hard at your requirements and then pick the motor that best meets those requirements. One thing to keep in mind when making that decision is that if high speed and deep cuts are on your required list, that you might also require a large spindle and a very large vacuum hold-down system to make deep, fast cuts a practical reality. One further point that must also be made is that making deep, fast cuts can cause a machine to flex substantially. The quality of cuts made at low to moderate speeds may be vastly superior to cuts made at top speed. My own machine is the PRT-Alpha that will soon have the 7.2:1 motors. Even though I know that I can build a custom belt-drive gearbox that would allow me to use a large stepper motor at 2X the torque of the 7.2:1 Alpha, I have been very pleased with the PRT-Alpha overall and hope to have substantially smoother cuts from the 7.1:1 motors than I had with the unmodified 1:1 motors that were originally shipped with my machine.