July 23, 2013
Motor Madness - Part III (...and final, I promise)
In the last two installments, we talked about simple DC motors and stepper motors. To recap, a DC motor is basically the simplest (think 'Lloyd' from the movie 'Dumb and Dumber') motor you can find. Simply apply voltage with sufficient current and it spins. You can reverse the spin direction by reversing voltage polarity, and you can control the speed by applying varying voltage levels or by sending short pulses of voltage. Stepper motors are a...'step up' (I hate puns) insofar as intelligence goes (think 'Harry' from 'Dumb and Dumber'). By which I mean, you can send them specific (countable) pulses and the motor will rotate in very predictable increments (steps). Steppers are natively 'open loop' but are oftem fitted with rotary encoders to provide position feedback. However...that position feedback is often an 'after the fact' reconcilliation on the number of steps commanded vs the number of pulses counted. It is not a 'monitor on the fly' feedback loop and that is by definition what a servo motor brings to the table.
In the most basic sense, a servo motor is a brushess DC motor fitted with a feedback sensor. The output of the feedback sensor is used to determine the final position of the motor. These devices can be very small (like hobby RC servos) or larger. In general, with increased size comes larger windings which enable more current and greater torque. Depending on the device the motor is attached to, you can expect to see a series of drive reduction gears or pulleys for even greater torque. Unlike hobby servos which have both gear reduction and circuitry embedded within their housings and use potentiometers for feedback, most industrial servo motors require a separate driver or control board that provides motor voltage and also monitors the encoder in real time. The image to the left represents a simple hobby servo which nicely illustrates the feeback loop concept. Both the input signal and the output from the sensor (potentiometer, in this case) are feed into a comparator circuit. Because the gearing is built in and known relative to the gear ratios, the comparator can essentially decide when the motor needs to be stopped in order to achieve the commanded motion (so many pulses should equal so much resistance).
The pulses sent to the motor are generally all the same voltage but number of pulses sent over time is what determines speed. This is called Pulse Wideth Modulation (PWM).
Troubleshooting a servo motor is not easy. If the motor and encoder are separate units, you can apply the rated voltage to motor (disconnect any pulley/belt or linkages first!!!). Encoders are a bit trickier and we will cover them in a future tutorial. You can try some basics like checking wiring connectors or blowing air the encoder to remove dust, dirt or grease. Beyond that...you will need to crack open the case and hook up a scope to see what the pulse train coming from the encoders look like. Most encoders are off the shelf devices and you can Google specs to compare with what you have. This will at least help you zone in on the encoder or the controller/board it is plugged into.
July 22, 2013
Motor Madness - Part II
Unlike a brushless DC motor which rotates continuously when a fixed DC voltage is applied to it, a step motor rotates in discrete step angles. Stepper Motors are manufactured with incremental steps per revolution of 12, 24, 72, 144, 180, and 200, resulting in stepping angles of 30, 15, 5, 2.5, 2, and 1.8 degrees per step. The stepper motor can be controlled with or without feedback.
Stepper motors work on the principle of electromagnetism. There is a soft iron or magnetic rotor shaft (rotor -= rotate...it spins) surrounded by the electromagnetic stators (stators = stationary...they're fixed locations). The rotor and stator have poles which may be teethed or not depending upon the type of stepper. When the stators are energized the rotor moves to align itself along with the stator (in case of a permanent magnet type stepper) or moves to have a minimum gap with the stator (in case of a variable reluctance stepper). This way the stators are energized in a sequence to rotate the stepper motor.
There are two basic types of steppers-- bipolar and unipolar.Unipolar
- A unipolar driver's output current direction cannot be changed.
- There are two sets of the coils for each phase in a motor.
- Only one set of the coils can be energized at a time.
- Each coil represents one phase.
- Driver's output current direction can be changed. 100% of the winding is utilized in the bipolar drive. That means the two sets of the coils in each phase can be connected either in series or in parallel to become one set of a coil
- Current direction changed from the driver creates another mechanical phase.
- The number of mechanical phases is always twice the number of electrical phases
- Bipolar drivers provide 40% more holding torque than unipolar drivers, but typically run at higher temperatures
For this last reason, proper heat dissipation is important with bipolar drivers. A bipolar stepper has 4 wires and Unipolar steppers have 5,6 or 8 wires. The rotor has a permanent magnet attached to it. The stator is made up of coils as shown in the image to the left. There are eight coils in this unipolar stepper motor. Every coil in the motor behaves as an electromagnet, when they are energized by electrical pulses. For this particular motor, the opposing coils are paired and each pair shares a common wire.You can see that there are five electrical conntections of the PCB (one common, and four to control each coil pairing).
Stepper motors tend to be less expensive than servo motors (more on servos in Part III), and they are easier to control due to the fact that their precise incremental movements do not require posistional feedback. Simply command this 'open loop' motor and the location is predictable. Now, if a pulley attached to motor shaft comes loose, or if a timing belt attached to the pulley were to lose a tooth due to wear, it is conceivable that the device being controlled could act in an unpredictable way (gets lost) and could fail (crash).
Stepper motor control circuits can be viewed with an oscilloscope.You would need to identify the driver chip or indexing controller being used and look at the pulse train (disconnect any pulleys first).with the motor attached. The motor itself can be tested with an ohmmeter. If you have a bipolar motor, place the ohmmeter between the positive and negative input terminals of each winding. The resistance of the two windings should be exactly the same. If not, the motor must be replaced. If you have a unipolar motor, place the ohmmeter between the positive terminal and the com terminal and then between the negative and com terminals. Do this for each winding. In each case, the resistance should be the same for all measurements; if not, the motor must be replaced.
July 15, 2013
Motor Madness - Part I
If it moves...it probably has a motor. There are a number of different types of motors within lab instruments and while repair or diagnosis of many of them are beyond the expertise even the most savvy field service technician, it is helpful to know a bit about them and what makes them tick (or spin).
The most common motors are simple AC or DC motors. As their names imply, each uses a different current scheme to achieve basic rotation but a simple brushed DC motor has five parts:
- Armature or rotor
- Axle (shaft)
- Field magnet
In many motors, the outer metal housing contains at least two field magnets (North and South).
The armature, also called the rotor as it rotates about the axel, is an electromagnet made by coiling thin wire around two or more poles of a metal core.
The commutator is a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet.
The commutator and the brushes enable for the "flipping" of the electric field" part of the motor.
Brushed DC Motors have two coils of wire around a rotor in the middle. Surrounding the coil are two magnets, both facing in the same direction. When the coils are facing the magnets, electricity flows into them. When electricity flows into a coil, it creates a magnetic field, and this magnetic field pushes the coils away from their magnets. As the rotor turns, the current shuts off. When the rotor has turned 180 degrees, each rotor faces the opposite magnet. The coils turn on again, this time with the electricity flowing in the opposite direction. This creates another pulse, pushing the rotor around again. The rotor has electric contacts on it, and there are small metal brushes that bump against the contacts. The brushes send in electricity, turning the motor on and off at the right times.
Operationally, all you need to do is apply the proper DC voltage at the nominally rated current and the motor will spin. For simple devices this can be done via an on/off switch.
A brushless DC motor has a permanent magnet on the inside of the rotor, such that its north and south poles are perpendicular to the axle. Coils surround the rotor. These coils function similar to a brushed motor in that hey give out timed pulses to push the magnet, spinning the rotor. Because there are no brushes however, the motor cannot control itself. Instead, it is attached to a speed controller ciruit, which gives pulses of electricity at a certain speed to control the motor. The faster the coils pulse, the faster the motor will spin. This is called Pulse Width Modulation or PWM (more on that in Part 3).
On a final note, other than brushes, there reall isn't much that can be easily fixed on a DC motor. For older devices that are no longer supported, you can find rebuild services that can repair toasted armature (more common on larger motors). The most tempting way to test a DC motor is of course to apply power...but please, if you do this make sure to disconnect the motor from any mechanical drive components (pulleys, bests, chains, linkages...etc) first. As always, if you choose to ignore this advice, please do not send nasty emails, legal notices or graphic images of your physical injuries...
Next Up: Part 2 - Stepper Motors