September 6, 2017
CRASH!!!! Your 8-channel liquid handling robot arm just raked across the deck and one of the z-axis rods looks bent. No problem, just call the manufacturer and have them come fix it, after all, it is still under warranty...right? Well, maybe...
Most instrument warranties cover parts and labor but, that usually comes with the expectation that the failure is due to normal wear and tear, not abuse or unintended usage. Using the liquid handler failure above as an example, the 8-channel arm likely got damaged because it failed to move to a safe Z-travel height before moving in X or Y. But, was that because the arm failed to execute that command or because the programmer failed to instruct the arm to do so? While a failure such as this might not occur in assays that have been running successfully for some period of time, they are more common when the user is still developing the assay or debugging it. This type of failure could also occur because an operator forgot to retract the arm after some assay interruption or error condition.
Many OEM's (Original Equipment Manufacturer) will work with you to get the instrument back online and some may even be tolerant of such failures to the point of covering the associated costs under their warranty..but, you will most likely find there is a limit to their understanding. If an instrument fails under normal usage, OEM's should and will cover repair costs but if an instrument fails again, or frequently due to operator error the OEM could and should charge for parts and labor and travel, even though the unit is under warranty. Although such a stance would be unpopular for end-users, it is really no different than what you might experience in other areas of your life. If you use your SUV to haul a boat that exceeds the vehicles gross towing rate you will probably damage your transmission or rear axle. Should you expect Ford or GM pay for that? The honest answer is, no.
Whether you bought the instrument new and are under the original warranty, or if you have purchased an extended warranty, make sure you understand just what kinds of failures are covered. Ask up front. Even if you purchase refurbished instruments, there is a limit to they nature of the failures that are covered (BTW - you should always insist on a minimum of a 6 month warranty on refurbished equipment). New or used, a warranty is a quality statement by the provider. Buying instruments "AS IS" or with a "Money Back Guarantee" should set off alarm bells that the low price option that looks so attractive today, could prove to be a costly investment in the future. Caveat Emptor...
What options should you consider when the warranty expires? That will be the subject of our next blog...
July 22, 2013
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.
June 28, 2013
Did you ever work with a field service engineer who was just plain awesome...someone who always went above and beyond to ensure your success? If so, you have probably asked yourself ''what would I do without them? ' Sadly, that hypothetical question, all too often, becomes reality.
For any number of reasons (some good, some not so good) people are transient. As the old saying goes, no one is truly irreplaceable, so the best protection vendors can provide for their customers is to ensure that more common procedures are documented. For example, irrespective of who is doing the work, a preventative maintenance procedure should always be the same. Each step, every tool, replacement part, lubrication or adjustment should be captured in a document that can be used to cross train FSE's so that your instruments always receive consistent maintenance.
Whether you are working with a new FSE to support a new install, or existing instrument don't hesitate to ask to see the procedure they will be following. Most vendors won't share all the details, but many will let you have a glance and most will provide checklist that highlights the work to be done.
If a vendor cannot produce documents for common procedures (like a PM). before they commence their work you should be concerned. I'm not saying that you are about to be mis-treated, however how can you be certain that the requisite work will be accomplished if there is no guideline? You wouldn't conduct an assay without a documented procedure and you shouldn't allow anyone to work on your instruments without one either.
If they can't show you 'the book', then throw the book at them!