April 27, 2015
Back in May of 2013, we talked about the use of third party tips on liquid handling robots. Well, we got a lot of emails on that one as it seems a lot of people are looking for alternative tip providers, for a number of reasons...but mostly cost savings.
We heard a lot of stories regarding OEM's who threatened to void instrument warranties or even refuse to service instruments that use third party tips. Is that legal? Probably not but, it certainly is unwise and I would imagine that is not a company policy so much as it is a regional sales rep or service engineer who does not want to lose a lucrative revenue stream.
Having worked on the supplier side (at Caliper Life Sciences, now part of Perkin Elmer), I can tell you that most OEM's want end users to buy their tips...and only their tips. Why is that? Well, the biggest reason is that they have invested heavily in the creation of precision injection molds and the logistics required to stock and ship tips. This is not inconsequential and often explains why OEM tips tend to cost more than third party tips. Even when the mold costs have been amortized, stocking and distributing tips is a costly endeavor, as is ongoing quality monitoring.
So, you might be asking, how could a third party tip cause an instrument to fail? The only conceivable scenarios I can think of are:
a) head crashes due to physical differences or
b) 0-rings on mandrels that might wear or deform due to physical differences (polymers or dimensions).
Other than that, it's really a red-herring argument. Our sister division, AssayGuru has performed performance analysis on a number of third party tips for various manufacturers and compared both pipetting accuracy and mechanical reliability. These tests were conducted on various brands of liquid handling robots using both third party and OEM tips. In most cases, there have been no issues and the third party manufacturers we have worked with were eager to find and correct any flaws, prior to launching their tip products.
In my next post, I will detail one such manufacturer who has impressed us so much that we have actually created a new partnership...stay tuned.
July 23, 2013
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.