Author Archives: Ben

New Tic T500 revision to address problem with missed steps

via Pololu Blog

One of the fun things you learn as an engineer is that physically laying out a circuit requires a lot more than simply confirming it matches the schematic. In practice, there are many more things to think about, and if you’ve seen datasheets with layout guidelines or recommendations, you’re probably aware of some of them. (Thermal considerations, adequate decoupling, minimizing parasitic inductance and capacitance…) What’s more, these considerations are often in conflict (e.g. is it better to have the decoupling capacitor closer to the IC or to have a wider trace for that high-current path?), and trouble can arise if you don’t correctly assess their relative importance. That is what happened to us with our Tic T500 stepper motor controller, where the location of a specific decoupling capacitor ended up being far more significant than we had anticipated.

The problem with our original Tic T500—as we found after a customer reported having issues—was that the driver sometimes seemed to reset and lose its position in its indexing table, causing the stepper motor to stutter and skip steps. This only happened under certain conditions, and it appeared to occur more often with low-resistance and low-inductance motors, high input voltages, and high current limits.

These oscilloscope captures show the difference between normal operation and the problematic behavior on the Tic (pink is the current in one motor coil, yellow is the voltage on the driver’s STEP pin). But even without a scope, it’s apparent when the problem occurs: you can see, hear, and feel the motor jolting and moving erratically.

Unlike blatant mistakes in the design of a board that prevent it from working at all, this type of problem can be difficult to detect since a very specific combination of factors needs to be present for it to occur. When we develop a new board that might be used in a wide range of conditions, we try to cover as much of that range as possible in our testing. For example, each of our switching voltage regulator designs goes through a characterization process that involves sweeping over the full input voltage range at many different loads while looking for instability in its output voltage. Some problems are resolved by more careful selection of component values, but occasionally, we have to do additional board revisions, sometimes even going to higher layer counts, before making a product available for sale. Unfortunately, we did not encounter the problem with the Tic T500 in our initial testing.

Our first indication that something might be wrong came last month when someone posted on our forum about some strange behavior they were observing, and we started trying to replicate the issue here. In an effort to understand what was happening, we duplicated the design of the Tic using an A-Star and an MP6500 carrier board (which has the same driver the Tic T500 uses) on a solderless breadboard. The expectation was that the relatively crude breadboard layout would make the problem worse by adding all sorts of stray capacitance, inductance, and contact resistance, and that would give us some better insight, but we were unable to reproduce the problem with this setup. This pointed to the issue being something subtle about the layout of the Tic rather than some fundamental error in its routing.

After trying various different modifications to the Tic T500, we found that we could reduce or eliminate the resets and skipped steps by adding extra capacitors near one of the driver’s VIN (motor voltage input) pins. We subsequently revised the PCB to include footprints for those additional capacitors, and that seems to have solved the problem. The left picture below shows the original PCB with a capacitor we added while tracking down this problem, and the right picture shows the new revision with that capacitor added in a classier way:

This picture indicates where all the VIN capacitors and pins are on all three boards:

Surprisingly, the new capacitor on the top side of the board (indicated by the dashed outline) actually seems to be much more effective, which is unfortunate, because it limits how big of a heat sink you can add there. Even though it is basically in the same place as the one visible on the bottom side on the board, the top-side capacitor alone was enough to prevent the resetting and skipping in our tests, while the bottom-side one alone did not.

One reason we designed our basic MP6500 carrier first was to make sure we had a good understanding of the driver before designing it into more complicated products, so it was more than a little frustrating when the same basic layout that worked on the carrier didn’t work on the Tic. Initially, we assumed the difference was due to the MP6500 carrier having a 4-layer PCB compared to two layers on the Tic T500, which generally lets everything be better connected, and that could still be a big part of the answer. However, while making the comparison image above, we realized the MP6500 IC is actually rotated 180° on the carrier compared to the Tics (the text on the IC is upside down on the carrier and right side up on the Tics). So the two 0.1 μF capacitors on the carrier are closer to the “bottom” of the chip, while the two on the original Tic are closer to the “top”. If the chip is sensitive to a lack of capacitance near the bottom VIN pin, maybe that helps explain the behavior we saw!

Even though we expect it to be somewhat rare for customers to encounter this problem on the Tic T500 (to my knowledge, we have only heard of two cases), we want to make sure our products represent our best work, so we’ve pulled all our stock of the original boards (tic03a), and we’re offering free replacements for any that have already been sold. If you bought a Tic T500 from us directly, we will be emailing you to let you know how to replace it with an updated board (tic03b), or you can just contact us directly for a replacement. We will also be working with our distributors to get them replacement units for any customers that got the original Tic T500.

New product: STSPIN220 Low-Voltage Stepper Motor Driver Carrier with 1/256 microstepping

via Pololu Blog

I am happy to announce our first new product of 2019, a carrier board for the STSPIN220 stepper motor driver, which operates all the way down to 1.8 V, making it our lowest-voltage stepper motor driver. And like its higher-voltage sibling, the STSPIN820 that we released a few months ago, it offers microstepping down to 1/256 steps. This new carrier board has the same 16-pin, 0.6″ × 0.8″ form factor as our other popular stepper motor drivers, and as with our STSPIN820 carrier, it inverts the enable input so that it has the more familiar functionality of those drivers (but be careful not to pop these into a 12 V or 24 V socket!).

By the way, keep in mind that you do not necessarily need a low-voltage stepper motor driver just because your stepper motor has a low rated voltage. The voltage rating is just the voltage at which your stepper motor will draw its rated current, and it’s really the current rating that you need to be careful about if you want to avoid damaging your stepper motor. All of our stepper motor drivers let you limit the maximum current, so as long as you set the limit below the rated current, you will be within spec for your motor, even if the voltage exceeds the rated voltage. In general, using a high supply voltage along with active current limiting allows for better performance, so the main reason for using a low-voltage stepper motor driver like the STSPIN220 is if your supply voltage is constrained to some low value by some other aspect of your system.

This new release brings our selection of stepper motor drivers in this compact form factor to eleven:


A4988
(original)

A4988,
Black Ed.

DRV8825

DRV8834

DRV8880

MP6500,
Pot. CC

MP6500,
Digital CC

TB67S279­FTG

TB67S249­FTG

STSPIN­820

STSPIN­220
Driver chip: A4988 DRV8825 DRV8834 DRV8880 MP6500 TB67S279­FTG TB67S249­FTG STSPIN­820 STSPIN­220
Min operating voltage: 8 V 8.2 V 2.5 V 6.5 V 4.5 V 10 V 10 V 7 V 1.8 V
Max operating voltage: 35 V 45 V 10.8 V 45 V 35 V 47 V 47 V 45 V 10 V
Max continuous current per phase:(1) 1 A 1.2 A 1.5 A 1.5 A 1 A 1.5 A 1.1 A 1.6 A 0.9 A 1.1 A
Peak current per phase:(2) 2 A 2.2 A 2 A 1.6 A 2.5 A 2 A 2 A 4.5 A 1.5 A 1.3 A
Microstepping down to: 1/16 1/32 1/32 1/16 1/8 1/32 1/32 1/256 1/256
Board layer count: 2 4 4 4 4 4 4 4 4 4
Special features: high current low-voltage
operation,
high current
AutoTune,
digital current
reduction
high current digital current
control,
high current
Auto Gain Control,
ADMD,
high max voltage
Auto Gain Control,
ADMD,
high max voltage,
high current
128 and 256
microsteps,
high max
voltage
64, 128, and
256 microsteps,
low-voltage
operation
1-piece price: $5.95 $7.49 $8.95 $5.95 $6.95 $5.95 $5.95 $7.75 $9.95 $7.75 $5.95
1 On Pololu carrier board, at room temperature, and without additional cooling.
2 Maximum theoretical current based on components on the board (additional cooling required).

Last year, we began offering introductory specials to celebrate each newly released product, and we are continuing with that this year: the first 100 customers that use coupon code STSPIN220INTRO can get up to five units at just $3.77 each.

New products: 16-channel QTR MD reflectance sensor arrays

via Pololu Blog

QTR-MD-16A Reflectance Sensor Array.

We now have 16-sensor, medium-density (8mm-pitch) versions of our new QTR reflectance sensor arrays. Like the versions already released, these new modules are available in analog and RC configurations and with two different sensor types, resulting in four new products in all:

Unlike the medium-density (MD) arrays we have released previously, which just use the high-density PCBs in partially populated configurations, these new 16-channel modules use PCBs specifically designed for an 8 mm pitch. As a result, these are the first MD versions that allow separate control of the odd and even emitters, which gives you extra options for detecting light reflected at various angles. They have the same board dimensions (125 × 16.5 mm) and mounting hole locations as the high-density (4mm-pitch) 31-channel arrays, but the pinout is different.

QTR-MD-16A Reflectance Sensor Array.

QTR-HD-31A Reflectance Sensor Array.

For more information on our new QTR sensor family, you can see some of our previous blog posts about the versions we have already released:

Don’t forget to get in on our QTR introductory promotion! Be one of the first 100 customers to use coupon code QTRINTRO and get any of these new sensors at half price! (Limit 3 per item per customer.)

(A little more than) twelve days of Christmas sale

via Pololu Blog


If you missed our Black Friday sale or realize you didn’t quite get everything you wanted, don’t fret: we have all active Pololu-brand and PCX products on sale for 12% off, and we are offering 15% off twelve broad categories of our products. The sale runs through Friday, December 21, but be careful to order early for delivery before Christmas. Save on your Christmas shopping, or stock up now on robot parts for the new year. Check out the sale page for all the discounts and coupon codes. Merry Christmas!