Not your father's power tools
Martin Rowe- February 1, 2009
Project engineer Jason Leh performs voltage, current, temperature, and other measurements to test motor controls.
Photo by Joshua Roberts/Getty Images
Towson, MD—DeWalt power tools such as drills, saws, and grinders help contractors worldwide build just about anything. Not long ago, power tools were electrically simple—a variable-speed power switch provided power to a motor. Today, electronics control motors, charge batteries, monitor a tool’s health, and add control features. Electronics testing is now just as important as mechanical testing in making a reliable tool.
At DeWalt headquarters near Baltimore, engineers design and test electronic modules for AC- and DC-powered tools. Instead of the mechanical switches used in older power tools, new tools have microcontrollers, triacs, and MOSFETS that provide pulsed power to motors. Electronics also contribute heavily to battery monitoring and management, especially as lithium-based batteries have begun replacing NiCd (nickel-cadmium) batteries in cordless DC-powered tools.
“With digital controls, we can monitor current, voltage, and temperature in a tool,” said engineering director Bhanu Gorti. “We can use the information to control the power applied to a motor based on load.” Today’s tools also record usage profiles that engineers can use to test tools by using them the way contractors do.
Project engineer Jason Leh and senior project engineer David Beers were part of a team that developed a method for controlling motor speed in grinders. “When contractors use our tools, they tend to run them into the ground,” noted Leh. “We found that tools often fail when their motors overheat.”
Unfortunately, reliability, manufacturing challenges, and added cost wouldn’t let DeWalt engineers use a motor-temperature sensor. To prevent overheating, they developed microcontroller code that estimates a motor’s temperature based on current and time.
Initially, they wrote code such that if the temperature exceeded a limit, the microcontroller would shut down the motor and wouldn’t restart it until the motor cooled to safe levels. Leh and Beers learned, however, that the heavy users don’t want a tool to remain off while the motor cools, as this could prevent them from finishing their work on time.
Leh changed the microcontroller code to give the user the ability to restart the tool by simply cycling the switch regardless of tool temperature. The user receives a warning that the tool may potentially overheat but still has the option to finish the job after the motor cools for several seconds. If the user immediately applies a heavy load to the tool, the microcontroller will shut down the motor to prevent damage.
Figure 1. An AC motor-control module controls motor speed by allowing only portions of an AC voltage cycle to pass through a triac.
Figure 1 shows how the microcontroller controls power in an AC-powered tool. A triac, under microcontroller control, conducts and applies voltage to the motor for a portion of an AC cycle only. In Figure 1, that occurs between the 60° and 180° points in the positive half cycle and between the 240° and 360° points in the negative half cycle.
The microcontroller delivers power based on information derived from measured current and estimated temperature. If the estimated motor temperature is low, the microcontroller applies the full AC cycle to the motor. As the temperature climbs, the microcontroller decreases the amount of voltage applied to the motor in proportion to the rise in temperature. If the temperature then decreases, the voltage supplied to the motor will increase.
Leh started the design by running simulations of the motor’s performance and temperature under load. From the simulations, he generated a series of curves that describe how the motor temperature changes under varying loads. From these curves, he derived coefficients that the microcontroller uses to estimate motor temperature. “We can use this method on other tools as well,” Leh said. “We just change the coefficients that model the motor’s thermal mass.”
To verify the accuracy of the simulation, Leh and Beers measure motor temperature by placing thermocouples at the best possible positions near the motor. Four thermocouples, connected to a Yokogawa or Fluke datalogger, measure the temperature once per second.
The motor temperature at the instant the tool turns on is important when estimating motor temperature while the tool runs. Because the microcontroller’s memory loses data when power is removed, the engineers needed a way to retain temperature. They solved the problem by placing a thermistor in the tool’s control module. The microcontroller measures temperature at tool startup and shutdown, storing the data in nonvolatile memory. Upon the restarting of the tool, the microcontroller retrieves the data from nonvolatile memory and uses it in calculations.
To verify temperature estimates, Leh developed an interface board that connects to the microcontroller’s SPI (serial peripheral interface) bus and converts the signals to RS-232 for connection to a PC. He can view the time-stamped data, in text format, using Windows HyperTerminal, and he can save the data to disk. Leh and Beers then time-correlated the measured and estimated temperatures and plotted them with Excel. Figure 2 shows how closely the estimate tracks the measured motor temperature.
Figure 2. Tests show how well a simulated motor temperature tracks a measured temperature.
During the verification test, DeWalt engineers simulated actual usage profiles obtained from users. In one profile, a user ramps up the load on the motor until it reaches 10 A, holds for 1 s, then ramps up to 15 A for 15 s. The motor then turns off for 2 min, turns back on, and ramps up to 20 A for 30 s.
Motor-control electronics reside in modules small enough to fit inside a tool. These modules need production test, and the motor-control engineering lab has testers identical to those used on the production lines. Using a bed-of-nails fixture, an in-circuit and functional tester verifies that a module is properly assembled. Known as “Tester 06,” the tester also calibrates a module’s internal peripherals such as the ADC (analog-to-digital converter) that makes motor current and voltage measurements. The functional tester also programs the module before potting and final test.
DeWalt engineers calibrate the internal peripherals by loading test routines into a module’s microcontroller. A programmable-logic controller controls the tester’s AC power supply, which delivers the voltage—through a calibrated resistive load—into the microcontroller. The microcontroller’s ADC measures load current by measuring voltage across a shunt resistor, making the measurements at both a low-current and a high-current setting.
Figure 3. An in-circuit functional tester loads test code into a microcontroller and calibrates the module.
After Tester 06 verifies a proper current reading, it calibrates the other ADC channels and the microcontroller’s internal oscillator. The tester compares all module measurements to its own calculated values and determines calibration values for the module (Figure 3). Following successful calibration, Tester 06 loads the microcontroller with its production code before it gets potted.
After potting, a motor-control module goes to final test. “Tester 02” lets engineers perform final tests in the same way as in production. Final test consists of performing hipot tests on the final assembly along with final verification of all inputs and outputs.
AC powered tools don’t always get clean power at a construction site, so their motor-control modules must work under all electrical conditions. “Sometimes on the job site,” noted Gorti, “AC-powered tools are powered by generators that have poor regulation. Because of that, we must test motor controls for voltage dips and surges. We test our tools under as many AC voltages as contractors will use.”
The motor-control lab has a California Instruments programmable AC source built into a tester with AC mains sockets for US, UK, Australia, Denmark, Germany, Norway, Finland, Sweden, Switzerland, Italy, South Africa, and India. Using the AC source, engineers can vary both voltage amplitude and frequency and test for line surges, dips, and clipping. This supply is also capable of delivering up to 12 kVA, which is useful for running a variety of tools.
Engineering director Bhanu Gorti oversees design and test of DeWalt motor controls, batteries, and battery chargers.
Photo by Joshua Roberts/Getty Images
Engineers use the tester to verify that tools will operate over a wide range of frequencies. “We design our tools to operate from 30 Hz to 80 Hz,” said Gorti.
Because DeWalt manufactures cordless tools, the company’s engineers must design and test battery packs, battery chargers, and the electronics to manage them. The company introduced its first 18-V cordless drill in 1996. Now, contractors spend more than $1 billion a year on batteries for DeWalt products.
Today, lithium-ion cells are replacing NiCd and NiMH (nickel-metal-hydride) batteries. Lithium batteries are lighter and don’t contain cadmium, a heavy metal. DeWalt’s lithium battery packs are backward compatible—both electrically and mechanically—with tools the company built for NiCd/NiMH batteries. A lithium battery pack will work with any DC-powered DeWalt tool, provided the tool is compatible with the battery’s output voltage. That’s important because it lets contractors use a battery pack of similar voltage from any DeWalt tool.
Figure 4. A lithium battery pack (right) is compatible with older NiCd battery packs. Chargers (left) can charge batteries in 15 min or 1 hr. Courtesy of DeWalt.
Lithium batteries (DeWalt currently uses lithium-ion phosphate technology) require electronics to monitor voltage and temperature while the battery charges and discharges. Because DeWalt’s lithium battery packs must be mechanically equivalent to its NiCd battery packs, engineers had to embed the battery-management electronics in the neck of the housing. Figure 4 shows a battery pack and a mating charger. The neck of the battery pack inserts into tools and chargers.
Battery testing includes testing of the power-management circuits. The battery-engineering lab has automated testers that provide power and loads to battery-management modules. Other stations test the complete battery packs (electronics included) for charging and discharging.
When testing battery-management modules and battery packs, engineering manager Dan White and his team simulate loads based on actual user profiles because DeWalt tools can record data on their power consumption. Working with a DeWalt tool, the engineers capture profiles with a laptop computer and duplicate them in the lab. An automated test station running Visual Basic contains numerous power resistors that the engineers can switch to the battery pack and use to record its voltage and current.
Engineering manager Dan White oversees design and test of battery packs. Here, he performs an ESD test on a battery pack.Photo by Joshua Roberts/Getty Images
Another station measures battery response during charging and discharging at –10°C to 90°C, even though the published specification is 0°C to 40°C. This station, which runs National Instruments’ LabView, contains several power supplies, an Agilent Technologies electronic load, and an Agilent multiplexing DMM (digital multimeter).
Engineers also evaluate batteries and control modules with a Fluke infrared temperature camera. The camera lets them pinpoint failed components. They will run tests designed to make the electronics fail. “We test it until it breaks because we want to know the conditions that our products can withstand,” said Gorti.
Technician Chris Held runs qualification tests on batteries to verify their microcontroller software. He developed a test bench that measures voltage, current, and temperature through a National Instruments’ data-acquisition chassis running LabView.
Using thermocouples on each lithium cell in a battery pack, Held performs measurements to verify that the power electronics properly measure voltage, current, and temperature when working in a tool. By connecting each cell in a battery pack to a channel in the data-acquisition system, Held looks for voltage differences among the cells during charging and discharging. Thermocouples and an infrared camera monitor the battery’s temperature during a test. The test verifies that a battery-management module will remove battery power from a tool when an overcurrent, overtemperature, or undervoltage condition occurs. Similarly, he tests the batteries for charging conditions to make sure the battery cells don’t overcharge.
Held also measures battery current under several loads. He runs a sequence of tests at 10 A and 15 A to verify that the battery electronics know how much current a load draws from the battery. An electronic load, controlled through a USB-to-GPIB interface, controls the battery’s load. In addition, Held uses a LeCroy oscilloscope and current probe to measure inrush current in tools at room temperature and at 60°C. Inrush currents in tools can exceed 200 A. Held’s tests verify that the battery-management module applies compensation to prevent the voltage from dropping to a level that will cause microcontroller lockup.
Like all electronics, battery-management modules are susceptible to RFI (radio frequency interference), both from external sources and from ESD (electrostatic discharge). White explained that, with the exception of the terminals, the batteries are housed in cases that are protected from ESD. Current from ESD can’t reach the cells or electronics directly, but static discharges produce electric fields that can couple into PCB traces and produce current.
To perform an ESD test, White wraps a copper foil around the neck of a battery pack (over the management module), leaving a spark gap in the foil. He then uses an ESD simulator from Thermo Keytek (now Thermo Fisher Scientific) to inject ESD into the foil ring. “It’s not a compliance test,” noted White, “but it provides a good indication of immunity.”
Charged and ready
Rechargeable batteries need charging, and engineers such as Geoff Howard design and test chargers, which provide constant current to discharged batteries. A battery charger can charge a battery in as little as 15 min, although typical charge time is 1 hr.
Chargers designed for lithium batteries differ from those designed for nickel-based batteries, because the battery electronics participate in the charge process by monitoring battery voltage. With NiCd batteries, chargers cut off current by monitoring the inflection point of the battery voltage because voltage will drop once the battery is fully charged. With lithium batteries, the management module needs to monitor voltage level and temperatures.
Bench testing of chargers lets Howard and others verify that a charger meets design specifications. Using an automated test setup, technicians measure charge current and battery voltage waveforms with a LeCroy oscilloscope. An Agilent electronic load with a capacitor across it simulates a battery. “For some tests, a resistor is enough,” said Howard, ”but the electronic load can switch ranges and it can create a switching waveform that can disrupt a charger. The capacitor, in parallel with the load, makes it behave more like a battery.”
The test bench also includes an Agilent waveform generator that simulates a tool’s power switch waveform, providing a trigger signal for the oscilloscope. Operating the oscilloscope in single-shot mode lets technicians measure spurious noise caused by a tool’s power switch that interacts with the battery electronics.
Figure 5. A battery charger built into a radio provides charged batteries and entertainment. Courtesy of DeWalt.
Some chargers are embedded into rugged radios, which provide on-the-job music and charge tool batteries (Figure 5). Technicians use the oscilloscope’s FFT (fast Fourier transform) feature to look at the charger in the frequency domain. Howard explained that chargers produce RF emissions from switching circuits that must not interfere with the radio. To make the RF measurements, engineer Aziz Iqbal built a “sniffer” probe from coax cable.
Battery engineers developed a failure test that uses a high-speed camera that can shoot a picture every 250 µs to show where a component will fail. In this test, an engineer forces a MOSFET—used in both battery management and motor control—into its linear region where the drain-to-source resistance is high. By pumping enough current through the device, the engineer makes the MOSFET fail. The bond wires inside the device package melt and cause the failure.
DeWalt’s tools, batteries, and chargers must withstand all the conditions on a construction site, which include temperature, vibration, shock, torque, and airflow. “Contractors drop our products all the time,” said Chris Sanford, who manages several test labs in the company’s reliability center.
Chris Sanford manages numerous test labs, including a drop-test lab in the reliability center. Photo by Joshua Roberts/Getty Images
Electromechanical test setups subject drills to heavy torque, just as a contractor will do. As Jason Leh noted earlier, contractors run tools to the point where motors can burn out. In the torque test lab, tools are attached to discs, much like disc brakes in a vehicle. Calipers, also much like those in disc brakes, apply friction to a spinning disc, which increases the load to the point where the disc stops. Tools must be able to shut down under overheating conditions caused by too much torque. The torque tests can run overnight and on weekends. If a tool fails, the tester can make a phone call to a technician who will come in to evaluate the failure.
Sanford also runs tests on the switches designed into tools. “A tool is only as good as its power switch,” he said. “If we specify that a power switch is rated for 200,000 cycles, we’ll test it for 1 million cycles.”
An automated tester for variable-speed-control switches looks for the weak point in a switch, which occurs at the point most likely to produce arcing across its contacts. The tester can home in on that point, then repeatedly test the switch around that point. Although DeWalt engineers test this condition for switches used in both AC- and DC-powered tools, the problem is more pronounced with DC-powered tools.
Other test labs focus on shock, vibration, sound, and airflow. The DeWalt facility includes rooms that contain wood, metal, and concrete where people spend hours cutting, drilling, and performing other tasks to test a tool’s reliability. DeWalt also has a lab in which technicians test batteries by constantly charging and discharging them under high-temperature conditions.
In the drop-test lab, DeWalt staffers subject tools to shock. The lab contains a tower that can raise a tool up to 20 ft and drop it to a concrete floor. While this test is important, contractors may drop tools from greater heights. So, after a tool passes an indoor drop test, technicians will go on the building roof and drop tools about 25 ft. “The building maintenance people keep telling us to get off the roof,” said Sanford.
DeWalt’s facility also includes a semianechoic chamber that lets engineers measure the sound a tool produces. “Sound measurements are important because European regulations require that tools not produce sound that can damage a person’s hearing,” said Sanford. “There are no such requirements in the US now, but they will come.”
An airflow lab lets engineers analyze how much air exits a tool. The engineers also control airflow to see how much air a tool needs in order to run cool enough to minimize motor burnouts.
In the sound and vibration lab, a shaker table simulates a tool transported in the back of a truck. “Our goal is to break everything and see that our tests exceed user expectations,” explained Sanford.
In addition to surviving drops, power tools used at construction sites must survive being run over by trucks. Engineers will drive a pickup truck over a battery or charger, then verify that it still works. These and other tests let DeWalt engineers learn how to improve their product so that contractors can keep on building.