The photo and YouTube video is used to determine a DC motor's properties. Called a dynamometer, one gathers torque and current draw over a range of motor speeds. Plots of these measurements yield the motor's back-EMF and torque constants. These properties are important to determine if a motor is suitable for a task. Below depicts data collected using the Lego-based Dynamometer (LBD). The plots show NXT motor properties like current versus torque, speed versus torque and mechanical power versus torque.

This tutorial presents the Parts List, Lego Build Plan, the mechanical and electrical construction, and operation. The circuit can be breadboarded. However, a PCB (KiCAD files included) makes for more compact and secure connections.

1. Motivation and Audience

This photo above is a typical setup of a DC motor lab required in engineering school. The lab has students capture measurements and apply fundamental DC motor theory to determine the motor's properties. Properties like stall torque, stall current, maximum power, and back-EMF constant are important. They are used to design robot arms, mobile carts, and drive trains in transfer lines like conveyor belts. One must select a motor that is well-suited for the device's torque and speed requirements. Ideally the motor comes with a datasheet from the manufacturer. But datasheets aren't always available or information is missing. This is especially true with cheap or surplus motors - and Lego motors.

This setup is a winch and works as follows as demonstrated in the above YouTube video. First, one ties one end of a string to a mass and the other end to a disk. Second, one secures the disk to the motor's axis. Knowing the values of the mass and disk radius, one can calculate the torque being applied to the motor. Third, the motor is mounted at a some height above the ground (e.g. on top of door frame). Fourth, an ammeter is connected in series to the DC motor. Lastly, the motor is turned on to wind the mass upwards. During the mass' ascent two measurements are written down; the ammeter current and the motor's speed (usually using a tachometer).

While winches work, it is cumbersome and prone to errors. First, the motor needs to be mounted above the ground e.g. clamped to a desk or door frame. The mounted height determines how much string one uses. But more importantly, it determines how much time one can read (and write down) the ammeter and tachometer readings. For small masses and fast motor speeds, the time can be only a few seconds. Adding to this cumbersomeness, is to repeat the process for a range of masses.

The Lego-based Dynamometer (LBD) presented here was designed with Lego parts that fit in a tackle box. This is so students could take the tackle box, quickly assemble it, and perform DC motor labs in the lab, the library, or even in their dorm. It can be dis-assembled back into the tackle box and easily stored for future use. Unlike winches, the LBD gives one time to to accurately read and write down torque, current and motor speed measurements. Lastly, everything fits in the tackle box and no additional devices like multimeters or tachometers are needed.

A prony brake, like bicycle caliper brakes, applies friction to a rotating disk. Below are some examples used with NXT motors:

• How to Make and Test a Prony Brake
• Team 45 Prony Brake Demo
• Team 14 ENGR 1523-01 Prony Brake
• Prony brake construction & operation with EV3 and Mindstorms software

These look like a class project or perhaps as capstone design that mimic the classic prony brake (Wikipedia). Here, as the thumb screws are tightened, the lever arm pushes harder into the scale. By knowing the lever arm's length and scale's mass reading, one can calculate the torque being applied to the motor. Unclear from these videos is how current and speed are measured.

A more compact way to couple the lever with bevel gears. A 3D printed bevel gear mechanism is shown in this Youtube video. One observes that the lever only applies force into the scale when friction is applied to rotating disk.

This LBD uses standard Lego parts to recreate this bevel-lever coupling. Additionally, thumb screws tighten a caliper made from Technic beams and rubber Lego parts. The thumb screws are locked into place using rivets that insert into the Technic beam. Lego rubber bands mimic springs that keep the calipers open. The ammeter, voltmeter, and tachometer are widely available parts that are encased in Lego plates. The net effect is a system that can is compact, self-contained, easily assembled and dis-assembled, and can be employed into engineering lab courses.

This Tutorial assumes the reader has the following background and interest:

• Wants to idenify Lego (and non-Lego) motor properties like maximum torque and efficiency
• Knows fundamental DC motor relationships between current, torque, and speed
• Has circuit building experience (needed for the ammeter, tachometer)

2. Parts List and Sources

3. Construction - Mechanical

Step 3-1:

Step 3-2:

Step 3-3:

4. Construction - Electrical

The above schematic (PDF | KiCAD SCH file) uses an NXT socket to connect the circuit to an NXT motor. A PCB (PDF | KiCAD PCB file) was fabricated with Technic-sized holes. This allows the PCB to fit snuggly on Lego Bricks. This KiCAD ZIP can be used if you wish to fabricate your own PCB boards at companies like Oshpark.com

One can breadboard the circuit. In that case, one may wish to use an NXT breadboard connector kit. The circuit integrates the LED Ammeter/Voltmeter, LED tachometer, and a barrel connector to plug in a voltage source that drives the NXT motor.

Step 4-1: LED Ammeter/Voltmeter

Popular and low-cost, this device is an LED Ammeter/Voltmeter. It has an internal shunt resistor and the on-board analog-to-digital converter calculates current going through the device. They are sold in a variety of shunt resistances. Make sure to use the one that can measure up to 10 A (as specified in the Parts List above). The minimum current it can measure is 0.10 A (i.e. 100 mA). Understandably, the NXT motor's no-load current is below 100 mA. However when loaded, it will easily go over 100 mA.

There are 2 options to power this device. The first is to use a shared voltage source. This means the voltage driving the load (i.e. the motor) is also used to power on the LED Ammeter/Voltmeter. The second, which is used in this circuit, uses an independent voltage supply, like a 9 V battery. The wire hookup is given in the above figure. Below is a breadboarded hookup.

Step 4-2: LED Tachometer

In Step 18 of the Lego Build Plan, one uses a Lego Rubber Band to secure the Hall Effect Sensor to the Lego Plate 2 x 6 Part 3795 (shown as Yellow in the photo). As the Brown Brick Round 4 x 4 Part 6222 rotates (because it's attached to the NXT motor), the Hall Effect Sensor's Signal pin will go HIGH once per revolution.

To display the motor's rotational speed, the LED tachometer above is used. It is sold with a magnet sensor. But because it is bulky, the 3-pin Hall Effect Sensor is used. It also comes with a JST 5-pin cable. It requires an external power supply to power the LED display. Hence the circuit employed a 9 V battery to power it, as well as the LED Ammeter/Voltmeter described in the previous step. Below is a step response of the NXT motor that captured the RPM data from the sensor.

5. Operations

As the photo shows, tilting the displays can make them easier to read. The lid can be lifted to access the electronics as needed. The PCB employed JST connectors for the LED Ammeter/Voltage and LED tachometer, to mate their polarities correctly. One sees the 9 V battery which powers the LED displays. The toggle switch is in series with this battery, so the displays can be turned on or off.

An 3-12V 2A Adjustable Power Supply was used to power the NXT motor. As such, one can capture Voltage [V] versus Motor Speed [RPM] data. Also, by fixing the voltage (e.g. 5.0 V), one can capture Current [A] versus Motor Speed [RPM] data as well as Torque [Nm] versus Motor Speed [RPM] data. Alternatively, the PCB includes banana sockets if one would rather use a different adjustable voltage supply.

6. Final Words

The NXT motor was used in the Build Plan. However, also use other Lego motors, as shown in the above photo. The EV3 Medium Motor Mount Build Plan (PDF | Studio IO file) again use the MILS principle. This allows the motor to be mounted on the base and easily snapped into LBD. Power Function motors like the XL-Motor Part 8882-1 can also be employed. But one needs the Power Functions Extension Wire (20 cm) Part 8886-1 and the NXT to RCX Part x1676 or Part 54690 cable. These will allow Power Function motors to connect into the LBD's NXT socket.

Conceivably one can connect non-Lego motors to the LBD. But this will require attaching the motor's axis to the Lego Axle on the LBD. Additionally, one would have to physically and securely mount the motor to the base. It may be simpler to re-create the LBD using non-Lego parts. The electronics can remain the same.