Brushed DC Motor Programmable Speed Regulator
This article explains how to design and build a low noise high performance low cost brushed DC motor programable speed regulator. The regulator is immune to the temperature, voltage, and motor load variation. Also, this circuit does not care about the motor coil resistance which allows using different motors in the same application without any adjustments.
Introduction
The circuit is designed for devices where a low power brushed DC motor is used. Where the stability of the motor speed and low noise are critical. The regulator is immune to the temperature, voltage, and motor load variation. Also, this circuit is not sensitive to the motor coil resistance which allows using different motors in the same application without any adjustments. The motor speed is pre-programmed and does not require trimming after assembly. Although, it is possible to re-program it to a different speed via I²C.
Below we described steps needed to understand how the solution has been programmed to design a brushed DC motor programable speed regulator using SLG47004. However, if you just want to get the result of programming, download Go Configure™ Software Hub, to view the already completed GreenPAK Design File.
Unlike traditional circuits (see Figure 1) this design does not require any feedback sensor. In this case, the motor itself is a sensor. Other, traditional designs (see Figure 2) are dependent on the motor coil resistance. Both require post-production trimming.
Some applications (audio, for example) are very noise sensitive. That makes any PWM motor controllers not suitable.
Every brushed DC motor produces interference when brushes switch from coil to coil. This is considered one of the biggest downsides of such a motor. Unfiltered it is capable to interrupt the normal work of the circuits powered from the same power source or simply situated close by. While it is possible to filter out to an acceptable level the interference using a capacitor connected in parallel to the motor (in some cases ferrite coil in series), it is impossible to get rid of it completely.
In the case of the design described in this document, the small leftover interference is used to determine the motor speed. In most cases, cheap low voltage low power brushed DC motors have three moving coils. Each of them produces interference in a form of spikes, thus the frequency of the spikes is three times the revolutions of the rotor. So, by counting the spikes it is possible to not only determine the speed but also control it by automatically adjusting to a pre-programmed value. All this (and more) can be achieved by using the SLG47004 a versatile programable mixed IC with only basic external components. See the schematic diagram in Figure 3.
Design Operation
Schematic Design
As previously mentioned, only one chip is used in this design. The SLG47004 IC combines all necessary analog and digital macrocells in a tiny 3 x 3 mm STQFN-24 package. See Figure 4 for a complete schematic diagram in the GreenPAK Designer project.
To understand how it works, see the simplified schematic diagram in Figure 5.
The analog part is a simple voltage regulator with a power P-channel MOSFET on the output. But instead of a voltage reference, it has a digital potentiometer controlled by a digital part of the device.
The digital part consists of the ACMP which detects the small spikes coming from motor brushes. Capacitor C2 blocks DC voltage but lets through the spikes. Amplified to a VDD level they go to the Counter, which is set to a frequency detection mode, where the spike frequency is compared to a reference frequency coming from the oscillator.
When the device is powered on the resistance of the digital potentiometer is as pre-set, so some small voltage goes through a current amplifier (voltage regulator) built out of the OPAMP and P-FET and comes to the motor. It starts rotating and produces spikes. With each detected spike the digital pot resistance will increase by one bit slowly increasing the motor voltage. As a result, the motor speed will be rising until the spike frequency matches the reference frequency. And when it does, the counter will signal the potentiometer to decrease its resistance by one bit with each spike. The voltage will drop along with the motor speed. The counter will detect the frequency drop and signal the pot to increase its resistance and the cycle does on.
In other words, the voltage on the motor rises while the spike frequency is lower than the reference frequency, and vice versa. Once the motor speed settles the potentiometer will go up and down one step (1 LSB) keeping the speed stable within ±1 LSB.
Ideally, it would take ±1 LSB to settle the motor speed, but due to the inertia, it may take up to ±50 LSB. The heavier the motor load, the more LSB it takes to settle the speed.
The frequency detector period can be calculated using the formula below:
Where:
T — period
RPS — revolutions per second (RPS = RPM/60)
n — number of motor coils
Should be noted that the oscillator frequency must be selected so the counter data is close to half the counter resolution given the desired RPM (for instance, 127 for the 8-bit counter). Leaving maximum headroom for regulating the motor speed.
Using P-FET on the output of the regulator has a benefit of a very low voltage drop, allowing the VDD margin as low as 100 mV.
See figures 6 to 9 for oscilloscope screenshots demonstrating the stability of the device at different VDD levels.
Legend:
· Yellow — test point output (Pin 12)
· Blue — ACMP input
· Purple — motor DC voltage
As an alternative design, an optical RPM feedback sensor can be used, see Figure 10. This method ensures greater motor speed accuracy and stability as there are more pulses per revolution. The frequency detector and ACMP Vref should be adjusted accordingly. In this case, in the formula for calculating the period «N», stands for the number of the gear tooths. The downside of this design is the cost of an extra sensor and increased power consumption.
In both designs, PIN 12 is used as a test point. Using an oscilloscope with probes connected to PIN 16 and PIN 12 it is possible to check if all input spikes are picked up by the ACMP. In some cases, the ACMP Vref should be adjusted so every input spike is picked up and converted to a logic-level pulse.
Additional Design Features
The suggested design is the simplest one. Its main goal is to explain its principles.
However, the SLG47004 has plenty of unused macrocells which allow for improving performance and adding new features to the design.
For example, instead of the 8-bit counter, the 16-bit counter can be used. This will drastically improve the frequency measuring accuracy and allow fine-tuning of the motor speed. In this case, the oscillator frequency should be increased accordingly.
Another improvement can be made by adding a feature of selecting different motor speeds. By adding more frequency detect macrocells and switching between them using simple logic (LUTs and DFFs), it is possible to create a device with user-selectable pre-programmed motor speeds.
Macrocell Configuration
I2C Settings: default
Conclusion
As can be seen, designing and building a low noise high performance low cost brushed DC motor programable speed regulator using the OPAMP PAK is very easy. The SLG47004 turned out to be the perfect IC for the design containing all necessary analog and digital macrocells. The design shown in this document is one of many versions of the device that can be built based on the SLG47004. Some unused macrocells can be used to design additional functions as suggested in section 4.2.
This slightly modified design was used as a DC motor speed regulator for a high-performance vinyl turntable. An optical feedback sensor and a 16-bit counter were used to sense and stabilize the speed. As a result, the rotating speed of the turntable was measured at exactly 33 1/3 RPM with a deviation of less than ±0.15%. Which far exceeds hi-fi requirements.
