Torque Control Circuit

Circuit :  Andy Collinson
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Description
This circuit is used to monitor a running motor. If the motor torque should increase the circuit can provide a visual indication or stop the motor, preventing damage. Applications like electric curtains, model cranes or lifts where a motor could stall can all benefit from this torque control circuit.

Notes
This circuit is available in two versions, one with an indicator which is useful if someone is present to stop the motor, and an automatic version which will remove power to the motor. Motors come in various shapes and sizes, the motor I used is from Ebay and is a 12V motor with integral gearbox running at 120RPM. These do move around on Ebay so always a good idea to do a search.

Torque or "twisting force" is a measure of the motors strength. Small electric motors have torque measured in grams per centimetre. With a light mechanical load the motor will draw its rated current. Should the mechanical load increase, then the motor torque (and current) will also increase. If the torque is severe then the motor could stall and overheat. As torque is proportional to armature current, its an easy matter to monitor the average motor current to provide indication of motor torque.

Measuring Torque
Some manufacturers do not publish motor torque, one method to measure motor torque is to calculate the time taken to raise a known weight as outlined by this web page, Lego Technic Motors Compared. As shown on the lower graphs of the link as torque increases, so does motor current. It is now a simple matter to monitor motor current to provide an indication of torque. To do this, a low value resistor is placed in series with the motor, as current flow through the motor (and resistor) increase, the voltage developed across the resistor also increases and is monitored by an op-amp wired as a comparator. Typical motor currents are shown below: When a voltage is first applied to a motor, its armature is static and a large current will flow to overcome the inertia of the armature. Once moving, motor current decreases to a lower value. Any increase in mechanical load or friction causes an increase of torque, and motor current will rise. These three distinct factors are shown in the time verses current plot below:

1) The starting current spike. ( A brief surge of current to overcome inertia and get the motor turning. )

2) The normal operating current. ( A lower current which can vary due to friction. )

3) Current under abnormal load. ( If the mechanical load becomes excessive torque and current increase. )

The graph is typical for any motor, but starting current and running current depend on the physical and electrical characteristics of the motor used, mechanical load and power supply.

It is section 3 the extra current consumed under load that this circuit works upon. The elevated current is proportional to higher motor torque and the circuit can be set to trigger anywhere in this region.

How it Works
The motor is controlled via the TC4427A MOSFET driver IC. This can driver motors up to 18V and 1.5A peak current. A small series resistor R6 is placed in series with the ground terminal of the TC4427A. R6 is chosen so that no more than 5% of the motors voltage is developed across it at normal load.

The starting current spike is filtered out by R4 and C1. The time constant just R4*C1 and with values shown is set to roughly half a second.

The CA3140 is a MOSFET input stage op-amp and works as a comparator, comparings the voltage across C1 to the voltage set by the 10k preset potentiometer. This is shown as the "blue square" in the real breadboard photograph and later, as black on the Fritzing breadboard layout. As soon as the voltage at C1 reaches the threshold, the op-amp output swings high to a couple of volts less than the supply voltage.

Basic Circuit
The circuit below has two switches S1 and S2. These control motor direction and apply a high input signal to pins 2 and 4 of The TC4427A. However the input signals could be supplied from an independent circuit, or micro-controller. The signalling requirement is that high voltage is defined as > 2.4 V and low < 0.8 V. The TC4427A can also withstand a high reverse current of up to 500mA and does not need any reverse diode protection.

In the basic circuit the 10k control is adjusted for desired torque, when reached the red LED will light and go out again when torque is reduced. This circuit does not stop the motor and serves as an indicator.

Modified Circuit with Motor Stop.
The modified circuit, below works as before, but has the addition of an output transistor and relay. When the CA3140 swings high, the BC108 is switched on, and the DPDT relay is energised. The relay has two contacts A1 and A2. Once energised relay A/2 is powered via its own contact A2 and contact A1 breaks power to the motor. The circuit can only be reset by pressing S3, a normally closed push button switch. Circuit Setup for different Motors.
To use a different motor (maximum 18V and no greater than 1.5A at maximum torque) first use a multimeter and measure normal running current. Then apply some torque using gentle pressure to the motor axle and note the current taken. R6 should not draw more than 5% of the motors voltage. With a 12v motor, 5% is 0.6V divide this by the normal running current to get the value for R6.

The motor used in this project from Ebay was rated 12V and had a no load current around 54mA. 5% of 12V is 0.6V and diving 0.6 by the no load current gives a value of 0.6 / 54mA = 11ohm. I did not have an 11 ohm resistor to hand but found the circuit worked well with a 15 ohm resistor and even a 4.7 ohm resistor. The lower the value the more electrical power is available to the motor.

To apply torque, my motor had a wheel with tyre attached. Applying pressure to the tyre increased current flow and torque. Under torque the motor current could rise to 130mA with the 4.7 ohm resistor. This means mean 4.7 * 0.13A = 0.61V would be developed across R6.

With such a low voltage the comparator input needs an adjustment range that is available easily. Potentiometers have a range of travel of about 270°. If the 10k control was wired across a 12V supply, then 1° would be about 0.044V and 0.6V would require a rotation of about 13° which would be fiddly. If you wanted to set the circuit to trigger at 80mA this would be almost impossible.

To facilitate adjustment, it is much easier to make the range of voltage available close to the value of maximum torque. This is the purpose of R3. With a 12V supply, the maximum voltage at Pin 2 is then 10k/(10k+47k)* 12V = 2.1V With a 270° trimmer each 1° of rotation equals 8mV and you will find it much easier to set precise torque levels.

Finally, below is a larger image of a breadbord layout I made using Fritzing. Fritzing is an open source project designed to make electronics accessible to everyone. The Fritzing program includes, breadboard layout, schematic capture and PCB in one package. Its cross platform and available for Linux, Mac and Windows. If you want to download the layout click the link below. Note this is the breadboard layout only.

TC4427 Pinout
The TC4427A manufactured by Microchip is available as an 8 Pin DIP package and the pinout is shown below:

Datasheet

TC4427 Datasheet