Long-stroke pumping unit

1. A pumping unit, comprising:

a tower;

a counterweight assembly movable along the tower;

a prime mover for reciprocating a rod string;

a belt having a first end connected to the counterweight assembly and having a second end connectable to the rod string;

a downhole pump connected to the rod string;

a load cell for measuring force exerted on the rod string;

a controller in communication with the load cell and operable to determine a load on the rod string,

wherein the load cell includes:

a tubular body disposed around the rod string; and

a strain gauge attached to the tubular body and in communication with the controller, and

wherein the controller is further operable to monitor for failure of the rod string or belt and control descent of the counterweight assembly in response to detection of the failure based on the load on the rod string.

2. The unit of claim 1 , wherein the load cell further comprises an accelerometer in communication with the controller.

3. The unit of claim 2 , wherein the accelerometer is configured to measure vibration of the rod string and is disposed in a chamber defined by a recess on an outer surface of the tubular body and a sleeve disposed around the recess.

4. The unit of claim 3 , wherein the accelerometer is a dual axis microelectromechanical system.

5. The unit of claim 3 , wherein the load cell includes an arm attached to the sleeve and a wire rope.

6. The unit of claim 3 , wherein the load cell is disposed in the chamber.

7. The unit of claim 3 , wherein the chamber further comprises an inert gas.

8. The unit of claim 1 , wherein the load cell is torsionally arrested relative to the rod string.

9. The unit of claim 1 , wherein the load cell includes a pair of washers for coupling the tubular body to the rod string.

10. The unit of claim 1 , further comprising a bar attached to the rod string, and wherein the load cell is disposed between the bar and an upper end of the rod string.

11. The unit of claim 1 , further comprising a sensor configured to detect a position of the rod string by detecting a position of the counterweight assembly.

12. The unit of claim 11 , wherein the sensor is an ultrasonic rangefinder comprising a long range transducer and a short range transducer.

13. The unit of claim 11 , wherein the sensor is a linear variable differential transformer (LVDT) comprising:

a string connected to the counterweight assembly and wound onto a spool;

a screw shaft engaged with a thread of the spool;

an LVDT core mounted to the screw shaft; and

an LVDT body at least partially receiving the LVDT core.

14. The unit of claim 1 , further comprising

a drive sprocket torsionally connected to the prime mover;

an idler sprocket connected to the tower;

a chain for orbiting around the sprockets; and

a carriage for longitudinally connecting the counterweight assembly to the chain while allowing relative transverse movement of the chain relative to the counterweight assembly.

15. The unit of claim 1 , wherein the controller is a programmable logic controller, application-specific integrated circuit, or field-programmable gate array.

16. The unit of claim 1 , wherein the controller is further operable to monitor for failure or imminent failure of the pumping unit and to shut down the pumping unit in response to detection of the failure or imminent failure.

17. The unit of claim 1 , wherein:

the prime mover is an electric three phase motor, and

further comprises a three phase variable speed motor driver.