Sunday, June 14, 2009

SDM students evaluate mowers for John Deere - SDM Pulse, Summer 2009

By Justin Kraft, SDM ’09

Editor’s note: In this article, Justin Kraft reviews the capstone project he completed with colleagues Tim Miller and Al Narveson to fulfill a core requirement of SDM’s certificate program (a oneyear program taught primarily at a distance). Kraft received his certificate in 2008 and is currently pursuing his SDM master’s degree.

Justin Kraft, SDM ’09
Companies interested in quickly infusing systems thinking into their technical operations often send several students simultaneously to SDM’s one-year certificate program. Tim Miller, Al Narveson, and I all came to the certificate program through the sponsorship of John Deere, and we teamed up for the required capstone project. Taking on a real company problem, we applied systems thinking to the development of a John Deere walk-behind lawnmower for golf greens.

Narveson and I are both senior engineers at the John Deere SouthEast Engineering Center in North Carolina, and Miller is a project engineer at John Deere Waterloo Works in Iowa. No one on the team was an expert in commercial turf mowing—yet the skills we learned through SDM enabled us to provide the company with a useful analysis of potential new mower designs.

We began our work with a look at stakeholder requirements. Using information from a stakeholder survey conducted by John Deere, we determined that customers have three core requirements for greens mowers.

1) No fluids. A leaky mower can damage golf greens, so customers prefer that no fluids be used in the equipment. Nevertheless, current state-of-the-art mowers include gasoline, grease, and hydraulic fluids.

2) Less vibration. Operators need to mow up to nine greens in a single day, so the process must be fast and efficient. Customers therefore prefer a machine with less vibration, because vibrations fatigue the operator.

3) Ease of serviceability. Mower blades need to be sharpened daily, so ease of serviceability can reduce overall maintenance costs. Auto sharpening of blades is not currently available on John Deere equipment, but it exists on some competitive machines.

As we learned in SDM, creating the target product specification is critical to the successful development of any product. We therefore used several tools and analysis techniques to ensure a robust, valid, and useful specification. This work included zeroing in on goals with a system problem statement:

To maintain the turf to an adjustable height from 5/64 inch to 1 inch, with no contamination and maximum operator comfort by cutting grass evenly using a John Deere walk-behind greens mower that is easy to service.

Other key goals included maximum safety for the operator and bystanders and least cost of manufacturing.

Our approach to the problem was to develop several alternative concepts, searching for ideas both externally (by reviewing patents, reading competitive product literature, and reviewing the best-in-class survey conducted by John Deere) and internally (by brainstorming as individuals and as a team). This concept generation process yielded five alternative designs.

We then employed the Pugh Selection Process taught in SDM to choose among design options. First we created a list of criteria weighted to reflect their importance to the target product specification. These ranged from cut quality (40 percent) and feasibility (20 percent) to noise level (5 percent) and controllability (2.5 percent). This process revealed that two of our initial concepts—an autonomous vehicle and a mower powered by a hydrogen fuel cell— were infeasible for commercial use at this time.

We therefore selected the following concepts for detailed review:

Hybrid gasoline-electric mower. A traditional internal combustion gasoline engine is used to create energy in the form of a mechanical drive and electric power.

Hybrid compressed air-electric engine. This concept uses a compressed air tank and engine to create energy in the form of electrical power. The air tank would need to be recharged between uses.

Battery powered electric mower with an electric drive. In this case, a battery is used to store electrical power.

All three options included the same blade-sharpening technology for easy serviceability. The air-electric engine would use the fewest fluids, and the electric motor would have the least vibration.

For the final selection process, we employed a weighted average approach using a Pugh selection matrix [see Figure 1]. This process eliminated the air-electric engine from consideration—but it was worthwhile to investigate its feasibility because this was the first time Deere had evaluated this design.

The score for the battery-powered electric drive was so close to that of the gas engine that we decided to perform sensitivity analyses of the concepts by changing the weight associated with each selection criterion.

In our first analysis, we looked to the voice of the customer and decided to place less weight on cut quality and more on noise and vibration. This change puts the battery-powered electric drive on top. A second analysis, adding to the weight for maintenance as well as for vibration and noise level, similarly benefited the electric mower.

However, power output was a critical concern in the battery-powered design. The lack of power meant a golf course would need to buy additional mowers and add operators to do the same work as a single gasoline mower—a major additional cost. We therefore adjusted our analysis to reflect this higher cost as well as the lower feasibility of this design option (the power output of current battery technology cannot truly compete with the internal combustion engine in commercial applications).

In the end, our product development team recommended the hybrid gas engine-electric motor as superior to the others in power output and feasibility. We found that the current state of the art does not allow for the other alternatives to be commercially viable at this time.

Interestingly, this capstone project was done in parallel with the actual production by Deere of a gas engine hybrid mower. I believe our work confirms that developing that mower was the correct strategy. In addition, our capstone project showed that the complete electric mower is probably closer to reality than most people at Deere believed. Therefore, an important takeaway is the future need to focus on evaluating developing energy storage technologies.

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